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[InstCombine] Negator: - (C - %x) --> %x - C (PR47997)

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This relaxes one-use restriction on that `sub` fold,
since apparently the addition of Negator broke
preexisting `C-(C2-X) --> X+(C-C2)` (with C=0) fold.
This commit is contained in:
Roman Lebedev 2020-11-03 13:34:38 +03:00
parent 7eb9baa95b
commit ca79221be4
3 changed files with 14 additions and 13 deletions
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17
lib/Transforms/InstCombine/InstCombineNegator.cpp
View File

@ -219,19 +219,22 @@ LLVM_NODISCARD Value *Negator::visitImpl(Value *V, unsigned Depth) {
break; // Other instructions require recursive reasoning.
}
if (I->getOpcode() == Instruction::Sub &&
(I->hasOneUse() || (isa<Constant>(I->getOperand(0)) &&
!isa<ConstantExpr>(I->getOperand(0))))) {
// `sub` is always negatible.
// However, only do this either if the old `sub` doesn't stick around, or
// it was subtracting from a constant. Otherwise, this isn't profitable.
return Builder.CreateSub(I->getOperand(1), I->getOperand(0),
I->getName() + ".neg");
}
// Some other cases, while still don't require recursion,
// are restricted to the one-use case.
if (!V->hasOneUse())
return nullptr;
switch (I->getOpcode()) {
case Instruction::Sub:
// `sub` is always negatible.
// But if the old `sub` sticks around, even thought we don't increase
// instruction count, this is a likely regression since we increased
// live-range of *both* of the operands, which might lead to more spilling.
return Builder.CreateSub(I->getOperand(1), I->getOperand(0),
I->getName() + ".neg");
case Instruction::SDiv:
// `sdiv` is negatible if divisor is not undef/INT_MIN/1.
// While this is normally not behind a use-check,

4
test/Transforms/InstCombine/icmp.ll
View File

@ -3831,9 +3831,7 @@ define i1 @pr47997(i32 %arg) {
; CHECK-NEXT: store i32 [[I]], i32* @x, align 4
; CHECK-NEXT: [[I1:%.*]] = sub nsw i32 1, [[ARG]]
; CHECK-NEXT: store i32 [[I1]], i32* @y, align 4
; CHECK-NEXT: [[I2:%.*]] = sub nsw i32 0, [[I1]]
; CHECK-NEXT: [[I3:%.*]] = icmp eq i32 [[I]], [[I2]]
; CHECK-NEXT: ret i1 [[I3]]
; CHECK-NEXT: ret i1 true
;
bb:
%i = add nsw i32 %arg, -1

6
test/Transforms/InstCombine/sub-of-negatible.ll
View File

@ -195,10 +195,10 @@ define i8 @neg_of_sub_from_constant(i8 %x) {
define i8 @neg_of_sub_from_constant_multi_use(i8 %x) {
; CHECK-LABEL: @neg_of_sub_from_constant_multi_use(
; CHECK-NEXT: [[S:%.*]] = sub i8 42, [[X:%.*]]
; CHECK-NEXT: [[S_NEG:%.*]] = add i8 [[X:%.*]], -42
; CHECK-NEXT: [[S:%.*]] = sub i8 42, [[X]]
; CHECK-NEXT: call void @use8(i8 [[S]])
; CHECK-NEXT: [[R:%.*]] = sub i8 0, [[S]]
; CHECK-NEXT: ret i8 [[R]]
; CHECK-NEXT: ret i8 [[S_NEG]]
;
%s = sub i8 42, %x
call void @use8(i8 %s)