[InstCombine] fix miscompile from multi-use cttz/ctlz transform

PR45762:
https://bugs.llvm.org/show_bug.cgi?id=45762

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index a5d11ce..b9d3274 100644 (file)
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -883,10 +883,11 @@ static Value *foldSelectCttzCtlz(ICmpInst *ICI, Value *TrueVal, Value *FalseVal,
     return SelectArg;
   }
 
-  // If the ValueOnZero is not the bitwidth, we can at least make use of the
-  // fact that the cttz/ctlz result will not be used if the input is zero, so
-  // it's okay to relax it to undef for that case.
-  if (II->hasOneUse() && !match(II->getArgOperand(1), m_One()))
+  // The ValueOnZero is not the bitwidth. But if the cttz/ctlz (and optional
+  // zext/trunc) have one use (ending at the select), the cttz/ctlz result will
+  // not be used if the input is zero. Relax to 'undef_on_zero' for that case.
+  if (II->hasOneUse() && SelectArg->hasOneUse() &&
+      !match(II->getArgOperand(1), m_One()))
     II->setArgOperand(1, ConstantInt::getTrue(II->getContext()));
 
   return nullptr;

diff --git a/llvm/test/Transforms/InstCombine/select-ctlz-to-cttz.ll b/llvm/test/Transforms/InstCombine/select-ctlz-to-cttz.ll
index e0cd667..44d284c 100644 (file)
--- a/llvm/test/Transforms/InstCombine/select-ctlz-to-cttz.ll
+++ b/llvm/test/Transforms/InstCombine/select-ctlz-to-cttz.ll
@@ -220,7 +220,7 @@ define <2 x i32> @select_clz_to_ctz_vec_with_undef(<2 x i32> %a) {
 
 define i4 @PR45762(i3 %x4) {
 ; CHECK-LABEL: @PR45762(
-; CHECK-NEXT:    [[T4:%.*]] = call i3 @llvm.cttz.i3(i3 [[X4:%.*]], i1 true), !range !2
+; CHECK-NEXT:    [[T4:%.*]] = call i3 @llvm.cttz.i3(i3 [[X4:%.*]], i1 false), !range !2
 ; CHECK-NEXT:    [[T7:%.*]] = zext i3 [[T4]] to i4
 ; CHECK-NEXT:    [[ONE_HOT_16:%.*]] = shl i4 1, [[T7]]
 ; CHECK-NEXT:    [[TMP1:%.*]] = icmp eq i3 [[X4]], 0