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[InstCombine] Fold sdiv exact X, -1<<C --> -(ashr exact X, C) 

While that does increases instruction count,
shift is obviously better than a division.

Name: base
Pre: (1<<C1) >= 0
%o0 = shl i8 1, C1
%r = sdiv exact i8 C0, %o0
  =>
%r = ashr exact i8 C0, C1

Name: neg
%o0 = shl i8 -1, C1
%r = sdiv exact i8 C0, %o0
  =>
%t0 = ashr exact i8 C0, C1
%r = sub i8 0, %t0

Name: reverse
Pre: C1 != 0 && C1 u< 8
%t0 = ashr exact i8 C0, C1
%r = sub i8 0, %t0
  =>
%o0 = shl i8 -1, C1
%r = sdiv exact i8 C0, %o0

https://rise4fun.com/Alive/MRplf
This commit is contained in:
Roman Lebedev 2020-08-06 21:08:30 +03:00
parent 47aec80e4a
commit 7ce76b06ec
No known key found for this signature in database
GPG Key ID: 083C3EBB4A1689E0
2 changed files with 19 additions and 5 deletions
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15
llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
View File

@ -1131,9 +1131,20 @@ Instruction *InstCombinerImpl::visitSDiv(BinaryOperator &I) {
return new ZExtInst(Builder.CreateICmpEQ(Op0, Op1), I.getType());
// sdiv exact X, 1<<C --> ashr exact X, C iff 1<<C is non-negative
if (I.isExact() && match(Op1, m_Power2()) && match(Op1, m_NonNegative()))
return BinaryOperator::CreateExactAShr(
// sdiv exact X, -1<<C --> -(ashr exact X, C)
if (I.isExact() && ((match(Op1, m_Power2()) && match(Op1, m_NonNegative())) ||
match(Op1, m_NegatedPower2()))) {
bool DivisorWasNegative = match(Op1, m_NegatedPower2());
if (DivisorWasNegative)
Op1 = ConstantExpr::getNeg(cast<Constant>(Op1));
auto *AShr = BinaryOperator::CreateExactAShr(
Op0, getLogBase2(I.getType(), cast<Constant>(Op1)), I.getName());
if (!DivisorWasNegative)
return AShr;
Builder.Insert(AShr);
AShr->setName(I.getName() + ".neg");
return BinaryOperator::CreateNeg(AShr, I.getName());
}
const APInt *Op1C;
if (match(Op1, m_APInt(Op1C))) {

9
llvm/test/Transforms/InstCombine/sdiv-exact-by-negative-power-of-two.ll
View File

@ -8,7 +8,8 @@
define i8 @t0(i8 %x) {
; CHECK-LABEL: @t0(
; CHECK-NEXT: [[DIV:%.*]] = sdiv exact i8 [[X:%.*]], -32
; CHECK-NEXT: [[DIV_NEG:%.*]] = ashr exact i8 [[X:%.*]], 5
; CHECK-NEXT: [[DIV:%.*]] = sub nsw i8 0, [[DIV_NEG]]
; CHECK-NEXT: ret i8 [[DIV]]
;
%div = sdiv exact i8 %x, -32
@ -25,7 +26,8 @@ define i8 @n1(i8 %x) {
define <2 x i8> @t2_vec_splat(<2 x i8> %x) {
; CHECK-LABEL: @t2_vec_splat(
; CHECK-NEXT: [[DIV:%.*]] = sdiv exact <2 x i8> [[X:%.*]], <i8 -32, i8 -32>
; CHECK-NEXT: [[DIV_NEG:%.*]] = ashr exact <2 x i8> [[X:%.*]], <i8 5, i8 5>
; CHECK-NEXT: [[DIV:%.*]] = sub nsw <2 x i8> zeroinitializer, [[DIV_NEG]]
; CHECK-NEXT: ret <2 x i8> [[DIV]]
;
%div = sdiv exact <2 x i8> %x, <i8 -32, i8 -32>
@ -34,7 +36,8 @@ define <2 x i8> @t2_vec_splat(<2 x i8> %x) {
define <2 x i8> @t3_vec(<2 x i8> %x) {
; CHECK-LABEL: @t3_vec(
; CHECK-NEXT: [[DIV:%.*]] = sdiv exact <2 x i8> [[X:%.*]], <i8 -32, i8 -16>
; CHECK-NEXT: [[DIV_NEG:%.*]] = ashr exact <2 x i8> [[X:%.*]], <i8 5, i8 4>
; CHECK-NEXT: [[DIV:%.*]] = sub <2 x i8> zeroinitializer, [[DIV_NEG]]
; CHECK-NEXT: ret <2 x i8> [[DIV]]
;
%div = sdiv exact <2 x i8> %x, <i8 -32, i8 -16>