[InstCombine] fold udiv with common factor from muls with nuw

Unfortunately, sdiv isn't as simple because of UB due to overflow.

This fold is mentioned in PR38239:
https://bugs.llvm.org/show_bug.cgi?id=38239

llvm-svn: 338059

1 files changed, 15 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
index d6adbec02ac..63761d42723 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
@@ -973,6 +973,21 @@ Instruction *InstCombiner::visitUDiv(BinaryOperator &I) {
   if (Instruction *NarrowDiv = narrowUDivURem(I, Builder))
     return NarrowDiv;
 
+  // If the udiv operands are non-overflowing multiplies with a common operand,
+  // then eliminate the common factor:
+  // (A * B) / (A * X) --> B / X (and commuted variants)
+  // TODO: The code would be reduced if we had m_c_NUWMul pattern matching.
+  // TODO: If -reassociation handled this generally, we could remove this.
+  Value *A, *B;
+  if (match(Op0, m_NUWMul(m_Value(A), m_Value(B)))) {
+    if (match(Op1, m_NUWMul(m_Specific(A), m_Value(X))) ||
+        match(Op1, m_NUWMul(m_Value(X), m_Specific(A))))
+      return BinaryOperator::CreateUDiv(B, X);
+    if (match(Op1, m_NUWMul(m_Specific(B), m_Value(X))) ||
+        match(Op1, m_NUWMul(m_Value(X), m_Specific(B))))
+      return BinaryOperator::CreateUDiv(A, X);
+  }
+
   // (LHS udiv (select (select (...)))) -> (LHS >> (select (select (...))))
   SmallVector<UDivFoldAction, 6> UDivActions;
   if (visitUDivOperand(Op0, Op1, I, UDivActions))