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[InstSimplify] Re-enable select ?, undef, X -> X transform when X is provably not poison

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Follow up from the transform being removed in D83360. If X is probably not poison, then the transform is safe.

Still plan to remove or adjust the code from ConstantFolding after this.

Differential Revision: https://reviews.llvm.org/D83440
This commit is contained in:
Craig Topper
2020-07-09 12:20:50 -07:00
parent 389b67b809
commit 469da663f2
2 changed files with 15 additions and 8 deletions
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11
llvm/lib/Analysis/InstructionSimplify.cpp
View File

@@ -4118,6 +4118,17 @@ static Value *SimplifySelectInst(Value *Cond, Value *TrueVal, Value *FalseVal,
if (TrueVal == FalseVal)
return TrueVal;
// If the true or false value is undef, we can fold to the other value as
// long as the other value isn't poison.
// select ?, undef, X -> X
if (isa<UndefValue>(TrueVal) &&
isGuaranteedNotToBeUndefOrPoison(FalseVal, Q.CxtI, Q.DT))
return FalseVal;
// select ?, X, undef -> X
if (isa<UndefValue>(FalseVal) &&
isGuaranteedNotToBeUndefOrPoison(TrueVal, Q.CxtI, Q.DT))
return TrueVal;
// Deal with partial undef vector constants: select ?, VecC, VecC' --> VecC''
Constant *TrueC, *FalseC;
if (TrueVal->getType()->isVectorTy() && match(TrueVal, m_Constant(TrueC)) &&

12
llvm/test/Transforms/InstSimplify/select.ll
View File

@@ -794,8 +794,7 @@ define <2 x i32> @true_undef_vec(i1 %cond, <2 x i32> %x) {
; These can be folded because the other value is guaranteed not to be poison.
define i32 @false_undef_true_constant(i1 %cond) {
; CHECK-LABEL: @false_undef_true_constant(
; CHECK-NEXT: [[S:%.*]] = select i1 [[COND:%.*]], i32 10, i32 undef
; CHECK-NEXT: ret i32 [[S]]
; CHECK-NEXT: ret i32 10
;
%s = select i1 %cond, i32 10, i32 undef
ret i32 %s
@@ -803,8 +802,7 @@ define i32 @false_undef_true_constant(i1 %cond) {
define i32 @true_undef_false_constant(i1 %cond) {
; CHECK-LABEL: @true_undef_false_constant(
; CHECK-NEXT: [[S:%.*]] = select i1 [[COND:%.*]], i32 undef, i32 20
; CHECK-NEXT: ret i32 [[S]]
; CHECK-NEXT: ret i32 20
;
%s = select i1 %cond, i32 undef, i32 20
ret i32 %s
@@ -830,8 +828,7 @@ define <2 x i32> @true_undef_false_constant_vec(i1 %cond) {
define i32 @false_undef_true_freeze(i1 %cond, i32 %x) {
; CHECK-LABEL: @false_undef_true_freeze(
; CHECK-NEXT: [[XF:%.*]] = freeze i32 [[X:%.*]]
; CHECK-NEXT: [[S:%.*]] = select i1 [[COND:%.*]], i32 [[XF]], i32 undef
; CHECK-NEXT: ret i32 [[S]]
; CHECK-NEXT: ret i32 [[XF]]
;
%xf = freeze i32 %x
%s = select i1 %cond, i32 %xf, i32 undef
@@ -841,8 +838,7 @@ define i32 @false_undef_true_freeze(i1 %cond, i32 %x) {
define i32 @false_undef_false_freeze(i1 %cond, i32 %x) {
; CHECK-LABEL: @false_undef_false_freeze(
; CHECK-NEXT: [[XF:%.*]] = freeze i32 [[X:%.*]]
; CHECK-NEXT: [[S:%.*]] = select i1 [[COND:%.*]], i32 undef, i32 [[XF]]
; CHECK-NEXT: ret i32 [[S]]
; CHECK-NEXT: ret i32 [[XF]]
;
%xf = freeze i32 %x
%s = select i1 %cond, i32 undef, i32 %xf