From decd49b20c24ab3c6f6490cb170e7f2795175149 Mon Sep 17 00:00:00 2001
From: jinpzhan <jinpzhan@amd.com>
Date: Wed, 12 Nov 2025 11:09:28 +0800
Subject: [PATCH] [InstCombine] Skip division sinking with allowReciprocal flag

---
 llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
index a9aacc707cc20..88700af60d633 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
@@ -795,7 +795,8 @@ Instruction *InstCombinerImpl::foldFMulReassoc(BinaryOperator &I) {
         return BinaryOperator::CreateFDivFMF(CC1, X, FMF);
     }
     if (match(Op0, m_FDiv(m_Value(X), m_Constant(C1)))) {
-      // FIXME: This seems like it should also be checking for arcp
+      // Constant folding is not affected by allowReciprocal,
+      // since constants are calculated at compile time.
       // (X / C1) * C --> X * (C / C1)
       Constant *CDivC1 =
           ConstantFoldBinaryOpOperands(Instruction::FDiv, C, C1, DL);
@@ -837,6 +838,12 @@ Instruction *InstCombinerImpl::foldFMulReassoc(BinaryOperator &I) {
                      m_Value(Z)))) {
     BinaryOperator *DivOp = cast<BinaryOperator>(((Z == Op0) ? Op1 : Op0));
     FastMathFlags FMF = I.getFastMathFlags() & DivOp->getFastMathFlags();
+    // Preserve (X / Y) * Z when reciprocal math is enabled to allow backend
+    // rcp instruction emission. Sinking to (X * Z) / Y would prevent this
+    // optimization, alter computation order and potentially affecting
+    // precision.
+    if (FMF.allowReciprocal())
+      return nullptr;
     if (FMF.allowReassoc()) {
       // Sink division: (X / Y) * Z --> (X * Z) / Y
       auto *NewFMul = Builder.CreateFMulFMF(X, Z, FMF);