[InstCombine][Docs] Update InstCombine contributor guide (#144228)
Update the guideline to reduce the chance of miscompilation/performance
regression.
---------
Co-authored-by: Nikita Popov <github@npopov.com>
Co-authored-by: Antonio Frighetto <me@antoniofrighetto.com>
diff --git a/llvm/docs/InstCombineContributorGuide.md b/llvm/docs/InstCombineContributorGuide.md
index b4041f8..cee0a7c 100644
--- a/llvm/docs/InstCombineContributorGuide.md
+++ b/llvm/docs/InstCombineContributorGuide.md
@@ -404,11 +404,32 @@
intrinsics, such as `TargetTransformInfo::instCombineIntrinsic()`. These are
already inherently target-dependent anyway.
+If some canonicalization narrow/widen the integer width of expressions, please
+check `shouldChangeType()` first. Otherwise, we may evaluate the expression
+in illegal/inefficient types.
+
For vector-specific transforms that require cost-modelling, the VectorCombine
pass can be used instead. In very rare circumstances, if there are no other
alternatives, target-dependent transforms may be accepted into
AggressiveInstCombine.
+Generally, we prefer unsigned operations over signed operations in the middle-end, even
+if signed operations are more efficient on some targets. The following is an incomplete
+list of canonicalizations that are implemented in InstCombine:
+
+| Original Pattern | Canonical Form | Condition |
+|------------------------------|----------------------------|-------------------------------|
+| `icmp spred X, Y` | `icmp samesign upred X, Y` | `sign(X) == sign(Y)` |
+| `smin/smax X, Y` | `umin/umax X, Y` | `sign(X) == sign(Y)` |
+| `sext X` | `zext nneg X` | `X >=s 0` |
+| `sitofp X` | `uitofp nneg X` | `X >=s 0` |
+| `ashr X, Y` | `lshr X, Y` | `X >=s 0` |
+| `sdiv/srem X, Y` | `udiv/urem X, Y` | `X >=s 0 && Y >=s 0` |
+| `add X, Y` | `or disjoint X, Y` | `(X & Y) != 0` |
+| `mul X, C` | `shl X, Log2(C)` | `isPowerOf2(C)` |
+| `select Cond1, Cond2, false` | `and Cond1, Cond2` | `impliesPoison(Cond2, Cond1)` |
+| `select Cond1, true, Cond2` | `or Cond1, Cond2` | `impliesPoison(Cond2, Cond1)` |
+
### PatternMatch
Many transforms make use of the matching infrastructure defined in
@@ -531,6 +552,19 @@
One-use checks can be performed using the `m_OneUse()` matcher, or the
`V->hasOneUse()` method.
+### Flag handling
+
+When possible, favour propagation of poison-generating flags like `nuw` and `nsw` since they may be
+hard to salvage later. Avoid doing so if it introduces additional complexity (e.g. requires querying `willNotOverflow`
+or KnownBits).
+
+Be careful with in-place operand/predicate changes, as poison-generating flags may not be valid for new
+operands. It is recommended to create a new instruction with careful handling of flags. If not
+applicable, call `Instruction::dropPoisonGeneratingFlags()` to clear flags in a conservative manner.
+
+Do not rely on fcmp's `nsz` flag to perform optimizations. It is meaningless for fcmp so it should not affect
+the optimization.
+
### Generalization
Transforms can both be too specific (only handling some odd subset of patterns,
@@ -558,6 +592,11 @@
use of ValueTracking queries. Whether this makes sense depends on the case,
but it's usually a good idea to only handle the constant pattern first, and
then generalize later if it seems useful.
+ * When possible, handle more canonical patterns as well. It is encouraged to avoid
+ potential phase-ordering issues. For example, if the motivating transform holds for
+ `add`, it also holds for `or disjoint`. See the canonicalization list above for details.
+ In most cases, it can be easily implemented with matchers like
+ `m_AddLike/m_SExtLike/m_LogicalAnd/m_LogicalOr`.
## Guidelines for reviewers
