[PATCH] D125887: [CGP] Freeze condition when despeculating ctlz/cttz

[Nikita Popov via Phabricator via llvm-commits]

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Freeze the condition of the newly introduced conditional branch, to avoid immediate undefined behavior if the input to ctlz/cttz was originally poison.


https://reviews.llvm.org/D125887

Files:
  llvm/lib/CodeGen/CodeGenPrepare.cpp
  llvm/test/Transforms/CodeGenPrepare/X86/cttz-ctlz.ll


Index: llvm/test/Transforms/CodeGenPrepare/X86/cttz-ctlz.ll
===================================================================
--- llvm/test/Transforms/CodeGenPrepare/X86/cttz-ctlz.ll
+++ llvm/test/Transforms/CodeGenPrepare/X86/cttz-ctlz.ll
@@ -13,7 +13,8 @@
 define i64 @cttz(i64 %A) {
 ; SLOW-LABEL: @cttz(
 ; SLOW-NEXT:  entry:
-; SLOW-NEXT:    [[CMPZ:%.*]] = icmp eq i64 [[A:%.*]], 0
+; SLOW-NEXT:    [[TMP0:%.*]] = freeze i64 [[A:%.*]]
+; SLOW-NEXT:    [[CMPZ:%.*]] = icmp eq i64 [[TMP0]], 0
 ; SLOW-NEXT:    br i1 [[CMPZ]], label [[COND_END:%.*]], label [[COND_FALSE:%.*]]
 ; SLOW:       cond.false:
 ; SLOW-NEXT:    [[Z:%.*]] = call i64 @llvm.cttz.i64(i64 [[A]], i1 true)
@@ -29,7 +30,8 @@
 ;
 ; FAST_LZ-LABEL: @cttz(
 ; FAST_LZ-NEXT:  entry:
-; FAST_LZ-NEXT:    [[CMPZ:%.*]] = icmp eq i64 [[A:%.*]], 0
+; FAST_LZ-NEXT:    [[TMP0:%.*]] = freeze i64 [[A:%.*]]
+; FAST_LZ-NEXT:    [[CMPZ:%.*]] = icmp eq i64 [[TMP0]], 0
 ; FAST_LZ-NEXT:    br i1 [[CMPZ]], label [[COND_END:%.*]], label [[COND_FALSE:%.*]]
 ; FAST_LZ:       cond.false:
 ; FAST_LZ-NEXT:    [[Z:%.*]] = call i64 @llvm.cttz.i64(i64 [[A]], i1 true)
@@ -46,7 +48,8 @@
 define i64 @ctlz(i64 %A) {
 ; SLOW-LABEL: @ctlz(
 ; SLOW-NEXT:  entry:
-; SLOW-NEXT:    [[CMPZ:%.*]] = icmp eq i64 [[A:%.*]], 0
+; SLOW-NEXT:    [[TMP0:%.*]] = freeze i64 [[A:%.*]]
+; SLOW-NEXT:    [[CMPZ:%.*]] = icmp eq i64 [[TMP0]], 0
 ; SLOW-NEXT:    br i1 [[CMPZ]], label [[COND_END:%.*]], label [[COND_FALSE:%.*]]
 ; SLOW:       cond.false:
 ; SLOW-NEXT:    [[Z:%.*]] = call i64 @llvm.ctlz.i64(i64 [[A]], i1 true)
@@ -57,7 +60,8 @@
 ;
 ; FAST_TZ-LABEL: @ctlz(
 ; FAST_TZ-NEXT:  entry:
-; FAST_TZ-NEXT:    [[CMPZ:%.*]] = icmp eq i64 [[A:%.*]], 0
+; FAST_TZ-NEXT:    [[TMP0:%.*]] = freeze i64 [[A:%.*]]
+; FAST_TZ-NEXT:    [[CMPZ:%.*]] = icmp eq i64 [[TMP0]], 0
 ; FAST_TZ-NEXT:    br i1 [[CMPZ]], label [[COND_END:%.*]], label [[COND_FALSE:%.*]]
 ; FAST_TZ:       cond.false:
 ; FAST_TZ-NEXT:    [[Z:%.*]] = call i64 @llvm.ctlz.i64(i64 [[A]], i1 true)
Index: llvm/lib/CodeGen/CodeGenPrepare.cpp
===================================================================
--- llvm/lib/CodeGen/CodeGenPrepare.cpp
+++ llvm/lib/CodeGen/CodeGenPrepare.cpp
@@ -2038,7 +2038,8 @@
     return false;
 
   // Bail if the value is never zero.
-  if (llvm::isKnownNonZero(CountZeros->getOperand(0), *DL))
+  Value *Op = CountZeros->getOperand(0);
+  if (llvm::isKnownNonZero(Op, *DL))
     return false;
 
   // The intrinsic will be sunk behind a compare against zero and branch.
@@ -2059,7 +2060,9 @@
   // Replace the unconditional branch that was created by the first split with
   // a compare against zero and a conditional branch.
   Value *Zero = Constant::getNullValue(Ty);
-  Value *Cmp = Builder.CreateICmpEQ(CountZeros->getOperand(0), Zero, "cmpz");
+  if (!isGuaranteedNotToBeUndefOrPoison(Op))
+    Op = Builder.CreateFreeze(Op); // Avoid introducing branch on poison.
+  Value *Cmp = Builder.CreateICmpEQ(Op, Zero, "cmpz");
   Builder.CreateCondBr(Cmp, EndBlock, CallBlock);
   StartBlock->getTerminator()->eraseFromParent();
 


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