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Enhance the shift propagator to handle the case when you have:

Browse Source 
A = shl x, 42
...
B = lshr ..., 38

which can be transformed into:
A = shl x, 4
...

iff we can prove that the would-be-shifted-in bits
are already zero.  This eliminates two shifts in the testcase
and allows eliminate of the whole i128 chain in the real example.

llvm-svn: 112314
This commit is contained in:
Chris Lattner 2010-08-27 22:53:44 +00:00
parent eb68981283
commit 3f880c2097
2 changed files with 71 additions and 22 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

78
lib/Transforms/InstCombine/InstCombineShifts.cpp
View File

@ -115,7 +115,7 @@ static bool CanEvaluateShifted(Value *V, unsigned NumBits, bool isLeftShift,
return CanEvaluateShifted(I->getOperand(0), NumBits, isLeftShift, IC) &&
CanEvaluateShifted(I->getOperand(1), NumBits, isLeftShift, IC);
case Instruction::Shl:
case Instruction::Shl: {
// We can often fold the shift into shifts-by-a-constant.
CI = dyn_cast<ConstantInt>(I->getOperand(1));
if (CI == 0) return false;
@ -125,10 +125,21 @@ static bool CanEvaluateShifted(Value *V, unsigned NumBits, bool isLeftShift,
// We can always turn shl(c)+shr(c) -> and(c2).
if (CI->getValue() == NumBits) return true;
// We can always turn shl(c1)+shr(c2) -> shl(c3)+and(c4), but it isn't
unsigned TypeWidth = I->getType()->getScalarSizeInBits();
// We can turn shl(c1)+shr(c2) -> shl(c3)+and(c4), but it isn't
// profitable unless we know the and'd out bits are already zero.
if (CI->getZExtValue() > NumBits) {
unsigned HighBits = CI->getZExtValue() - NumBits;
if (MaskedValueIsZero(I->getOperand(0),
APInt::getHighBitsSet(TypeWidth, HighBits)))
return true;
}
return false;
case Instruction::LShr:
}
case Instruction::LShr: {
// We can often fold the shift into shifts-by-a-constant.
CI = dyn_cast<ConstantInt>(I->getOperand(1));
if (CI == 0) return false;
@ -139,10 +150,19 @@ static bool CanEvaluateShifted(Value *V, unsigned NumBits, bool isLeftShift,
// We can always turn lshr(c)+shl(c) -> and(c2).
if (CI->getValue() == NumBits) return true;
unsigned TypeWidth = I->getType()->getScalarSizeInBits();
// We can always turn lshr(c1)+shl(c2) -> lshr(c3)+and(c4), but it isn't
// profitable unless we know the and'd out bits are already zero.
return false;
if (CI->getZExtValue() > NumBits) {
unsigned LowBits = CI->getZExtValue() - NumBits;
if (MaskedValueIsZero(I->getOperand(0),
APInt::getLowBitsSet(TypeWidth, LowBits)))
return true;
}
return false;
}
case Instruction::Select: {
SelectInst *SI = cast<SelectInst>(I);
return CanEvaluateShifted(SI->getTrueValue(), NumBits, isLeftShift, IC) &&
@ -209,16 +229,23 @@ static Value *GetShiftedValue(Value *V, unsigned NumBits, bool isLeftShift,
// We turn shl(c)+lshr(c) -> and(c2) if the input doesn't already have
// zeros.
assert(CI->getValue() == NumBits);
APInt Mask(APInt::getLowBitsSet(TypeWidth, TypeWidth - NumBits));
V = IC.Builder->CreateAnd(I->getOperand(0),
ConstantInt::get(I->getContext(), Mask));
if (Instruction *VI = dyn_cast<Instruction>(V)) {
VI->moveBefore(I);
VI->takeName(I);
if (CI->getValue() == NumBits) {
APInt Mask(APInt::getLowBitsSet(TypeWidth, TypeWidth - NumBits));
V = IC.Builder->CreateAnd(I->getOperand(0),
ConstantInt::get(I->getContext(), Mask));
if (Instruction *VI = dyn_cast<Instruction>(V)) {
VI->moveBefore(I);
VI->takeName(I);
}
return V;
}
return V;
// We turn shl(c1)+shr(c2) -> shl(c3)+and(c4), but only when we know that
// the and won't be needed.
assert(CI->getZExtValue() > NumBits);
I->setOperand(1, ConstantInt::get(I->getType(),
CI->getZExtValue() - NumBits));
return I;
}
case Instruction::LShr: {
unsigned TypeWidth = I->getType()->getScalarSizeInBits();
@ -238,16 +265,23 @@ static Value *GetShiftedValue(Value *V, unsigned NumBits, bool isLeftShift,
// We turn lshr(c)+shl(c) -> and(c2) if the input doesn't already have
// zeros.
assert(CI->getValue() == NumBits);
APInt Mask(APInt::getHighBitsSet(TypeWidth, TypeWidth - NumBits));
V = IC.Builder->CreateAnd(I->getOperand(0),
ConstantInt::get(I->getContext(), Mask));
if (Instruction *VI = dyn_cast<Instruction>(V)) {
VI->moveBefore(I);
VI->takeName(I);
if (CI->getValue() == NumBits) {
APInt Mask(APInt::getHighBitsSet(TypeWidth, TypeWidth - NumBits));
V = IC.Builder->CreateAnd(I->getOperand(0),
ConstantInt::get(I->getContext(), Mask));
if (Instruction *VI = dyn_cast<Instruction>(V)) {
VI->moveBefore(I);
VI->takeName(I);
}
return V;
}
return V;
// We turn lshr(c1)+shl(c2) -> lshr(c3)+and(c4), but only when we know that
// the and won't be needed.
assert(CI->getZExtValue() > NumBits);
I->setOperand(1, ConstantInt::get(I->getType(),
CI->getZExtValue() - NumBits));
return I;
}
case Instruction::Select:

15
test/Transforms/InstCombine/shift.ll
View File

@ -425,3 +425,18 @@ entry:
; CHECK: ret i128 %ins
}
define i64 @test37(i128 %A, i32 %B) {
entry:
%tmp27 = shl i128 %A, 64
%tmp22 = zext i32 %B to i128
%tmp23 = shl i128 %tmp22, 96
%ins = or i128 %tmp23, %tmp27
%tmp45 = lshr i128 %ins, 64
%tmp46 = trunc i128 %tmp45 to i64
ret i64 %tmp46
; CHECK: %tmp23 = shl i128 %tmp22, 32
; CHECK: %ins = or i128 %tmp23, %A
; CHECK: %tmp46 = trunc i128 %ins to i64
}