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[InstCombine] use freeze to enable poison-safe logic->select fold

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Without a freeze, this transform can leak poison to the output:
https://alive2.llvm.org/ce/z/GJuF9i

This makes the transform as uniform as possible, and it can help
reduce patterns like issue #58313 (although that particular
example probably still needs another transform).

Differential Revision: https://reviews.llvm.org/D136527
This commit is contained in:
Sanjay Patel
2022-10-22 10:12:34 -04:00
parent db40f9b774
commit 8628e6df70
2 changed files with 13 additions and 10 deletions
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12
llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
View File

@@ -2853,15 +2853,17 @@ Instruction *InstCombinerImpl::visitSelectInst(SelectInst &SI) {
// (C && A) || (!C && B) --> sel C, A, B
// (A && C) || (!C && B) --> sel C, A, B
// (C && A) || (B && !C) --> sel C, A, B
// (A && C) || (B && !C) --> sel C, A, B (only with real 'and' ops)
// (A && C) || (B && !C) --> sel C, A, B (may require freeze)
if (match(FalseVal, m_c_LogicalAnd(m_Not(m_Value(C)), m_Value(B))) &&
match(CondVal, m_c_LogicalAnd(m_Specific(C), m_Value(A)))) {
auto *SelCond = dyn_cast<SelectInst>(CondVal);
auto *SelFVal = dyn_cast<SelectInst>(FalseVal);
if (!SelCond || !SelFVal ||
!match(SelFVal->getTrueValue(),
m_Not(m_Specific(SelCond->getTrueValue()))))
return SelectInst::Create(C, A, B);
bool MayNeedFreeze = SelCond && SelFVal &&
match(SelFVal->getTrueValue(),
m_Not(m_Specific(SelCond->getTrueValue())));
if (MayNeedFreeze)
C = Builder.CreateFreeze(C);
return SelectInst::Create(C, A, B);
}
// (!C && A) || (C && B) --> sel C, B, A

11
llvm/test/Transforms/InstCombine/select-safe-transforms.ll
View File

@@ -529,14 +529,12 @@ define i1 @bools2_logical_commute2_and1_and2(i1 %a, i1 %c) {
ret i1 %or
}
; This is not safe to transform if 'c' could be poison.
; Freeze 'c' to prevent poison from leaking.
define i1 @bools2_logical_commute3(i1 %a, i1 %b, i1 %c) {
; CHECK-LABEL: @bools2_logical_commute3(
; CHECK-NEXT: [[NOT:%.*]] = xor i1 [[C:%.*]], true
; CHECK-NEXT: [[AND1:%.*]] = select i1 [[A:%.*]], i1 [[C]], i1 false
; CHECK-NEXT: [[AND2:%.*]] = select i1 [[B:%.*]], i1 [[NOT]], i1 false
; CHECK-NEXT: [[OR:%.*]] = select i1 [[AND1]], i1 true, i1 [[AND2]]
; CHECK-NEXT: [[TMP1:%.*]] = freeze i1 [[C:%.*]]
; CHECK-NEXT: [[OR:%.*]] = select i1 [[TMP1]], i1 [[A:%.*]], i1 [[B:%.*]]
; CHECK-NEXT: ret i1 [[OR]]
;
%not = xor i1 %c, -1
@@ -546,6 +544,9 @@ define i1 @bools2_logical_commute3(i1 %a, i1 %b, i1 %c) {
ret i1 %or
}
; No freeze needed when 'c' is guaranteed not be poison.
; Intermediate logic folds may already reduce this.
define i1 @bools2_logical_commute3_nopoison(i1 %a, i1 %b, i1 noundef %c) {
; CHECK-LABEL: @bools2_logical_commute3_nopoison(
; CHECK-NEXT: [[OR:%.*]] = select i1 [[C:%.*]], i1 [[A:%.*]], i1 [[B:%.*]]