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[InstCombine][Docs] Update InstCombine contributor guide #144228

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[InstCombine][Docs] Update InstCombine contributor guide
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39 changes: 39 additions & 0 deletions llvm/docs/InstCombineContributorGuide.md
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Expand Up @@ -404,11 +404,32 @@ The use of TargetTransformInfo is only allowed for hooks for target-specific
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 types, follow the instructions below.

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 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
Expand Down Expand Up @@ -531,6 +552,19 @@ need to add a one-use check for the inner instruction.
One-use checks can be performed using the `m_OneUse()` matcher, or the
`V->hasOneUse()` method.

### Flag handling

When possible, propagate poison-generating flags like `nuw` and `nsw` since they may be
hard to salvage later. If it is not free (e.g. requires additional complexity like `willNotOverflow`
or KnownBits queries), don't do that.

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 carefully 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.
Comment on lines +565 to +566
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@artagnon artagnon Jun 15, 2025

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Is nsz meaningless for fcmp? I thought there are some optimizations already that take nsz on fcmp into account, with the property that -0. == +0.?

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@dtcxzyw dtcxzyw Jun 15, 2025

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0.0 and -0.0 are considered equal, even if nsz is absent.

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@artagnon artagnon Jun 15, 2025

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Perhaps I'm confused about something? See CmpInst::isEquivalence, which is used in some places:

// Returns true if either operand of CmpInst is a provably non-zero
// floating-point constant.
static bool hasNonZeroFPOperands(const CmpInst *Cmp) {
  auto *LHS = dyn_cast<Constant>(Cmp->getOperand(0));
  auto *RHS = dyn_cast<Constant>(Cmp->getOperand(1));
  if (auto *Const = LHS ? LHS : RHS) {
    using namespace llvm::PatternMatch;
    return match(Const, m_NonZeroNotDenormalFP());
  }
  return false;
}

// Floating-point equality is not an equivalence when comparing +0.0 with
// -0.0, when comparing NaN with another value, or when flushing
// denormals-to-zero.
bool CmpInst::isEquivalence(bool Invert) const {
  switch (Invert ? getInversePredicate() : getPredicate()) {
  case CmpInst::Predicate::ICMP_EQ:
    return true;
  case CmpInst::Predicate::FCMP_UEQ:
    if (!hasNoNaNs())
      return false;
    [[fallthrough]];
  case CmpInst::Predicate::FCMP_OEQ:
    return hasNonZeroFPOperands(this);
  default:
    return false;
  }
}

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@dtcxzyw dtcxzyw Jun 15, 2025

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equal is not equivalence. The latter one means one of the operands can be replaced by the other.

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@nikic nikic Jun 16, 2025

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@artagnon Note that this does not actually checks the nsz flag, it checks whether the operands are non-zero. That's fine.


### Generalization

Transforms can both be too specific (only handling some odd subset of patterns,
Expand Down Expand Up @@ -558,6 +592,11 @@ guidelines.
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

Expand Down
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