flang/docs/ReleaseNotes.md

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# Flang {{version}} {{in_progress}}Release Notes

::::{only} PreRelease
:::{warning}
These are in-progress notes for the upcoming LLVM {{version}} release.
Release notes for previous releases can be found on [the Download
Page](https://releases.llvm.org/download.html).
:::
::::

## Introduction

This document contains the release notes for the Flang Fortran frontend,
part of the LLVM Compiler Infrastructure, release {{version}}. Here we
describe the status of Flang in some detail, including major
improvements from the previous release and new feature work. For the
general LLVM release notes, see [the LLVM
documentation](https://llvm.org/docs/ReleaseNotes.html). All LLVM
releases may be downloaded from the [LLVM releases web
site](https://llvm.org/releases/).

Note that if you are reading this file from a Git checkout, this
document applies to the *next* release, not the current one. To see the
release notes for a specific release, please see the [releases
page](https://llvm.org/releases/).

## Major New Features

## Bug Fixes

- Fixed `fir::getTypeSizeAndAlignment` returning the wrong allocation size for
  **packed `fir::RecordType`s** (produced by the AIX lowering of `BIND(C)`
  derived types, or declared directly in textual FIR). Fields in a packed
  record are placed back-to-back using each component's allocation size
  (`alignTo(storeSize, ABIalign)`), not its raw store size, and the record's
  ABI alignment is 1. For example, a packed `{i32, f64}` on x86-64 now
  correctly reports 12 bytes instead of 16.
  ([#220377](https://github.com/llvm/llvm-project/pull/220377))

- Fixed `fir::getTypeSizeAndAlignment` omitting **tail padding** from unpacked
  derived types. The returned size is now rounded up to the record's own ABI
  alignment, matching the allocation extent used by array element strides, CUDA
  shared-memory layout, and stack/heap allocation placement. For example,
  `{i32, i8}` (store size 5 bytes, align 4) now correctly reports 8 bytes
  instead of 5.
  ([#220377](https://github.com/llvm/llvm-project/pull/220377))

- Fixed a **`BIND(C)` / `VALUE` argument-passing ABI bug** on SystemZ:
  derived types whose allocation size fits in a GPR were incorrectly passed
  indirectly (by reference) instead of as an integer register value, because
  `getTypeSizeAndAlignment` was returning the unpadded store size rather than
  the allocation size. For example, `{i32, i8}` (allocation size 8 bytes) is
  now correctly passed as `i64`, and `{i16, i8}` (4 bytes) as `i32`, matching
  the C ABI.
  Fortran programs with `BIND(C)` `VALUE` derived-type arguments of these shapes
  that interoperate with C were already producing incorrect results; programs
  compiled entirely in Fortran that relied on the old (incorrect) convention
  must be recompiled.
  ([#220377](https://github.com/llvm/llvm-project/pull/220377))

- Fixed a **`BIND(C)` / `VALUE` argument-passing ABI bug** on PPC64le:
  derived types were classified using the unpadded store size rather than the
  allocation size, producing the wrong number of GPR slots. The argument was
  already passed by value; only the slot count was wrong. For example,
  `{f128, i8}` (allocation size 32 bytes) is now correctly passed as
  `[4 x i64]` instead of `[3 x i64]`, matching the C ABI.
  Fortran programs with `BIND(C)` `VALUE` derived-type arguments of these shapes
  that interoperate with C were already producing incorrect results; programs
  compiled entirely in Fortran that relied on the old (incorrect) convention
  must be recompiled.
  ([#220377](https://github.com/llvm/llvm-project/pull/220377))

- Fixed the `TRANSFER` intrinsic inline path to compare sizes using
  `getTypeSizeAndAlignment` (which includes tail padding for `RecordType`,
  matching the allocation extent used by `STORAGE_SIZE` and the runtime
  `TRANSFER` path). For `RecordType` sources, the inline path now copies
  the record data via `fir.copy` into result-aligned storage when the source
  alignment is less than the result alignment, preserving both internal and
  tail padding bytes and satisfying the F2023 16.9.212 requirement that the
  result's physical representation be identical to the source's when both have
  the same length.
  ([#220377](https://github.com/llvm/llvm-project/pull/220377))


## Non-comprehensive list of changes in this release

- Added support for the OpenMP implementation-defined extension sentinels
  (OpenMP 5.2, section 3.1): `!$omx`, `c$omx` and `*$omx` in fixed source form
  and `!$ompx` in free source form. These sentinels are recognized like their
  `omp` counterparts when OpenMP is enabled.
  
- Change source path in -Rpass remarks (e.g., -Rpass=loop-vectorize) from a
  (mostly) full path to clang's behavior which is to use the source filename
  as specified on the command line (except that ./foo.f90 removes the ./
  prefix).

- Fortran-standard-compliant reassociation within individual `REAL` and
  `COMPLEX` sum expressions is now enabled by default at all optimization
  levels. This may change exact floating-point results. Flang users can
  restore left-to-right evaluation with `-fno-fp-sum-reassociation`.

- The legacy array-value operations (`fir.array_load`, `fir.array_fetch`,
  `fir.array_update`, `fir.array_modify`, `fir.array_access`,
  `fir.array_amend`, `fir.array_merge_store`) have been removed from FIR,
  together with the `array-value-copy` pass that legalized them and its
  `-mmlir -disable-avc` option. Nothing in flang has produced these
  operations since the legacy (non-HLFIR) expression lowering was deleted.
  Downstream projects that still construct them must migrate to HLFIR (or
  their own legalization) before rebasing. `fir.array_coor` is unrelated
  and remains supported.

- Added support for compressed DWARF debug sections. Flang now supports
  compressing DWARF debug info in ELF object files using zlib or zstd,
  reducing debug information size in compiled binaries.

- The FIR loop invariant code motion pass (`flang-licm`) is now enabled by
  default at optimization levels above `-O0`. It can be turned off with
  `-mmlir -disable-fir-licm`. The `-mmlir -enable-fir-licm` option that
  previously opted into the pass has been removed.

- Named constants (`PARAMETER`) now appear in the debug information, so a
  debugger can print them by name. A constant is described only in the
  compilation unit that defines it: one declared in a module is described
  where that module is compiled, and one declared in a procedure is local to
  that unit. Constants of an intrinsic module such as `iso_fortran_env` are
  not described yet, because no compilation unit defines them.

- A reference with a constant subscript that is out of range is now accepted with
  a warning instead of being rejected with an error. A subscript is required to be
  within its bounds only when the reference is executed (F'2023 9.5.3.1 paragraph
  2), and that cannot be determined in general, so programs that keep such a
  reference in a branch or procedure that never runs are no longer rejected. The
  same applies to array section endpoints, but not to cosubscripts, which remain
  errors. Use `-fno-out-of-bounds-subscripts` to get an error again, or
  `-Wno-out-of-bounds-subscripts` to silence the warning.

## New Compiler Flags
- Added `-fno-out-of-bounds-subscripts`, which restores the previous behavior of
  rejecting an out-of-range constant subscript with an error. See the entry above
  for the change in default behavior.

- Added the gfortran-compatible `-ffpe-trap=` flag, which sets the initial
  floating-point exception halting mode of the main program. It takes a
  comma-separated list of `invalid`, `zero`, `overflow`, `underflow`, `inexact`,
  and the extension `denormal`, or `none` to disable halting. See the Flang
  command line reference for the supported targets and details.

- Added `-gz` and `-gz=<format>` flags to enable compression of DWARF debug
  sections. Supported formats are `zlib`, `zstd`, and `none`.

- Added `-finit-local=<val>` to initialize automatic local
  variables that have no explicit or default initialization. Accepted values
  are `zero` and `0x<hex-byte>` (e.g. `0xAA`). The gfortran
  compatibility alias `-finit-local-zero` is equivalent to `-finit-local=zero`.
  Both modes fill every storage byte including struct padding.
  PowerPC vector locals (`vector(real(4))` etc.) are not initialized.

## Windows Support

## Fortran Language Changes in Flang
- Standard intrinsic functions are now classified as `SIMPLE` (F2023 16.1 p2).
  A procedure pointer or dummy procedure whose interface is an intrinsic
  function, such as `procedure(sqrt)`, can no longer be associated with a
  procedure that is `PURE` but not `SIMPLE` (F2023 10.2.2.4 p3,
  F2023 15.5.2.10 p1). Declare the target `SIMPLE` if it satisfies the
  `SIMPLE` constraints, or give the pointer or dummy an explicit `PURE`
  interface.

## Build System Changes

## New Issues Found

## Additional Information

Flang's documentation is located in the `flang/docs/` directory in the
LLVM monorepo.

If you have any questions or comments about Flang, please feel free to
contact us on the [Discourse
forums](https://discourse.llvm.org/c/subprojects/flang/33).