docs/InlineRuntimeChecks.md

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# Inline runtime checks in Flang with `fir.assert`

Status: draft design, for discussion.
Context: [llvm/llvm-project#223287](https://github.com/llvm/llvm-project/pull/223287).
Source references are to `llvm-project` at `fe1de64cb3c2` (2026-10-02). Line
numbers drift, so each reference also names the function or pattern.

Abbreviations used in the tables:

| Short | Path |
|---|---|
| `SHI` | `flang/lib/Optimizer/HLFIR/Transforms/SimplifyHLFIRIntrinsics.cpp` |
| `OB` | `flang/lib/Optimizer/HLFIR/Transforms/OptimizedBufferization.cpp` |
| `IHA` | `flang/lib/Optimizer/HLFIR/Transforms/InlineHLFIRAssign.cpp` |
| `SAA` | `flang/lib/Optimizer/HLFIR/Transforms/SeparateAllocatableAssign.cpp` |
| `C2F` | `flang/lib/Optimizer/HLFIR/Transforms/ConvertToFIR.cpp` |
| `SI` | `flang/lib/Optimizer/Transforms/SimplifyIntrinsics.cpp` (FIR level) |
| `IC` | `flang/lib/Optimizer/Builder/IntrinsicCall.cpp` |
| `X2H` | `flang/lib/Lower/ConvertExprToHLFIR.cpp` |
| `RT/` | `flang-rt/lib/runtime/` |

Terminology:

- **Lowering** is the translation of the Fortran parse tree to HLFIR, in
  `flang/lib/Lower`.
- **Emitting** an assert means creating a `fir.assert` operation, either in
  lowering or in an HLFIR or FIR pass.
- **Code generation** is the conversion of FIR to the LLVM dialect
  (`FIRToLLVMLowering` in `flang/lib/Optimizer/CodeGen/CodeGen.cpp`), where
  `fir.assert` becomes a branch and a runtime call.

## 1. Problem

Flang relies on the Fortran runtime for almost all dynamic checks: shape
conformance, allocation status, argument consistency of intrinsics. Every
time an HLFIR or FIR pass replaces a runtime call with inline code, the check
silently disappears. Three consequences:

1. **Behavior depends on the optimization level.** The inlining passes mostly
   run at `-O1` and above, so the same non-conforming program stops with a
   clear message at `-O0` and does something else at `-O2`.
2. **The inline code is often memory-unsafe on non-conforming input, not just
   wrong.** Loop bounds are taken from one operand and used to index another
   (`fir::factory::deduceOptimalExtents`). A size mismatch becomes an
   out-of-bounds read or write instead of an error.
3. **Some checks the standard requires are missing on every path.** For
   example, the inline ALLOCATE/DEALLOCATE of intrinsic-type allocatables
   does not check the allocation status (F2023 9.7.1.3, 9.7.3.2, 9.7.4).

The few inline checks that exist today (MOD/MODULO `P==0`, NEAREST `S==0`,
STORAGE_SIZE, the realloc-with-scalar-RHS case) use `fir.if` plus
`fir.call @_FortranAReportFatalUserError`. That pattern:

- has no data dependence on the operation it protects, so passes may move the
  protected operation above the check (MLIR models neither "may not return"
  nor non-local control flow: `mlir/docs/Rationale/SideEffectsAndSpeculation.md`);
- introduces a call with unknown memory effects, which blocks alias-based
  optimizations;
- does not tell LLVM the call never returns (the FIR declaration has no
  `noreturn`), so LLVM cannot use the checked condition afterwards.

## 2. Goals and non-goals

Goals:

- Keep every user-facing check the runtime performs when the call is inlined,
  and report it with the runtime's message format,
  `fatal Fortran runtime error(<file>:<line>): <message>`, and with message
  text as close as possible to the runtime's.
- Make non-conforming programs behave the same at `-O0` and `-O2`, so that
  tests disabled because of that difference (for example in the
  llvm-test-suite gfortran tests) can be re-enabled.
- Keep the checks cheap and transparent to optimization. They must not block
  store-to-load forwarding, CSE of loads, LICM of loads, or alias analysis.
- Let checks be hoisted, merged, folded, and dropped under an option.
- Provide one mechanism for future opt-in checks (`-fcheck=bounds`,
  `pointer`, `do`, `bits`).

Non-goals:

- Detecting undefined POINTER association status (only disassociated, that
  is null, is detectable).
- Checking explicit-shape dummy size against the actual argument in general
  (the callee only receives an address).
- Changing the runtime's own checks, apart from the bugs listed in the
  appendix.

## 3. The `fir.assert` operation

### 3.1 Semantics and syntax

```
%g:N = fir.assert %ok, <kind>, "<message>" [values(%v0, ... : T0, ...)] [guard(%x0, ... : U0, ...)]
```

- `%ok` (`i1`) is the condition that must hold.
- `<kind>` is a check category (section 4). Options act on categories.
- `<message>` is the error text, a `printf` format.
- `values(%v0, ... : T0, ...)` are optional `i8`, `i16`, `i32`, `i64`, or
  `index` SSA values (`T0`, ... are their types) printed as signed integers
  into the message, one per `%lld` directive. Boolean diagnostics must be
  explicitly extended by the producer to select their signed or unsigned
  interpretation. Values are only read on the failure path. For example, the two
  extents in `"DOT_PRODUCT: SIZE(VECTOR_A) is %lld but SIZE(VECTOR_B) is %lld"`.
- `guard(%x0, ... : U0, ...)` are the optional guarded values, of any type.
  The op has one result per guarded value, with the same type (`%g#i` has
  type `Ui`).
- If `%ok` is true, each result equals the corresponding guarded value and
  nothing else happens. Otherwise the program terminates through the Fortran
  runtime with the formatted message.
- Operations that must not run before the check consume the guarded results.
  The ordering is then enforced by SSA dominance, for every pass, without any
  pass having to know about `fir.assert`. Asserts without a guard only order
  themselves with respect to I/O and calls (section 3.2).
- The condition itself must be well defined on the inputs it rejects. It
  must not depend on an operation that has undefined behavior on them, such
  as a division by a zero stride, an overflowing `nsw` or `nuw` operation,
  or a read of an absent OPTIONAL argument outside `fir.if`. Such operations
  come after the assert and use its guarded results.

What to guard: the operand of the protected operation, such as a divisor,
a DO step, a subscript, or a box that is dereferenced. For conformance
checks, it is the **array being accessed** (a variable, or an
`!hlfir.expr`), at the point where it is combined with another array:

- In an elemental operation or an elemental procedure reference, guard the
  array operands that do not provide the iteration space.
- In an array assignment, guard both sides. The pass that expands the
  assignment may iterate over either one.

Conformance checks do not guard shapes or extents. HLFIR can re-derive them
without going
through a guarded value: from static types (`hlfir.shape_of` folds to a
constant shape), from the preference for constant extents
(`fir::factory::deduceOptimalExtents`), or from the other side of an
assignment. Leaving them unguarded also keeps constant loop bounds visible
to MLIR loop optimizations. Array bases are never compile-time constants, so
guarding them hides no constant. It does hide their origin (global, local,
dummy) from the analyses that find it through the defining operation, which
must look through the guard (section 3.2). Section 7 explains how the guards
follow an expression when it is inlined.

### 3.2 Traits

- `MemoryEffects<[MemWrite<FortranRuntimeResource>]>`, where
  `FortranRuntimeResource` is a non-addressable resource (like the existing
  `fir::DebuggingResource`).
  - The op is never trivially dead, so DCE keeps asserts without uses.
  - It stays ordered with calls and I/O, whose unknown effects cover every
    resource.
  - It does not touch user memory. FIR alias analysis skips non-addressable
    resources (`flang/lib/Optimizer/Analysis/AliasAnalysis.cpp`, `getModRef`),
    and MLIR CSE ignores writes to resources disjoint from a load's resource.
    So asserts do not block load CSE, store-to-load forwarding, or FIR LICM of
    loads. This also means `SimplifyHLFIRIntrinsics` can emit asserts in its
    first run (`allowNewSideEffects=false`, `SHI:3321-3333`): the concern
    there is new effects that stop CSE from merging loads, which this op does
    not have.
- Not speculatable.
- **Not transparent to other operations' folds and patterns.** The assert's
  own canonicalization removes it when `%ok` is a constant true (section 3.4).
  But no other rewrite may look through an assert to its guarded operand,
  because that drops the dependence. For example, in
  `%a = fir.box_addr %b; %a_ok = fir.assert ... guard(%a); %c = fir.embox %a_ok`,
  a fold that rewrites an `fir.embox` of a `fir.box_addr` into the original
  box must not replace `%c` with `%b`. In practice:
  - the op does not implement `ViewLikeOpInterface`;
  - it is not added to the helpers that skip `fir.declare` or `fir.convert`
    to find a defining op;
  - it is not given a folder that returns a guarded operand when `%ok` is
    unknown.

  Analyses, on the other hand, must look through the assert, since they do
  not rewrite uses. This covers alias analysis, and the helpers that find the
  origin, Fortran attributes, contiguity, or optionality of a variable
  through its defining operation (for example the matching done by
  `OptimizedBufferization`). Otherwise guarding both sides of every array
  assignment (A1) would pessimize them, for example by adding overlap tests
  and temporaries, so this is a prerequisite of the conformance checks
  (section 8). Shape and length queries may also look through `conformance`
  and `argument` guards, but not through `pointer` guards, which protect the
  validity of the guarded value itself.

### 3.3 TableGen sketch

```
def fir_AssertOp : fir_Op<"assert", [AttrSizedOperandSegments,
    MemoryEffects<[MemWrite<FortranRuntimeResource>]>]> {
  let arguments = (ins I1:$condition,
                       fir_RuntimeCheckKindAttr:$kind,
                       StrAttr:$message,
                       Variadic<AnySignlessIntegerOrIndex>:$values,
                       Variadic<AnyType>:$guarded);
  let results = (outs Variadic<AnyType>:$results); // types == guarded types
  let hasVerifier = 1;
  let hasCanonicalizer = 1;
}
```

### 3.4 Canonicalization and redundancy elimination

- **Constant true:** erase the assert and replace its results with the guarded
  operands. This replaces the hand-written `fir::getIntIfConstant` early-outs
  of today's checks (`IC:6929-6931`). It must be a canonicalization pattern
  rather than only a `fold`: for an assert with no guard, an empty fold result
  means an in-place update, not an erasure.
- **Constant false:** keep it, with no diagnostic. A false condition is common
  in code that is unreachable but hard to prove unreachable at compile time
  (for example, after inlining or specialization on constant arguments), so a
  warning would be noisy.

  A later pass could erase the code strictly dominated by a constant-false
  assert. This is not proposed for the first version, and it must not run in
  any pipeline that may drop asserts (for example, device code with checks
  disabled). There, the assert would disappear together with the pruned
  code, and a program that should have stopped would silently produce
  wrong results.
- **Dominating assert with the same condition and kind:** remove the later
  one, mapping its results to the earlier one's results. This is valid
  regardless of what lies between them and regardless of the message, because
  the earlier check fails first. Two restrictions:
  - The kinds must be the same. Some pipelines drop asserts by kind after
    this point (section 4), and removing a check because of an earlier one
    that is dropped later would lose it.
  - The earlier assert must guard everything the later one guards; otherwise
    the consumers would lose their dependence.

  Upstream CSE cannot do this: it skips ops with write effects
  (`mlir/lib/Transforms/Utils/CSE.cpp`, `simplifyOperation`). So this needs a
  small dominance-based pass (section 6). It is not required to start with:
  once the runtime call is declared `noreturn`, LLVM already removes many
  redundant checks (GVN, jump threading). The MLIR pass is for cleaning up
  checks early, before the MLIR loop analyses and transformations that would
  otherwise have to step around them.

### 3.5 Code generation

`fir.assert` is converted in `FIRToLLVMLowering`, where the IR is already a
control-flow graph:

```
// fir.assert %ok, conformance, "..." values(%na, %nb : index, index) guard(%b : !fir.box<...>)
llvm.cond_br %ok weights([2000, 1]), ^cont, ^fail
^fail:
  llvm.call @_FortranAReportFatalUserErrorValues(%msg, %file, %line, %na64, %nb64, ...)
  llvm.unreachable
^cont:   // uses of the result are replaced with %b
```

- **Runtime entry.** `_FortranAReportFatalUserError(msg, source, line)`
  (`flang/include/flang/Runtime/stop.h`) is `[[noreturn]]` and device-callable
  (`RT_API_ATTRS`). It takes no values. Assertions with up to four values use
  `ReportFatalUserErrorValues(msg, source, line, v0..v3)` with fixed
  `long long` arguments, padding unused arguments with zeros. Assertions with
  more values use `ReportFatalUserErrorValueArray(msg, source, line, count,
  values)`, where `count` is a `size_t` and `values` points to an array of
  `long long`. Code generation packs that array on the failure path; the
  assertion's diagnostic operands remain SSA values until then. Both entries
  have fixed signatures to keep the ABI simple on devices. Messages use only
  `%lld` and `%%` (for a literal percent sign). The array entry prints the
  diagnostic in pieces through the existing runtime termination machinery,
  without allocating a formatted message. Concurrent failures may interleave
  their output.
- **Declaration.** The runtime function declaration must carry `noreturn`,
  so LLVM can use the checked condition on the continuation path.
- **Source location.** Take it from the assert's own location. Resolve
  `FusedLoc` and `CallSiteLoc` to the innermost `FileLineColLoc`: today
  `fir::factory::locationToFilename` returns null for them, so inlined checks
  lose file and line.
- **Device code.** String globals must be created in the nearest symbol table
  (a `gpu.module` for device code), and the conversion must also exist in
  any device code generation pipeline. If a device runtime lacks the entry point,
  fall back to `printf` plus `llvm.trap`. The function is not in the OpenMP
  offload API group today, unlike `Abort`.

Converting asserts earlier, before `CFGConversion` (to `fir.if` plus call), is
also possible, but `fir.if` regions cannot end with `fir.unreachable`, and
asserts would stop being visible to the passes after that point.

### 3.6 Alternatives considered

**`fir.if` plus a runtime call (today's pattern).** Not retained, for the
reasons in section 1: there is no data dependence on the protected operation,
the call's unknown memory effects block alias-based optimizations, and LLVM
is not told that the call never returns.

**`cf.assert`.** Not retained. It writes the default resource, which is how
it models program termination, so it blocks store-to-load forwarding and
load CSE across it. It still does not order non-speculatable operations
without memory effects, such as `arith.remsi`, after it. Its conversion to
LLVM prints with `puts` and calls `abort`, rather than reporting through the
Fortran runtime. It also has no guarded results.

**A result-less `fir.assert` (the current revision of
[#223287](https://github.com/llvm/llvm-project/pull/223287)).** The PR
defines `fir.assert` as a non-speculatable op with a
`MemWrite<FortranRuntimeResource>` effect, converts it to `cf.assert`, and
teaches Flang LICM not to hoist non-speculatable operations across a
preceding assert in the same block. This design keeps that effect model
(section 3.2), which gives the right DCE, alias-analysis, and forwarding
behavior. It is not retained as is, because the ordering of the protected
operations is not expressed in the IR:

- MLIR has no notion of an operation that may not return. Every pass may
  assume that all operations in a block execute together, and may move a
  `fir.load` or an `arith.remsi` above an assert whose effects do not
  conflict with it. The LICM change protects one pass. Every other pass that
  moves or recreates operations would need the same fix, and future passes
  would have to remember it. Examples are sinking, scheduling, and rewrite
  patterns that build new operations at a different insertion point.
- Once a protected load or remainder is above the check, LLVM may infer from
  its undefined behavior that the check cannot fail, and delete it.
- The guarded results make the ordering a matter of SSA dominance, so no pass
  has to know about `fir.assert`. Code generation then calls the Fortran
  runtime entry directly, so messages match the runtime's format (section
  3.5).

The design in this document is therefore the PR's op, with guarded results
added and code generation calling the runtime directly.

**A region-based assert, like `shape.assuming`.** The protected operations go
inside a region attached to the assert, and the values used afterwards are
yielded out:

```
%r = fir.assert %ok, conformance, "..." -> (f32) {
  // inlined DOT_PRODUCT loop nest
  fir.result %sum : f32
}
```

Ordering is structural, so it is as safe as the guarded-result form. It is
not retained because the region gets in the way of the optimizations the
checks must not block:

- The protected code is usually a whole loop nest or the rest of a
  statement, so everything it defines has to be yielded through the region.
  An allocation-status check would wrap the rest of the block.
- Passes that work on a single block or match operation sequences see a
  region boundary instead. Examples are the `OptimizedBufferization` pattern
  that matches an `hlfir.elemental` and its `hlfir.assign`, store-to-load
  forwarding, and LICM of loads out of the region.
- Merging two redundant asserts means merging or inlining regions. Hoisting
  one means moving its whole region, or splitting it.
- Several checks on the same statement nest regions inside each other.

The guarded-result form gives the same guarantee at the granularity of a
value: guarding the arrays a loop accesses protects every access in the
loop, without moving the loop into a region.

## 4. Check categories and default policy

| Kind | Meaning | Default | Option |
|---|---|---|---|
| `status` | Allocation status required by the standard (ALLOCATE/DEALLOCATE, assignment to an unallocated or disassociated left-hand side) and allocation failure | On, all optimization levels | Option to disable, for diagnosing regressions |
| `conformance` | Shape or size agreement that the runtime checks when not inlined | On, all optimization levels (same behavior as `-O0`) | `-fno-check=conformance` (name to be decided) |
| `argument` | Intrinsic argument values the runtime checks (DIM range, RESHAPE SHAPE values) | On | same mechanism |
| `bounds` | Subscript, section, substring, and character-dummy-length checks | Off | `-fcheck=bounds` |
| `pointer` | Dereference of a disassociated POINTER, an unallocated ALLOCATABLE, or an absent OPTIONAL | Off | `-fcheck=pointer` |
| `do` | DO loop with zero step | Off | `-fcheck=do` |
| `bits` | Bit-intrinsic position and shift ranges | Off | `-fcheck=bits` |
| `divide` | Integer MOD/MODULO with `P==0` | Off | existing `-fcheck-integer-mod-zero-divisor` |

Gating:

- Asserts are gated where they are emitted. Lowering and the passes that
  emit asserts do not emit the ones whose kind is disabled, whether it is a
  default-on kind disabled by an option or an opt-in kind that is not
  enabled. Disabled checks therefore never affect the MLIR passes, and cost
  nothing.
- The enabled kinds reach lowering and the passes through a module
  attribute, as `-fcheck-integer-mod-zero-divisor` does today
  (`fir.check_integer_mod_zero_divisor`). A single `fir.runtime_checks`
  attribute listing the enabled kinds would scale better.
- Dropping asserts after they are emitted is only needed when one function
  is compiled for several targets with different policies. For example, for
  an OpenACC `acc routine`, lowering emits a single `func.func` that later
  becomes a host and a device variant, and some asserts may have to be
  dropped or simplified in the device variant. A small pass drops asserts by
  kind, replacing their results with the guarded operands. It runs only in
  such pipelines, after the variants are split, not in the normal pipeline.
- `-fcheck=` currently exists only as a gfortran pass-through in
  `clang/include/clang/Options/FlangOptions.td`. It would become a Flang
  option with gfortran-compatible sub-options.

## 5. Checks to emit

Each table gives the condition in IR terms, the runtime message to reuse, the
insertion point, and what to guard. `ext(x,d)` is the extent of dimension `d`
(0-based) from `hlfir::genExtentsVector` or `fir.box_dims`. `null(p)` is a null
test of a `fir.box_addr` result. Messages are adapted from the runtime's rather
than quoted: `%s` is substituted at compile time, values use `%lld`, and some
texts are new (for example the per-dimension A1 message).

### 5.1 Checks lost by HLFIR and FIR inlining (`status`, `conformance`, `argument`)

These checks exist in the runtime today and disappear at `-O1` and above.

#### Assignment and copy

| ID | Operation | Condition (`%ok`) | Runtime message | Inserted in | Guard |
|---|---|---|---|---|---|
| A1 | Array assignment, no realloc, whatever the right-hand side (variable, `hlfir.elemental`, or `hlfir.eval_in_mem` such as an inlined MATMUL) | for each `d`: `ext(lhs,d) == ext(rhs,d)` | `Assign: mismatching element counts in array assignment (to %lld, from %lld)` (`RT/assign.cpp:453-461`; `AssignSimple` at `:921-928`) | Lowering, before `hlfir.assign` (`flang/lib/Lower/Bridge.cpp`, `genDataAssignment`, `:5884`) | Both sides |
| A2 | Assignment to a POINTER or non-realloc ALLOCATABLE left-hand side | `!null(lhs) \|\| size(lhs) == 0` | `Assign: left-hand side variable is neither allocated nor allocatable` (`RT/assign.cpp:371-374`) | `IHA` `InlineHLFIRAssignConversion`; `OB` `ElementalAssignBufferization`, `EvaluateIntoMemoryAssignBufferization` | Left-hand-side box |
| A5 | Scalar broadcast to an unallocated allocatable (realloc) or disassociated pointer | `!null(lhs)` | `Assign: mismatched ranks (%lld != %lld) in assignment to unallocated allocatable` (`RT/assign.cpp:361-370`), or the existing inline text `array left hand side must be allocated when the right hand side is a scalar` (`MutableBox.cpp:947-954`) | `OB` `BroadcastAssignBufferization` (`:514-612`); it fires even with `realloc` | Left-hand-side box |
| A6 | Inline (re)allocation for assignment | `!null(newStorage)` after `fir.allocmem` | `Memory allocation failed` / `AssignSimple: allocation failed (stat=%lld)` (`RT/assign.cpp:130`, `:1050`) | `SAA` (runs at every optimization level) and `IHA` `InlineAllocatableExprAssignConversion`, through `fir::factory::genReallocIfNeeded` / `allocateAndInitNewStorage`; `C2F` scalar allocatable path (`:123-150`), which creates the `fir.allocmem` directly | New storage address |

Not lost: `InlineCopyInConversion` replaces `ShallowCopyDirect`, whose checks
are internal, because the temporary is built from the variable's own shape.

Notes on A1:

- It is emitted once, in lowering, rather than in each pass that expands
  `hlfir.assign` (`InlineHLFIRAssign`, the `OptimizedBufferization` patterns,
  `BufferizeHLFIR`, the runtime-call path). Those passes only see the guarded
  sides. `OptimizedBufferization` inlines the right-hand side's elemental
  body through the guard (section 7), and `tryUsingAssignLhsDirectly` writes
  through the guarded left-hand side.
- At `-O0`, the runtime then checks the assignment again, which is harmless.
- Assignments created by passes (`temporary_lhs`, the ones produced by
  `SeparateAllocatableAssign`) conform by construction and are not checked.
- Assignments in WHERE and FORALL, and to vector-subscripted left-hand
  sides, use `hlfir.region_assign`. The runtime does not check them either,
  so their check is new (B7).
- The runtime only compares **element counts**. A `2x3 = 3x2` assignment
  passes the runtime and gives a wrong result in element order. The inline
  loop nest would access out of bounds, so the assert must compare
  **per-dimension** extents. That is stricter than the runtime, and it is
  exactly F2023 10.2.1.2 conformance. The message then also names the
  dimension (see the A1 example in section 5.4).

#### Reductions, MATMUL, DOT_PRODUCT

| ID | Operation | Condition | Runtime message | Inserted in | Guard |
|---|---|---|---|---|---|
| A7 | DOT_PRODUCT | `ext(a,0) == ext(b,0)` | `DOT_PRODUCT: SIZE(VECTOR_A) is %lld but SIZE(VECTOR_B) is %lld` (`RT/dot-product.h:63-67`) | `SHI` `DotProductConversion::genProductExtent`, before `deduceOptimalExtents`; `SI` DOT_PRODUCT at the call site (the generated body always loops over the first vector) | Both vectors (VECTOR_B is enough for `SI`) |
| A8 | MATMUL | `ext(a, rank(a)-1) == ext(b,0)` | `MATMUL: unacceptable operand shapes (%lldx%lld, %lldx%lld)` and the vector forms (`RT/matmul.h:272-293`) | `SHI` `MatmulConversion::genResultShape`, before `deduceOptimalExtents` | Both matrices |
| A9 | MATMUL_TRANSPOSE | `ext(a,0) == ext(b,0)` | `MATMUL-TRANSPOSE: unacceptable operand shapes (%lldx%lld, %lldx%lld)` (`RT/matmul-transpose.h:201-209`) | same | Both matrices |
| A10 | Array MASK of SUM, PRODUCT, MAXVAL, MINVAL, MAXLOC, MINLOC (a scalar MASK conforms with anything) | `!present(mask) \|\|` for each `d`: `ext(mask,d) == ext(array,d)`, with the extents read inside `fir.if present` | `Incompatible array arguments to %s: dimension %lld of ARRAY has extent %lld but MASK has extent %lld` (`RT/tools.cpp:97-104`) | `SHI` `ReductionAsElementalConverter::convert` (`:1142-1266`); `SI` `simplifyMinMaxlocReduction` | MASK |
| A11 | Non-constant DIM on a rank-1 argument of ALL, ANY, COUNT, MAXLOC, MINLOC | `dim == 1` | `%s: bad DIM=%lld for ARRAY with rank 1` (`RT/reduction.cpp:256`, `RT/tools.cpp:345`) | `SHI` `AllAnyAsElementalConverter`, `CountAsElementalConverter`, `MinMaxlocAsElementalConverter`; `SI` `simplifyLogicalDim1Reduction`, `simplifyMinMaxlocReduction` | None (DIM is not used for addressing) |

Not lost:

- Partial reductions with a non-constant DIM are not inlined (`getConstDim`).
- A constant DIM is checked by semantics.
- MAXLOC and MINLOC with a BACK argument are not inlined by HLFIR.
- FIR-level SUM, MAXVAL and COUNT only fire without DIM (and, for SUM and
  MAXVAL, without MASK).
- For SUM, PRODUCT, MAXVAL and MINVAL on rank-1 arrays, the runtime path already
  ignores DIM, so an assert there would be a new check (kind `argument`).

#### Transformational intrinsics

| ID | Operation | Condition | Runtime message | Inserted in | Guard |
|---|---|---|---|---|---|
| A12 | CSHIFT and EOSHIFT with an array SHIFT | for each `k`, with `j` the matching ARRAY dimension (skipping DIM): `ext(shift,k) == ext(array,j)` | `%s: on dimension %lld, SHIFT= has extent %lld but ARRAY= has extent %lld` (`RT/transformational.cpp:48-53`) | `SHI` `ArrayShiftConversion`, after the ARRAY extents are computed, before `hlfir.elemental` or `hlfir.eval_in_mem` | SHIFT |
| A13 | EOSHIFT with an array BOUNDARY | `!present(boundary) \|\|` same per-dimension test | `EOSHIFT: BOUNDARY= has extent %lld on dimension %lld but must conform with extent %lld of ARRAY=` (`RT/transformational.cpp:630-643`) | same | BOUNDARY |
| A14 | EOSHIFT on CHARACTER with BOUNDARY | `len(boundary) == len(array)` | `EOSHIFT: BOUNDARY= has element byte length %lld, but ARRAY= has length %lld` (`RT/transformational.cpp:625-629`, `:700-704`) | same; the inline path pads or truncates instead of failing | None |
| A15 | RESHAPE: SHAPE values | for each `i`: `shape(i) >= 0` | `RESHAPE: bad value for SHAPE(%lld)=%lld` (`RT/transformational.cpp:818-821`) | `SHI` `ReshapeAsElementalConversion`, after the SHAPE loads, before `fir.shape` (`:3130-3137`) | SHAPE extents fed to `fir.shape` |
| A16 | RESHAPE: enough elements | `product(shape) <= size(array) \|\| (present(pad) && size(pad) > 0)` | `RESHAPE: not enough elements, need %lld but only have %lld` (`RT/transformational.cpp:831-836`) | same | SOURCE and PAD, and the dynamic extents used as divisors |

A16 also prevents an unsigned division by zero in the inline index
computation when ARRAY or PAD is empty (`SHI:3262-3266`). The divisions do
not access memory, so guarding SOURCE does not order them: the extents they
divide by are guarded too. Constant extents need no guard, because MLIR
already treats a division by a non-zero constant as speculatable.

Not lost: TRANSPOSE (rank only, static), character comparison
(`hlfir.cmpchar`), and INDEX. Their runtime entry points have no user-facing
checks.

### 5.2 Checks missing in code generated inline during lowering

| ID | Kind | Operation | Condition | Message | Inserted in | Guard |
|---|---|---|---|---|---|---|
| B1 | `status` | ALLOCATE of an allocated allocatable ([#223287](https://github.com/llvm/llvm-project/pull/223287)) | `null(box)` | `the object '%s' is already allocated` (runtime stat text: `Base address is not null`) | `flang/lib/Lower/Allocatable.cpp`, `AllocateStmtHelper`, before `genInlinedAllocation` (`:562-574`) | None needed (the program terminates before the leak is observable) |
| B2 | `status` | DEALLOCATE of an unallocated allocatable | `!null(box)` | `the object '%s' is not allocated` (runtime: `Base address is null`) | same file, inline branch of `genDeallocate`, before `genFreemem` (`:1015-1020`) | None needed (`free(NULL)` is harmless) |
| B3 | `divide` | Integer MOD/MODULO, `P==0` | `p != 0` | `MOD with P==0` / `MODULO with P==0` | `IC` `genIntegerZeroDivisorCheck`: replace `fir.if`; also add it to the UNSIGNED path (`genMod`, `genModulo`), which has none | `p` (the `arith.remsi` operand) |
| B4 | migrate | NEAREST `S==0`, IEEE `RADIX/=2`, STORAGE_SIZE of an unallocated polymorphic, realloc with scalar RHS | existing conditions | existing messages | `IC` `genNearest`, `checkRadix`, `genStorageSize`; `MutableBox.cpp` `genReallocIfNeeded` | Operand of the protected op |
| B5 | `bits` | BTEST, IBCLR/IBSET, IBITS, ISHFTC, MASKL/MASKR, MVBITS (out of range gives poison); ISHFT and SHIFTL/SHIFTR/SHIFTA/DSHIFTL/DSHIFTR (clamped, so silently accepted) | `0 <= pos < bit_size`, `pos + len <= bit_size`, and similar, computed on the arguments' original kind (before any conversion) and without overflow | gfortran `-fcheck=bits` style | `IC` per intrinsic | Shift or position operand |
| B6 | `conformance` | Elemental operations and elemental procedure references on arrays | for each `d`: `ext(x,d) == ext(s,d)` for every array operand `x`, where `s` is the operand providing the iteration space | New, no runtime equivalent | `X2H` binary and unary elemental operations (`:1838-1886`), elemental calls in `flang/lib/Lower/ConvertCall.cpp` (`:2440-2472`); both carry a `TODO: merge shape` | Operands other than `s` |
| B7 | `conformance` | WHERE mask and assignments, FORALL assignments, assignments to vector-subscripted left-hand sides (`hlfir.region_assign`) | mask, left-hand side, and right-hand side extents agree (F2023 10.2.1.2, 10.2.3.1) | New | `LowerHLFIROrderedAssignments`, which builds the loops | Arrays accessed in the loops, including vector subscripts |

B1 and B2 are required by the standard and should be unconditional. The
runtime path already checks them (`RT/allocatable.cpp:142-143`, `:198-199`);
`-mllvm -use-alloc-runtime` works around the problem today.

B6 and B7 are new behavior at every optimization level: the runtime is never
involved, and Flang checks neither elemental expressions nor these
assignments today. gfortran only checks conformance under `-fcheck=bounds`,
and ifx under `-check shape`. Whether they belong to `conformance` or to a
separate opt-in kind is open (section 9).
The choice does not affect the other checks, which protect their own
accesses. An elemental-call argument with dynamic OPTIONAL handling
(`ConvertCall.cpp:2452-2471`) may be absent, so its condition must include
presence.

Also note: the CSHIFT shift normalization calls `genModulo` with the extent
hidden behind a `select`, so with `-fcheck-integer-mod-zero-divisor` every
CSHIFT gets a dead `MODULO with P==0` check (`SHI` `normalizeShiftValue`).
Constant folding of the assert, or emitting `arith.remsi` directly there,
fixes it.

### 5.3 New opt-in checks (`bounds`, `pointer`, `do`)

`inb(i, lb, ext)` is the single unsigned test `(i - lb) <u ext`. It is
equivalent to `lb <= i <= lb + ext - 1` and false when `ext == 0`.

| ID | Kind | Check | Condition | Inserted in | Guard | Notes |
|---|---|---|---|---|---|---|
| C1 | `bounds` | Array element subscript | for each `d`: `inb(i_d, lb_d, ext_d)`; for the last dimension of an assumed-size array, only `i >= lb` | `X2H` `HlfirDesignatorBuilder::visit(ArrayRef)`, scalar-subscript branch, after `genSubscript` (`:737-743`) | Subscript value | Skip constant subscripts with constant bounds (checked by semantics). Keep the legacy `dimension(1)` dummy idiom working. |
| C2 | `bounds` | Section triplet `lo:hi:st` | `st != 0`; then `n == 0 \|\| (inb(lo) && inb(last))`, `last = lo + (n-1)*st`, with the extent `n` computed from the guarded `st` | same function, triplet branch, before the extent is computed with `genExtentFromTriplet` (`:713-736`) | `st` first, then `lo`, `hi` | Check `last`, not `hi` (`a(1:11:3)` is valid for `ub=10`). Zero-size sections are exempt. |
| C3 | `bounds` | Vector subscript elements | `inb(v(k), lb, ext)` | `X2H` `createVectorSubscriptElementAddrOp`, after the element load (`:995-1003`) | Loaded subscript | O(n). A MINVAL/MAXVAL pre-check is cheaper but loses the index in the message. |
| C4 | `bounds` | Substring `s(lo:hi)` | `hi < lo \|\| (lo >= 1 && hi <= len)` | `X2H` `gen(Substring)`, before the length is replaced (`:549-555`) | `lo`, `hi` | Must be done before `ConvertToFIR`, which drops `hi`. |
| C5 | `bounds` | Character dummy length | As required by F2023 15.5.2.5: `passedLen >= declaredLen` for scalars of default or C character kind, equality for assumed-shape and assumed-rank dummies and other kinds, no check for sequence-associated arrays | `flang/lib/Lower/ConvertVariable.cpp`, dummy mapping (non-box `:3129-3142`, box `:2898-2939`) | Dummy base address | Skip bare-reference dummies (no length) and absent OPTIONAL. |
| C6 | `pointer` | POINTER dereference | `!null(box_addr(load p))` | Context-aware call sites, **not** inside `hlfir::derefPointersAndAllocatables`: `X2H` `visit(SymbolRef)` (`:648-653`), the pointer-component paths (`:808-809`, `:889`), and `flang/lib/Lower/ConvertCall.cpp` for a non-optional, non-pointer dummy (`:1497-1498`) | Loaded box or base address | Several callers of `derefPointersAndAllocatables` dereference a possibly null box on purpose (a disassociated actual passed to an OPTIONAL dummy counts as absent, F2023 15.5.2.13 p1). |
| C7 | `pointer` | ALLOCATABLE dereference | same as C6 | same sites | same | Exempt realloc assignment, `ALLOCATED`, `MOVE_ALLOC`, and actuals passed to ALLOCATABLE or OPTIONAL dummies. A zero-size allocation has a non-null address, so "allocated" means "non-null". |
| C8 | `pointer` | Use of an absent OPTIONAL dummy | `fir.is_present %x` | Part references (`X2H` `visit(SymbolRef)`), value uses, actuals passed to non-optional dummies (`ConvertCall.cpp`, `genIsPresentIfArgMaybeAbsent`) | Declare result at the use site | Guard per use, not once at entry (uses under `if (present(x))` are legal). Exempt `PRESENT` and forwarding to OPTIONAL dummies. |
| C9 | `do` | Zero step in DO, DO CONCURRENT, FORALL, and array-constructor and I/O implied-DOs | `step != 0` | Where lowering computes the step, e.g. `flang/lib/Lower/Bridge.cpp` for DO (`:3094-3099`) | Step value | A zero step currently divides by zero in the trip-count computation (SIGFPE at every optimization level). Section triplets are covered by C2. |

Out of scope for now: the explicit-shape dummy size versus the actual (callee
only has an address), and checks of undefined POINTER status.

### 5.4 IR examples

**A7, DOT_PRODUCT** (`SHI` `DotProductConversion`):

```
%da:3 = fir.box_dims %a, %c0 : (!fir.box<!fir.array<?xf32>>, index) -> (index, index, index)
%db:3 = fir.box_dims %b, %c0 : (!fir.box<!fir.array<?xf32>>, index) -> (index, index, index)
%eq   = arith.cmpi eq, %da#1, %db#1 : index
%a_ok, %b_ok = fir.assert %eq, conformance,
          "DOT_PRODUCT: SIZE(VECTOR_A) is %lld but SIZE(VECTOR_B) is %lld"
          values(%da#1, %db#1 : index, index)
          guard(%a, %b : !fir.box<!fir.array<?xf32>>, !fir.box<!fir.array<?xf32>>)
// The loop bound is not guarded, and stays a constant when an extent is one.
%r = fir.do_loop %i = %c1 to %da#1 step %c1 iter_args(%acc = %zero) -> (f32) {
  %ea = hlfir.designate %a_ok (%i) : (!fir.box<!fir.array<?xf32>>, index) -> !fir.ref<f32>
  %eb = hlfir.designate %b_ok (%i) : (!fir.box<!fir.array<?xf32>>, index) -> !fir.ref<f32>
  ...
}
```

**A1, assignment of an elemental expression** (`x = y + 1.0`, lowering, then
`OB` `ElementalAssignBufferization`):

```
%e = hlfir.elemental %shy unordered : (!fir.shape<1>) -> !hlfir.expr<?xf32> {
^bb0(%i: index):
  %yi = hlfir.designate %y#0 (%i) : (!fir.box<!fir.array<?xf32>>, index) -> !fir.ref<f32>
  ...
}
%dx:3 = fir.box_dims %x#0, %c0 : (!fir.box<!fir.array<?xf32>>, index) -> (index, index, index)
%eq   = arith.cmpi eq, %dx#1, %ny : index
%x_ok, %e_ok = fir.assert %eq, conformance,
          "Assign: mismatching extents on dimension %lld in array assignment (to %lld, from %lld)"
          values(%c1, %dx#1, %ny : index, index, index)
          guard(%x#0, %e : !fir.box<!fir.array<?xf32>>, !hlfir.expr<?xf32>)
hlfir.assign %e_ok to %x_ok : !hlfir.expr<?xf32>, !fir.box<!fir.array<?xf32>>

// After OptimizedBufferization, the guard on %e has moved to the array that
// its body accesses (section 7), and the loop is free of asserts.
%x_ok, %y_ok = fir.assert %eq, conformance, "..." values(...)
          guard(%x#0, %y#0 : !fir.box<!fir.array<?xf32>>, !fir.box<!fir.array<?xf32>>)
fir.do_loop %i = %c1 to %ny step %c1 unordered {
  %yi = hlfir.designate %y_ok (%i) : (!fir.box<!fir.array<?xf32>>, index) -> !fir.ref<f32>
  ...
  %xi = hlfir.designate %x_ok (%i) : (!fir.box<!fir.array<?xf32>>, index) -> !fir.ref<f32>
  hlfir.assign %v to %xi : f32, !fir.ref<f32>
}
```

When the elemental shape comes from `x` itself (`x = x + 1.0`), the two
extents are the same SSA value and the assert folds away.

**A10, SUM with an OPTIONAL MASK** (`SHI` `ReductionAsElementalConverter`):

```
%present = fir.is_present %mask : (!fir.box<!fir.array<?x!fir.logical<4>>>) -> i1
%m:2 = fir.if %present -> (i1, index) {
  %dm:3 = fir.box_dims %mask, %c0 : (...) -> (index, index, index)
  %eq   = arith.cmpi eq, %dm#1, %na : index
  fir.result %eq, %dm#1 : i1, index
} else {
  fir.result %true, %na : i1, index
}
%mask_ok = fir.assert %m#0, conformance,
           "Incompatible array arguments to SUM: dimension %lld of ARRAY has extent %lld but MASK has extent %lld"
           values(%c1, %na, %m#1 : index, index, index)
           guard(%mask : !fir.box<!fir.array<?x!fir.logical<4>>>)
// reduction loop over 1..%na, MASK read through %mask_ok at the same index
```

**A15/A16, RESHAPE without PAD** (`SHI` `ReshapeAsElementalConversion`):

```
%s1  = fir.load %p1 : !fir.ref<i32>
%nn  = arith.cmpi sge, %s1, %c0_i32 : i32
%s1i = fir.convert %s1 : (i32) -> index
%da:3 = fir.box_dims %a, %c0 : (!fir.box<!fir.array<?xf32>>, index) -> (index, index, index)
%enough = arith.cmpi ule, %s1i, %da#1 : index
%ok  = arith.andi %nn, %enough : i1
%ext, %a_ok = fir.assert %ok, argument, "RESHAPE: bad SHAPE= or not enough elements in SOURCE="
         guard(%s1i, %a : index, !fir.box<!fir.array<?xf32>>)
%shape = fir.shape %ext : (index) -> !fir.shape<1>
%r = hlfir.elemental %shape unordered : (!fir.shape<1>) -> !hlfir.expr<?xf32> { ... reads %a_ok ... }
```

The SHAPE values are guarded because they size the result, which may be
allocated as a temporary. This is not an iteration-space guard: SOURCE is
guarded separately, so its accesses stay protected wherever the elemental
is inlined.

Two separate asserts give the two exact runtime messages. One combined assert
gives a smaller IR.

**B1, ALLOCATE status** (`Allocatable.cpp`):

```
%bx   = fir.load %a#0 : !fir.ref<!fir.box<!fir.heap<!fir.array<?xi32>>>>
%addr = fir.box_addr %bx : (!fir.box<!fir.heap<!fir.array<?xi32>>>) -> !fir.heap<!fir.array<?xi32>>
%allocated = fir.is_present %addr : (!fir.heap<!fir.array<?xi32>>) -> i1   // non-null test
%free = arith.xori %allocated, %true : i1
fir.assert %free, status, "the object 'array' is already allocated"
%mem  = fir.allocmem !fir.array<?xi32>, %n {uniq_name = "_QFEarray.alloc"}
```

**C1, array element `a(i)`** (`X2H` `HlfirDesignatorBuilder`):

```
%d:3 = fir.box_dims %a#0, %c0 : (!fir.box<!fir.array<?xf32>>, index) -> (index, index, index)
%off = arith.subi %i, %d#0 : index
%ok  = arith.cmpi ult, %off, %d#1 : index
%i_ok = fir.assert %ok, bounds, "subscript %lld is out of bounds for dimension 1 of 'a' (%lld:%lld)"
          values(%i, %d#0, %ub : index, index, index) guard(%i : index)
%e = hlfir.designate %a#0 (%i_ok) : (!fir.box<!fir.array<?xf32>>, index) -> !fir.ref<f32>
```

**C6, POINTER dereference `p(i)`**:

```
%bx  = fir.load %p#0 : !fir.ref<!fir.box<!fir.ptr<!fir.array<?xf32>>>>
%adr = fir.box_addr %bx : (!fir.box<!fir.ptr<!fir.array<?xf32>>>) -> !fir.ptr<!fir.array<?xf32>>
%ok  = fir.is_present %adr : (!fir.ptr<!fir.array<?xf32>>) -> i1
%bx_ok = fir.assert %ok, pointer, "POINTER 'p' is not associated" guard(%bx : !fir.box<!fir.ptr<!fir.array<?xf32>>>)
%e = hlfir.designate %bx_ok (%i) : (!fir.box<!fir.ptr<!fir.array<?xf32>>>, index) -> !fir.ref<f32>
```

## 6. Where asserts are emitted, optimized, and converted

Pipeline (`flang/lib/Optimizer/Passes/Pipelines.cpp`), with the `fir.assert`
steps added:

| Stage | Pass (existing, unless marked new) | Optimization level | `fir.assert` role |
|---|---|---|---|
| Lowering | `flang/lib/Lower` (assignments, elemental expressions, designators, ALLOCATE, DEALLOCATE, DO, intrinsics in `IC`) | all | Emit A1, B1-B6, and C1-C9 |
| HLFIR | `SimplifyHLFIRIntrinsics` (twice) | O1+ | Emit A7-A16 |
| HLFIR | `InlineElementals`, `SeparateAllocatableAssign` | all | Emit A6 (`SeparateAllocatableAssign`); move guards from inlined expressions to the arrays they access (section 7) |
| HLFIR | CSE, canonicalize | O1+ | Fold constant asserts; merges the conditions |
| HLFIR | `OptimizedBufferization`, `InlineHLFIRAssign` (twice), `InlineHLFIRCopy` (O3) | O1+ (`InlineHLFIRAssign` also at O0 for device code, scalar right-hand sides only) | Emit A2, A5, A6; move guards as `InlineElementals` does |
| HLFIR | `LowerHLFIROrderedAssignments` | all | Emit B7; move guards as `InlineElementals` does |
| HLFIR | `BufferizeHLFIR` | all | Move guards from expressions to their buffers |
| HLFIR | `ConvertHLFIRtoFIR` | all | Emit A6 (scalar allocatable assignment) |
| FIR | CSE, canonicalize | all | Fold constant asserts |
| FIR | **new, optional: redundant-assert elimination** | O1+ | Dominance-based merging (section 3.4) |
| FIR | `SimplifyIntrinsics` | O1+ | Emit A7, A10, A11 at the call site |
| FIR | flang LICM, extended | O1+ | Hoist invariant asserts (section 7) |
| FIR | **new, optional: redundant-assert elimination** (second run) | O1+ | Merge asserts that hoisting made comparable |
| FIR | `SimplifyFIROperations`, `CFGConversion` | all | Nothing |
| Device | **new: drop asserts by kind**, only in pipelines that split one function into host and device variants (for example OpenACC `acc routine`) | all | Drop the kinds disabled for the device variant (section 4) |
| Code generation | `FIRToLLVMLowering` | all | Convert to a branch plus a noreturn call (section 3.5) |

At `-O0`, the passes marked "all" above still emit asserts or move guards,
and CSE and canonicalization still run, but hoisting and redundancy
elimination do not. The code generation pattern must work on unoptimized IR.

## 7. Optimization interactions

- **No interference with memory optimizations.** The assert writes only the
  non-addressable runtime resource (section 3.2). Passes that inspect memory
  effects themselves, instead of asking alias analysis, must also skip
  effects on non-addressable resources (`Resource::isAddressable`), as FIR
  alias analysis does. For example, `InlineElementals` (`isConflictingWrite`)
  and `ScheduleOrderedAssignments` currently treat any write without a value
  as a conflict, so an assert between an `hlfir.elemental` and its
  `hlfir.apply` would block inlining.
- **Inlining a guarded expression.** `InlineElementals`,
  `OptimizedBufferization`, and `LowerHLFIROrderedAssignments` replace
  `hlfir.apply %e, %j`, or an `hlfir.assign` of `%e`, with the body of the
  elemental `%e` evaluated at the consumer's index.
  `LowerHLFIROrderedAssignments` does the same for the
  `hlfir.elemental_addr` of a vector-subscripted designator. When `%e` is
  guarded by an assert, the guard must survive. The body's accesses run at a
  foreign index, so they are exactly what the assert protects. These passes
  go through `hlfir::inlineElementalOp`, which applies one rule:
  - Rewrite the assert in place so that it guards the arrays that the body
    accesses at an index derived from its index argument: variables, and
    expressions read through nested `hlfir.apply`. Then map them to the
    guarded results when cloning the body. The arrays are defined before
    the elemental, so they dominate the assert and nothing has to move.
  - All accesses must be covered, not just one. Index arithmetic (lower
    bounds, CSHIFT, RESHAPE, vector subscripts) does not change that,
    because the accessed arrays are still defined outside the body.
  - Operations that are not speculatable and do not access memory, such as
    divisions by extents, are protected by guarding their divisor too. Their
    rare forms with only constant operands outside the body fall back to
    guarding the index, inside the loop.
  - Operations with unknown effects, such as calls, are already ordered with
    the assert by its effect.

  A guard on a nested expression is handled by the same rule when that
  expression is inlined. `BufferizeHLFIR` also uses `inlineElementalOp`, but
  to evaluate an elemental at its own definition, so there the guard simply
  moves to the buffer, as for `hlfir.no_reassoc`. A pass that does not
  apply the rule must not inline through the assert.
  That loses an optimization but does not produce wrong code. The asserts
  stay before the loops, and the loop bounds remain the unguarded extents.
  `InlineElementals` must also find the elemental through the assert
  (`getTwoUses` already looks through `hlfir.declare` and `fir.convert`).
- **Hoisting.** Conformance asserts are already outside the loops. Asserts
  inside loops come from per-element checks (section 5.3), from checks on
  operands that vary per element, and from the fallback above. FIR LICM
  hoists only pure ops and loads, so asserts stay in the loop, and so do the
  operations depending on their guarded results. A dedicated rule is needed.
  Moving an assert earlier is legal only if (a) it executes whenever the
  new position does: the loop runs at least once (or the hoisted condition
  accounts for an empty loop), and the assert runs on every iteration, which
  a conditional CYCLE or EXIT can break; and (b) no operation it crosses
  conflicts with its effect (calls, I/O, STOP, other asserts). Its users can
  then be hoisted too. LLVM could also do this after code generation, but
  doing it in Flang leaves simpler loops for the MLIR loop optimizations.
- **Range checks for subscripts (C1).** For an affine subscript in a
  `fir.do_loop`, check the range of values it takes before the loop instead
  of each value. Two variants:
  - *Predication*: report the failure before the loop. It needs the same
    conditions as hoisting, and output written before the failing iteration
    is lost.
  - *Versioning*: the range test only selects an unchecked fast loop, and
    the original checked loop runs when it fails, so messages are exact and
    the fast path vectorizes. The range test must be conservative and
    computed without overflow. It can reuse the cloning infrastructure of
    `fir::LoopVersioning`. Recommended for `-O2 -fcheck=bounds`.
- **Vectorization.** After code generation, an assert inside a loop is an early exit
  to a noreturn call, which the LLVM loop vectorizer does not handle. The
  A checks, and the B checks on loop-invariant operands, are placed before
  the loop nests, so they cost nothing per element. B checks on operands
  that vary per element (for example MOD with an array P) and the C checks
  can end up in hot loops.
- **Never check compiler-generated accesses.** Element accesses in loops
  created by `BufferizeHLFIR` and the inlining passes are in bounds by
  construction once the A checks hold. That is why bounds checks belong in
  `HlfirDesignatorBuilder` and not in a pass over all `hlfir.designate`, or in
  the code generation of `fir.array_coor`.
- **Device code.** Per-element checks in kernels cost registers and block
  unrolling and vectorized loads. For `-fcheck=bounds` in offloaded loops,
  prefer range checks before the kernel launch, and keep in-kernel checks for
  non-affine subscripts.

## 8. Suggested implementation order

1. `fir.assert` with `FortranRuntimeResource`, verifier, canonicalization,
   code generation, the runtime entry with values, and a `noreturn`
   declaration. Make the passes that inspect effects skip non-addressable
   ones, make the analyses look through guards (section 3.2), and implement
   the rule for inlining guarded expressions in `hlfir::inlineElementalOp`,
   with the `BufferizeHLFIR` pattern for asserts on expressions (section 7).
2. B1 and B2 (standard-required, one compare per statement). This supersedes
   the current version of
   [#223287](https://github.com/llvm/llvm-project/pull/223287).
3. A1, A2, A5, and A7-A9 (out-of-bounds risks in the most common inlined
   code).
4. A10-A16, A6, B6, and B7.
5. Migrate the existing checks (B3, B4) and add the UNSIGNED MOD check.
6. LICM support, then redundant-assert elimination if MLIR loop
   optimizations need it (LLVM already removes many redundant checks).
7. `-fcheck=` plumbing, then C9, C1, C2, C4, C5, then C6-C8, then C3 and B5.
8. Range checks and versioning for C1.

## 9. Open questions

1. Diagnostic detail: report only the failing dimension, or include complete
   shapes and subscript tuples? The fixed and count-and-array entries support
   both. Inline messages keep the runtime's text as closely as possible, and
   only differ where the inline check is stricter (per-dimension conformance,
   section 5.1).
2. When a default-on kind (`conformance`, `argument`) is disabled by an
   option, should its checks be emitted as assumptions (`llvm.intr.assume` at
   code generation) instead of not at all? An assumption gives more
   optimization, but a false condition becomes undefined behavior. This seems
   acceptable when the standard only makes the program non-conforming, which
   is the case for these kinds. It does not apply to `status`: there the
   standard requires error termination, and disabling the kind is only meant
   for diagnosing regressions, so its checks are simply not emitted. Input
   from users on the expected behavior would help. Opt-in kinds that are not
   enabled are never emitted, as assertions or as assumptions.
3. Option names for disabling `status`, `conformance`, and `argument`.
   Should the checks that are new at every optimization level (B6, B7) be in
   `conformance` (on by default) or in a separate opt-in kind?
4. Allocation failure (A6): check after each `fir.allocmem`, or give
   `fir.allocmem` a "fail on null" attribute handled in code generation?
5. Device runtime availability of the error entry point in each offload
   configuration (CUDA, OpenACC, OpenMP offload).

## Appendix: runtime issues found during the survey

- `RT/character.cpp`, `CharacterCompare`: `yChars` uses `shift<char>` instead
  of `shift<CHAR>`, so KIND=2 and KIND=4 array comparisons read past the end
  of the second operand. This descriptor-based entry point is not called by
  Flang lowering, which compares characters element by element with the
  scalar entry points.
- `RT/tools.cpp`, `CheckIntegerKind`: the format arguments are passed in the
  wrong order.
- `RT/reduction.cpp`: the rank-1 DIM check of ALL, ANY, COUNT and PARITY
  accepts `DIM=0`.
- `ReportFatalUserError` is not in the OpenMP offload API group, unlike
  `Abort`.