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[InstCombine] Fold abs of known negative operand

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If we know that the abs operand is known negative, we can replace
it with a neg.

To avoid computing known bits twice, I've removed the fold for the
non-negative case from InstSimplify. Both the non-negative and the
negative case are handled by InstCombine now, with one known bits call.

Differential Revision: https://reviews.llvm.org/D87196
This commit is contained in:
Nikita Popov 2020-09-05 17:23:48 +02:00
parent 7cc27a28b7
commit 938cb930df
4 changed files with 30 additions and 16 deletions
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3
lib/Analysis/InstructionSimplify.cpp
View File

@ -5274,9 +5274,6 @@ static Value *simplifyBinaryIntrinsic(Function *F, Value *Op0, Value *Op1,
// on the outer abs.
if (match(Op0, m_Intrinsic<Intrinsic::abs>(m_Value(), m_Value())))
return Op0;
// If the sign bit is clear already, then abs does not do anything.
if (isKnownNonNegative(Op0, Q.DL, 0, Q.AC, Q.CxtI, Q.DT))
return Op0;
break;
case Intrinsic::smax:

19
lib/Transforms/InstCombine/InstCombineCalls.cpp
View File

@ -657,6 +657,19 @@ InstCombinerImpl::foldIntrinsicWithOverflowCommon(IntrinsicInst *II) {
return nullptr;
}
static Optional<bool> getKnownSign(Value *Op, Instruction *CxtI,
const DataLayout &DL, AssumptionCache *AC,
DominatorTree *DT) {
KnownBits Known = computeKnownBits(Op, DL, 0, AC, CxtI, DT);
if (Known.isNonNegative())
return false;
if (Known.isNegative())
return true;
return isImpliedByDomCondition(
ICmpInst::ICMP_SLT, Op, Constant::getNullValue(Op->getType()), CxtI, DL);
}
/// CallInst simplification. This mostly only handles folding of intrinsic
/// instructions. For normal calls, it allows visitCallBase to do the heavy
/// lifting.
@ -791,11 +804,9 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
if (match(IIOperand, m_Select(m_Value(), m_Neg(m_Value(X)), m_Deferred(X))))
return replaceOperand(*II, 0, X);
if (Optional<bool> Imp = isImpliedByDomCondition(
ICmpInst::ICMP_SGE, IIOperand,
Constant::getNullValue(IIOperand->getType()), II, DL)) {
if (Optional<bool> Sign = getKnownSign(IIOperand, II, DL, &AC, &DT)) {
// abs(x) -> x if x >= 0
if (*Imp)
if (!*Sign)
return replaceInstUsesWith(*II, IIOperand);
// abs(x) -> -x if x < 0

7
test/Transforms/InstCombine/abs-intrinsic.ll
View File

@ -233,7 +233,7 @@ define i32 @abs_assume_neg(i32 %x) {
; CHECK-LABEL: @abs_assume_neg(
; CHECK-NEXT: [[CMP:%.*]] = icmp slt i32 [[X:%.*]], 0
; CHECK-NEXT: call void @llvm.assume(i1 [[CMP]])
; CHECK-NEXT: [[ABS:%.*]] = call i32 @llvm.abs.i32(i32 [[X]], i1 false)
; CHECK-NEXT: [[ABS:%.*]] = sub i32 0, [[X]]
; CHECK-NEXT: ret i32 [[ABS]]
;
%cmp = icmp slt i32 %x, 0
@ -245,9 +245,8 @@ define i32 @abs_assume_neg(i32 %x) {
define i32 @abs_known_neg(i16 %x) {
; CHECK-LABEL: @abs_known_neg(
; CHECK-NEXT: [[EXT:%.*]] = zext i16 [[X:%.*]] to i32
; CHECK-NEXT: [[NEG:%.*]] = xor i32 [[EXT]], -1
; CHECK-NEXT: [[ABS:%.*]] = call i32 @llvm.abs.i32(i32 [[NEG]], i1 false)
; CHECK-NEXT: ret i32 [[ABS]]
; CHECK-NEXT: [[NEG_NEG:%.*]] = add nuw nsw i32 [[EXT]], 1
; CHECK-NEXT: ret i32 [[NEG_NEG]]
;
%ext = zext i16 %x to i32
%neg = sub nsw i32 -1, %ext

17
test/Transforms/InstSimplify/abs_intrinsic.ll
View File

@ -47,11 +47,14 @@ define i32 @test_abs_abs_3(i32 %x) {
}
; If the sign bit is known zero, the abs is not needed.
; These cases are only folded by InstCombine, to avoid computing known bits
; twice, for the non-negative and the negative case.
define i32 @zext_abs(i31 %x) {
; CHECK-LABEL: @zext_abs(
; CHECK-NEXT: [[ZEXT:%.*]] = zext i31 [[X:%.*]] to i32
; CHECK-NEXT: ret i32 [[ZEXT]]
; CHECK-NEXT: [[ABS:%.*]] = call i32 @llvm.abs.i32(i32 [[ZEXT]], i1 false)
; CHECK-NEXT: ret i32 [[ABS]]
;
%zext = zext i31 %x to i32
%abs = call i32 @llvm.abs.i32(i32 %zext, i1 false)
@ -61,7 +64,8 @@ define i32 @zext_abs(i31 %x) {
define <3 x i82> @lshr_abs(<3 x i82> %x) {
; CHECK-LABEL: @lshr_abs(
; CHECK-NEXT: [[LSHR:%.*]] = lshr <3 x i82> [[X:%.*]], <i82 1, i82 1, i82 1>
; CHECK-NEXT: ret <3 x i82> [[LSHR]]
; CHECK-NEXT: [[ABS:%.*]] = call <3 x i82> @llvm.abs.v3i82(<3 x i82> [[LSHR]], i1 true)
; CHECK-NEXT: ret <3 x i82> [[ABS]]
;
%lshr = lshr <3 x i82> %x, <i82 1, i82 1, i82 1>
%abs = call <3 x i82> @llvm.abs.v3i82(<3 x i82> %lshr, i1 true)
@ -71,7 +75,8 @@ define <3 x i82> @lshr_abs(<3 x i82> %x) {
define i32 @and_abs(i32 %x) {
; CHECK-LABEL: @and_abs(
; CHECK-NEXT: [[AND:%.*]] = and i32 [[X:%.*]], 2147483644
; CHECK-NEXT: ret i32 [[AND]]
; CHECK-NEXT: [[ABS:%.*]] = call i32 @llvm.abs.i32(i32 [[AND]], i1 true)
; CHECK-NEXT: ret i32 [[ABS]]
;
%and = and i32 %x, 2147483644
%abs = call i32 @llvm.abs.i32(i32 %and, i1 true)
@ -81,7 +86,8 @@ define i32 @and_abs(i32 %x) {
define <3 x i82> @select_abs(<3 x i1> %cond) {
; CHECK-LABEL: @select_abs(
; CHECK-NEXT: [[SEL:%.*]] = select <3 x i1> [[COND:%.*]], <3 x i82> zeroinitializer, <3 x i82> <i82 2147483647, i82 42, i82 1>
; CHECK-NEXT: ret <3 x i82> [[SEL]]
; CHECK-NEXT: [[ABS:%.*]] = call <3 x i82> @llvm.abs.v3i82(<3 x i82> [[SEL]], i1 false)
; CHECK-NEXT: ret <3 x i82> [[ABS]]
;
%sel = select <3 x i1> %cond, <3 x i82> zeroinitializer, <3 x i82> <i82 2147483647, i82 42, i82 1>
%abs = call <3 x i82> @llvm.abs.v3i82(<3 x i82> %sel, i1 false)
@ -94,7 +100,8 @@ define i32 @assume_abs(i32 %x) {
; CHECK-LABEL: @assume_abs(
; CHECK-NEXT: [[ASSUME:%.*]] = icmp sge i32 [[X:%.*]], 0
; CHECK-NEXT: call void @llvm.assume(i1 [[ASSUME]])
; CHECK-NEXT: ret i32 [[X]]
; CHECK-NEXT: [[ABS:%.*]] = call i32 @llvm.abs.i32(i32 [[X]], i1 true)
; CHECK-NEXT: ret i32 [[ABS]]
;
%assume = icmp sge i32 %x, 0
call void @llvm.assume(i1 %assume)