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(1 - X) * (-2) -> (x - 1) * 2, for all positive nonzero powers of 2

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rdar://problem/6501862


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@132316 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Stuart Hastings 2011-05-30 20:00:33 +00:00
parent 67dc11352d
commit acbf107d9b
2 changed files with 41 additions and 0 deletions
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17
lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
View File

@ -135,6 +135,23 @@ Instruction *InstCombiner::visitMul(BinaryOperator &I) {
return BinaryOperator::CreateAdd(Add, Builder->CreateMul(C1, CI)); return BinaryOperator::CreateAdd(Add, Builder->CreateMul(C1, CI));
} }
} }
// (1 - X) * (-2) -> (x - 1) * 2, for all positive nonzero powers of 2
// The "* 2" thus becomes a potential shifting opportunity.
{
const APInt & Val = CI->getValue();
const APInt &PosVal = Val.abs();
if (Val.isNegative() && PosVal.isPowerOf2()) {
Value *X = 0;
if (match(Op0, m_Sub(m_One(), m_Value(X)))) {
// ConstantInt::get(Op0->getType(), 2);
Value *Sub = Builder->CreateSub(X, ConstantInt::get(X->getType(), 1),
"dec1");
return BinaryOperator::CreateMul(Sub, ConstantInt::get(X->getType(),
PosVal));
}
}
}
} }
// Simplify mul instructions with a constant RHS. // Simplify mul instructions with a constant RHS.

24
test/Transforms/InstCombine/2011-05-28-swapmulsub.ll Normal file
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@ -0,0 +1,24 @@
; ModuleID = 'test1.c'
; RUN: opt -S -instcombine < %s | FileCheck %s
target triple = "x86_64-apple-macosx10.6.6"
define zeroext i16 @foo(i32 %on_off, i16* %puls) nounwind uwtable ssp {
entry:
%on_off.addr = alloca i32, align 4
%puls.addr = alloca i16*, align 8
%a = alloca i32, align 4
store i32 %on_off, i32* %on_off.addr, align 4
store i16* %puls, i16** %puls.addr, align 8
%tmp = load i32* %on_off.addr, align 4
; CHECK-NOT: sub
; CHECK-NOT: mul
; (1 - %tmp) * (-2) -> (%tmp - 1) * 2
%sub = sub i32 1, %tmp
%mul = mul i32 %sub, -2
; CHECK: shl
; CHECK-NEXT: add
store i32 %mul, i32* %a, align 4
%tmp1 = load i32* %a, align 4
%conv = trunc i32 %tmp1 to i16
ret i16 %conv
}