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[InstCombine] Fold freeze(undef) into a proper constant
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This is a simple patch that folds freeze(undef) into a proper constant after inspecting its uses.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D84948
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@aqjune
aqjune committed Aug 6, 2020
1 parent 54a1097 commit c771087
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Showing 4 changed files with 41 additions and 36 deletions.
28 changes: 28 additions & 0 deletions llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
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Expand Up @@ -3387,6 +3387,34 @@ Instruction *InstCombinerImpl::visitFreeze(FreezeInst &I) {
return NV;
}

if (match(Op0, m_Undef())) {
// If I is freeze(undef), see its uses and fold it to the best constant.
// - or: pick -1
// - select's condition: pick the value that leads to choosing a constant
// - other ops: pick 0
Constant *BestValue = nullptr;
Constant *NullValue = Constant::getNullValue(I.getType());
for (const auto *U : I.users()) {
Constant *C = NullValue;

if (match(U, m_Or(m_Value(), m_Value())))
C = Constant::getAllOnesValue(I.getType());
else if (const auto *SI = dyn_cast<SelectInst>(U)) {
if (SI->getCondition() == &I) {
APInt CondVal(1, isa<Constant>(SI->getFalseValue()) ? 0 : 1);
C = Constant::getIntegerValue(I.getType(), CondVal);
}
}

if (!BestValue)
BestValue = C;
else if (BestValue != C)
BestValue = NullValue;
}

return replaceInstUsesWith(I, BestValue);
}

return nullptr;
}

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3 changes: 1 addition & 2 deletions llvm/test/Transforms/InstCombine/freeze-phi.ll
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Expand Up @@ -154,14 +154,13 @@ define i32 @one_undef(i8 %cond) {
; CHECK-NEXT: i8 1, label [[C:%.*]]
; CHECK-NEXT: ]
; CHECK: A:
; CHECK-NEXT: [[PHI_FR:%.*]] = freeze i32 undef
; CHECK-NEXT: br label [[D:%.*]]
; CHECK: B:
; CHECK-NEXT: br label [[D]]
; CHECK: C:
; CHECK-NEXT: br label [[D]]
; CHECK: D:
; CHECK-NEXT: [[Y:%.*]] = phi i32 [ [[PHI_FR]], [[A]] ], [ 32, [[B]] ], [ 0, [[C]] ]
; CHECK-NEXT: [[Y:%.*]] = phi i32 [ 0, [[A]] ], [ 32, [[B]] ], [ 0, [[C]] ]
; CHECK-NEXT: ret i32 [[Y]]
;
switch i8 %cond, label %A [
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25 changes: 7 additions & 18 deletions llvm/test/Transforms/InstCombine/freeze.ll
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Expand Up @@ -21,9 +21,7 @@ define i32 @make_const() {

define i32 @and_freeze_undef(i32 %x) {
; CHECK-LABEL: @and_freeze_undef(
; CHECK-NEXT: [[F:%.*]] = freeze i32 undef
; CHECK-NEXT: [[RES:%.*]] = and i32 [[F]], [[X:%.*]]
; CHECK-NEXT: ret i32 [[RES]]
; CHECK-NEXT: ret i32 0
;
%f = freeze i32 undef
%res = and i32 %x, %f
Expand All @@ -34,10 +32,8 @@ declare void @use_i32(i32)

define i32 @and_freeze_undef_multipleuses(i32 %x) {
; CHECK-LABEL: @and_freeze_undef_multipleuses(
; CHECK-NEXT: [[F:%.*]] = freeze i32 undef
; CHECK-NEXT: [[RES:%.*]] = and i32 [[F]], [[X:%.*]]
; CHECK-NEXT: call void @use_i32(i32 [[F]])
; CHECK-NEXT: ret i32 [[RES]]
; CHECK-NEXT: call void @use_i32(i32 0)
; CHECK-NEXT: ret i32 0
;
%f = freeze i32 undef
%res = and i32 %x, %f
Expand All @@ -47,9 +43,7 @@ define i32 @and_freeze_undef_multipleuses(i32 %x) {

define i32 @or_freeze_undef(i32 %x) {
; CHECK-LABEL: @or_freeze_undef(
; CHECK-NEXT: [[F:%.*]] = freeze i32 undef
; CHECK-NEXT: [[RES:%.*]] = or i32 [[F]], [[X:%.*]]
; CHECK-NEXT: ret i32 [[RES]]
; CHECK-NEXT: ret i32 -1
;
%f = freeze i32 undef
%res = or i32 %x, %f
Expand All @@ -58,10 +52,8 @@ define i32 @or_freeze_undef(i32 %x) {

define i32 @or_freeze_undef_multipleuses(i32 %x) {
; CHECK-LABEL: @or_freeze_undef_multipleuses(
; CHECK-NEXT: [[F:%.*]] = freeze i32 undef
; CHECK-NEXT: [[RES:%.*]] = or i32 [[F]], [[X:%.*]]
; CHECK-NEXT: call void @use_i32(i32 [[F]])
; CHECK-NEXT: ret i32 [[RES]]
; CHECK-NEXT: call void @use_i32(i32 0)
; CHECK-NEXT: ret i32 [[X:%.*]]
;
%f = freeze i32 undef
%res = or i32 %x, %f
Expand All @@ -73,10 +65,7 @@ declare void @use_i32_i1(i32, i1)

define void @or_select_multipleuses(i32 %x, i1 %y) {
; CHECK-LABEL: @or_select_multipleuses(
; CHECK-NEXT: [[F:%.*]] = freeze i1 undef
; CHECK-NEXT: [[A:%.*]] = select i1 [[F]], i32 [[X:%.*]], i32 32
; CHECK-NEXT: [[B:%.*]] = or i1 [[F]], [[Y:%.*]]
; CHECK-NEXT: call void @use_i32_i1(i32 [[A]], i1 [[B]])
; CHECK-NEXT: call void @use_i32_i1(i32 32, i1 [[Y:%.*]])
; CHECK-NEXT: ret void
;
%f = freeze i1 undef
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21 changes: 5 additions & 16 deletions llvm/test/Transforms/InstCombine/select.ll
View file
Expand Up @@ -2490,9 +2490,7 @@ define <2 x i32> @true_undef_vec(i1 %cond, <2 x i32> %x) {

define i8 @cond_freeze(i8 %x, i8 %y) {
; CHECK-LABEL: @cond_freeze(
; CHECK-NEXT: [[COND_FR:%.*]] = freeze i1 undef
; CHECK-NEXT: [[S:%.*]] = select i1 [[COND_FR]], i8 [[X:%.*]], i8 [[Y:%.*]]
; CHECK-NEXT: ret i8 [[S]]
; CHECK-NEXT: ret i8 [[X:%.*]]
;
%cond.fr = freeze i1 undef
%s = select i1 %cond.fr, i8 %x, i8 %y
Expand All @@ -2501,9 +2499,7 @@ define i8 @cond_freeze(i8 %x, i8 %y) {

define i8 @cond_freeze2(i8 %x, i8 %y) {
; CHECK-LABEL: @cond_freeze2(
; CHECK-NEXT: [[COND_FR:%.*]] = freeze i1 undef
; CHECK-NEXT: [[S:%.*]] = select i1 [[COND_FR]], i8 [[X:%.*]], i8 1
; CHECK-NEXT: ret i8 [[S]]
; CHECK-NEXT: ret i8 1
;
%cond.fr = freeze i1 undef
%s = select i1 %cond.fr, i8 %x, i8 1
Expand All @@ -2512,9 +2508,7 @@ define i8 @cond_freeze2(i8 %x, i8 %y) {

define i8 @cond_freeze3(i8 %x) {
; CHECK-LABEL: @cond_freeze3(
; CHECK-NEXT: [[COND_FR:%.*]] = freeze i1 undef
; CHECK-NEXT: [[S:%.*]] = select i1 [[COND_FR]], i8 1, i8 [[X:%.*]]
; CHECK-NEXT: ret i8 [[S]]
; CHECK-NEXT: ret i8 1
;
%cond.fr = freeze i1 undef
%s = select i1 %cond.fr, i8 1, i8 %x
Expand All @@ -2523,9 +2517,7 @@ define i8 @cond_freeze3(i8 %x) {

define <2 x i8> @cond_freeze_vec(<2 x i8> %x) {
; CHECK-LABEL: @cond_freeze_vec(
; CHECK-NEXT: [[COND_FR:%.*]] = freeze <2 x i1> undef
; CHECK-NEXT: [[S:%.*]] = select <2 x i1> [[COND_FR]], <2 x i8> <i8 1, i8 2>, <2 x i8> [[X:%.*]]
; CHECK-NEXT: ret <2 x i8> [[S]]
; CHECK-NEXT: ret <2 x i8> <i8 1, i8 2>
;
%cond.fr = freeze <2 x i1> <i1 undef, i1 undef>
%s = select <2 x i1> %cond.fr, <2 x i8> <i8 1, i8 2>, <2 x i8> %x
Expand All @@ -2536,10 +2528,7 @@ declare void @foo2(i8, i8)

define void @cond_freeze_multipleuses(i8 %x, i8 %y) {
; CHECK-LABEL: @cond_freeze_multipleuses(
; CHECK-NEXT: [[COND_FR:%.*]] = freeze i1 undef
; CHECK-NEXT: [[S:%.*]] = select i1 [[COND_FR]], i8 [[X:%.*]], i8 [[Y:%.*]]
; CHECK-NEXT: [[S2:%.*]] = select i1 [[COND_FR]], i8 [[Y]], i8 [[X]]
; CHECK-NEXT: call void @foo2(i8 [[S]], i8 [[S2]])
; CHECK-NEXT: call void @foo2(i8 [[X:%.*]], i8 [[Y:%.*]])
; CHECK-NEXT: ret void
;
%cond.fr = freeze i1 undef
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