maxjhandsome
/
llvm-project
forked from OSchip/llvm-project
1
0
Fork 0
Code Issues Pull Requests Packages Releases Wiki Activity

[CGP] Freeze condition when despeculating ctlz/cttz 

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

Differential Revision: https://reviews.llvm.org/D125887
This commit is contained in:
Nikita Popov 2022-05-18 15:34:12 +02:00
parent 572fc7d2fd
commit 5126c38012
2 changed files with 14 additions and 6 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

8
llvm/lib/CodeGen/CodeGenPrepare.cpp
View File

@ -2038,7 +2038,8 @@ static bool despeculateCountZeros(IntrinsicInst *CountZeros,
return false;
// Bail if the value is never zero.
if (llvm::isKnownNonZero(CountZeros->getOperand(0), *DL))
Value *Op = CountZeros->getOperand(0);
if (isKnownNonZero(Op, *DL))
return false;
// The intrinsic will be sunk behind a compare against zero and branch.
@ -2059,7 +2060,10 @@ static bool despeculateCountZeros(IntrinsicInst *CountZeros,
// 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");
// Avoid introducing branch on poison.
if (!isGuaranteedNotToBeUndefOrPoison(Op))
Op = Builder.CreateFreeze(Op, Op->getName() + ".fr");
Value *Cmp = Builder.CreateICmpEQ(Op, Zero, "cmpz");
Builder.CreateCondBr(Cmp, EndBlock, CallBlock);
StartBlock->getTerminator()->eraseFromParent();

12
llvm/test/Transforms/CodeGenPrepare/X86/cttz-ctlz.ll
View File

@ -13,7 +13,8 @@ target datalayout = "e-n32:64"
define i64 @cttz(i64 %A) {
; SLOW-LABEL: @cttz(
; SLOW-NEXT: entry:
; SLOW-NEXT: [[CMPZ:%.*]] = icmp eq i64 [[A:%.*]], 0
; SLOW-NEXT: [[A_FR:%.*]] = freeze i64 [[A:%.*]]
; SLOW-NEXT: [[CMPZ:%.*]] = icmp eq i64 [[A_FR]], 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 @@ define i64 @cttz(i64 %A) {
;
; FAST_LZ-LABEL: @cttz(
; FAST_LZ-NEXT: entry:
; FAST_LZ-NEXT: [[CMPZ:%.*]] = icmp eq i64 [[A:%.*]], 0
; FAST_LZ-NEXT: [[A_FR:%.*]] = freeze i64 [[A:%.*]]
; FAST_LZ-NEXT: [[CMPZ:%.*]] = icmp eq i64 [[A_FR]], 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 @@ entry:
define i64 @ctlz(i64 %A) {
; SLOW-LABEL: @ctlz(
; SLOW-NEXT: entry:
; SLOW-NEXT: [[CMPZ:%.*]] = icmp eq i64 [[A:%.*]], 0
; SLOW-NEXT: [[A_FR:%.*]] = freeze i64 [[A:%.*]]
; SLOW-NEXT: [[CMPZ:%.*]] = icmp eq i64 [[A_FR]], 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 @@ define i64 @ctlz(i64 %A) {
;
; FAST_TZ-LABEL: @ctlz(
; FAST_TZ-NEXT: entry:
; FAST_TZ-NEXT: [[CMPZ:%.*]] = icmp eq i64 [[A:%.*]], 0
; FAST_TZ-NEXT: [[A_FR:%.*]] = freeze i64 [[A:%.*]]
; FAST_TZ-NEXT: [[CMPZ:%.*]] = icmp eq i64 [[A_FR]], 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)