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InstCombine: Reduce "X shift (A srem B)" to "X shift (A urem B)" iff B is positive.

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This allows to transform the rem in "1 << ((int)x % 8);" to an and.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@120028 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Benjamin Kramer 2010-11-23 18:52:42 +00:00
parent e104f1bccb
commit b70ebd2aa3
3 changed files with 28 additions and 10 deletions
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12
lib/Target/README.txt
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@ -1698,22 +1698,14 @@ from gcc.
//===---------------------------------------------------------------------===//
Missed instcombine transformation:
define i32 @a(i32 %x) nounwind readnone {
entry:
%rem = srem i32 %x, 32
%shl = shl i32 1, %rem
ret i32 %shl
}
The srem can be transformed to an and because if x is negative, the shift is
undefined. Here's a more general case that occurs on 64-bit targets:
%382 = srem i32 %tmp14.i, 64 ; [#uses=1]
%383 = zext i32 %382 to i64 ; [#uses=1]
%384 = shl i64 %381, %383 ; [#uses=1]
%385 = icmp slt i32 %tmp14.i, 64 ; [#uses=1]
Testcase derived from 403.gcc.
The srem can be transformed to an and because if x is negative, the shift is
undefined. Testcase derived from 403.gcc.
//===---------------------------------------------------------------------===//

15
lib/Transforms/InstCombine/InstCombineShifts.cpp
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@ -53,6 +53,21 @@ Instruction *InstCombiner::commonShiftTransforms(BinaryOperator &I) {
if (ConstantInt *CUI = dyn_cast<ConstantInt>(Op1))
if (Instruction *Res = FoldShiftByConstant(Op0, CUI, I))
return Res;
// X shift (A srem B) -> X shift (A urem B) iff B is positive.
// Because shifts by negative values are undefined.
if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Op1))
if (BO->getOpcode() == Instruction::SRem && BO->getType()->isIntegerTy()) {
// Make sure the divisor's sign bit is zero.
APInt Mask = APInt::getSignBit(BO->getType()->getPrimitiveSizeInBits());
if (MaskedValueIsZero(BO->getOperand(1), Mask)) {
Value *URem = Builder->CreateURem(BO->getOperand(0), BO->getOperand(1),
BO->getName());
I.setOperand(1, URem);
return &I;
}
}
return 0;
}

11
test/Transforms/InstCombine/shift.ll
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@ -441,3 +441,14 @@ entry:
; CHECK: %ins = or i128 %tmp23, %A
; CHECK: %tmp46 = trunc i128 %ins to i64
}
define i32 @test38(i32 %x) nounwind readnone {
entry:
%rem = srem i32 %x, 32
%shl = shl i32 1, %rem
ret i32 %shl
; CHECK: @test38
; CHECK-NOT: srem
; CHECK: ret i32
}