[llvm] [Reassociate]Reject fma pairs with a multi-use fmul addend (PR #220640)

[Alexey Bataev via llvm-commits]

https://github.com/alexey-bataev created https://github.com/llvm/llvm-project/pull/220640

Keeping such pairs frozen hid shared partial sums from reassociation.
Fixes the vbor regression from #215873 left over after #218675.

Assisted-by: Cursor


>From da07688b9e5044fbd9825255bd9f933da03f966a Mon Sep 17 00:00:00 2001
From: Alexey Bataev <a.bataev at outlook.com>
Date: Wed, 2 Sep 2026 09:20:17 -0700
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Created using spr 1.3.7
---
 llvm/lib/Transforms/Scalar/Reassociate.cpp    |  9 ++++---
 llvm/test/Transforms/Reassociate/fma-pairs.ll | 26 +++++++++++++++++++
 2 files changed, 31 insertions(+), 4 deletions(-)

diff --git a/llvm/lib/Transforms/Scalar/Reassociate.cpp b/llvm/lib/Transforms/Scalar/Reassociate.cpp
index 59c29ebeb21ee..5e5b0a8ae1d8f 100644
--- a/llvm/lib/Transforms/Scalar/Reassociate.cpp
+++ b/llvm/lib/Transforms/Scalar/Reassociate.cpp
@@ -194,13 +194,14 @@ static BinaryOperator *isFMulAddCandidate(Value *V) {
     return m_CombineAnd(m_AllowContract(m_OneUse(m_FMul(m_Value(), m_Value()))),
                         m_BinOp(FMul));
   };
-  BinaryOperator *Mul = nullptr, *OtherMul = nullptr;
+  BinaryOperator *Mul = nullptr;
   Value *OtherOp = nullptr;
-  // Keep constants and nested additions visible to the enclosing expression so
-  // they can participate in folding and reassociation.
+  // Keep constants, nested additions and other contractible multiplies visible
+  // to the enclosing expression so they can participate in folding,
+  // reassociation and factorization.
   if (!match(FAdd, m_c_FAdd(ContractableFMul(Mul), m_Value(OtherOp))) ||
       isa<Constant>(OtherOp) || isReassociableOp(OtherOp, Instruction::FAdd) ||
-      match(OtherOp, ContractableFMul(OtherMul)))
+      match(OtherOp, m_AllowContract(m_FMul(m_Value(), m_Value()))))
     return nullptr;
   return Mul;
 }
diff --git a/llvm/test/Transforms/Reassociate/fma-pairs.ll b/llvm/test/Transforms/Reassociate/fma-pairs.ll
index e3e330ebce039..f7c9c9b86e730 100644
--- a/llvm/test/Transforms/Reassociate/fma-pairs.ll
+++ b/llvm/test/Transforms/Reassociate/fma-pairs.ll
@@ -454,3 +454,29 @@ define double @nested_fma_subtraction(double %acc, double %a, double %b, double
   ret double %result
 }
 
+
+define float @multi_use_fmul_addend(float %a, float %c, float %d, float %e, float %f) {
+; CHECK-LABEL: define float @multi_use_fmul_addend(
+; CHECK-SAME: float [[A:%.*]], float [[C:%.*]], float [[D:%.*]], float [[E:%.*]], float [[F:%.*]]) {
+; CHECK-NEXT:    [[AC:%.*]] = fmul fast float [[C]], [[A]]
+; CHECK-NEXT:    [[AD:%.*]] = fmul fast float [[D]], [[A]]
+; CHECK-NEXT:    [[AE:%.*]] = fmul fast float [[E]], [[A]]
+; CHECK-NEXT:    [[E_SUM:%.*]] = fadd fast float [[AD]], [[AC]]
+; CHECK-NEXT:    [[T1:%.*]] = fmul fast float [[E_SUM]], [[E]]
+; CHECK-NEXT:    [[F_SUM0:%.*]] = fadd fast float [[AD]], [[AC]]
+; CHECK-NEXT:    [[F_SUM:%.*]] = fadd fast float [[F_SUM0]], [[AE]]
+; CHECK-NEXT:    [[T2:%.*]] = fmul fast float [[F_SUM]], [[F]]
+; CHECK-NEXT:    [[R:%.*]] = fadd fast float [[T2]], [[T1]]
+; CHECK-NEXT:    ret float [[R]]
+;
+  %ac = fmul fast float %a, %c
+  %ad = fmul fast float %a, %d
+  %ae = fmul fast float %a, %e
+  %e.sum = fadd fast float %ad, %ac
+  %t1 = fmul fast float %e.sum, %e
+  %f.sum0 = fadd fast float %ae, %ad
+  %f.sum = fadd fast float %f.sum0, %ac
+  %t2 = fmul fast float %f.sum, %f
+  %r = fadd fast float %t1, %t2
+  ret float %r
+}