[PATCH] D141031: [InstCombine] Combine (zext a) mul (zext b) to llvm.umul.with.overflow only if mul has NUW flag

[luxufan via Phabricator via llvm-commits]

StephenFan created this revision.
StephenFan added reviewers: nikic, spatel.
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Fixes: https://github.com/llvm/llvm-project/issues/59836


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D141031

Files:
  llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
  llvm/test/Transforms/InstCombine/overflow-mul.ll


Index: llvm/test/Transforms/InstCombine/overflow-mul.ll
===================================================================
--- llvm/test/Transforms/InstCombine/overflow-mul.ll
+++ llvm/test/Transforms/InstCombine/overflow-mul.ll
@@ -241,3 +241,43 @@
   %cmp = icmp ne i32 %mul, %and
   ret i1 %cmp
 }
+
+; Negative test: mul(zext x, zext y) may overflow.
+define i32 @mul_may_overflow(i32 %x, i32 %y) nounwind {
+; CHECK-LABEL: @mul_may_overflow(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[L:%.*]] = zext i32 [[X:%.*]] to i34
+; CHECK-NEXT:    [[R:%.*]] = zext i32 [[Y:%.*]] to i34
+; CHECK-NEXT:    [[MUL34:%.*]] = mul i34 [[L]], [[R]]
+; CHECK-NEXT:    [[OVERFLOW:%.*]] = icmp ult i34 [[MUL34]], 4294967296
+; CHECK-NEXT:    [[RETVAL:%.*]] = zext i1 [[OVERFLOW]] to i32
+; CHECK-NEXT:    ret i32 [[RETVAL]]
+;
+entry:
+  %l = zext i32 %x to i34
+  %r = zext i32 %y to i34
+  %mul34 = mul i34 %l, %r
+  %overflow = icmp ule i34 %mul34, 4294967295
+  %retval = zext i1 %overflow to i32
+  ret i32 %retval
+}
+
+; Negative test: mul(zext x, zext y) may overflow.
+define i32 @mul_may_overflow_commute(i32 %x, i32 %y) nounwind {
+; CHECK-LABEL: @mul_may_overflow_commute(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[L:%.*]] = zext i32 [[X:%.*]] to i34
+; CHECK-NEXT:    [[R:%.*]] = zext i32 [[Y:%.*]] to i34
+; CHECK-NEXT:    [[MUL34:%.*]] = mul i34 [[L]], [[R]]
+; CHECK-NEXT:    [[OVERFLOW:%.*]] = icmp ult i34 [[MUL34]], 4294967296
+; CHECK-NEXT:    [[RETVAL:%.*]] = zext i1 [[OVERFLOW]] to i32
+; CHECK-NEXT:    ret i32 [[RETVAL]]
+;
+entry:
+  %l = zext i32 %x to i34
+  %r = zext i32 %y to i34
+  %mul34 = mul i34 %l, %r
+  %overflow = icmp uge i34 4294967295, %mul34
+  %retval = zext i1 %overflow to i32
+  ret i32 %retval
+}
Index: llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -6338,11 +6338,11 @@
     }
 
     // (zext a) * (zext b)  --> llvm.umul.with.overflow.
-    if (match(Op0, m_Mul(m_ZExt(m_Value(A)), m_ZExt(m_Value(B))))) {
+    if (match(Op0, m_NUWMul(m_ZExt(m_Value(A)), m_ZExt(m_Value(B))))) {
       if (Instruction *R = processUMulZExtIdiom(I, Op0, Op1, *this))
         return R;
     }
-    if (match(Op1, m_Mul(m_ZExt(m_Value(A)), m_ZExt(m_Value(B))))) {
+    if (match(Op1, m_NUWMul(m_ZExt(m_Value(A)), m_ZExt(m_Value(B))))) {
       if (Instruction *R = processUMulZExtIdiom(I, Op1, Op0, *this))
         return R;
     }


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