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Make use of the exact bit when optimizing '(X >>exact 3) << 1' to eliminate the 

'and' that would zero out the trailing bits, and to produce an exact shift
ourselves.

llvm-svn: 147391
This commit is contained in:
Nick Lewycky 2011-12-31 21:30:22 +00:00
parent 1c65a21ec4
commit b59008c694
2 changed files with 29 additions and 5 deletions
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16
llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
View File

@ -536,12 +536,11 @@ Instruction *InstCombiner::FoldShiftByConstant(Value *Op0, ConstantInt *Op1,
if (ShiftAmt1 == 0) return 0; // Will be simplified in the future.
Value *X = ShiftOp->getOperand(0);
uint32_t AmtSum = ShiftAmt1+ShiftAmt2; // Fold into one big shift.
IntegerType *Ty = cast<IntegerType>(I.getType());
// Check for (X << c1) << c2 and (X >> c1) >> c2
if (I.getOpcode() == ShiftOp->getOpcode()) {
uint32_t AmtSum = ShiftAmt1+ShiftAmt2; // Fold into one big shift.
// If this is oversized composite shift, then unsigned shifts get 0, ashr
// saturates.
if (AmtSum >= TypeBits) {
@ -603,9 +602,16 @@ Instruction *InstCombiner::FoldShiftByConstant(Value *Op0, ConstantInt *Op1,
// (X >>? C1) << C2 --> X >>? (C1-C2) & (-1 << C2)
if (I.getOpcode() == Instruction::Shl &&
ShiftOp->getOpcode() != Instruction::Shl) {
Value *Shift = Builder->CreateBinOp(ShiftOp->getOpcode(), X,
ConstantInt::get(Ty, ShiftDiff));
ConstantInt *ShiftDiffCst = ConstantInt::get(Ty, ShiftDiff);
if (ShiftOp->isExact()) {
// (X >>?exact C1) << C2 --> X >>?exact (C1-C2)
BinaryOperator *NewShr = BinaryOperator::Create(ShiftOp->getOpcode(),
X, ShiftDiffCst);
NewShr->setIsExact(true);
return NewShr;
}
Value *Shift = Builder->CreateBinOp(ShiftOp->getOpcode(),
X, ShiftDiffCst);
APInt Mask(APInt::getHighBitsSet(TypeBits, TypeBits - ShiftAmt2));
return BinaryOperator::CreateAnd(Shift,
ConstantInt::get(I.getContext(),Mask));

18
llvm/test/Transforms/InstCombine/shift.ll
View File

@ -542,3 +542,21 @@ define i32 @test45(i32 %a) nounwind {
; CHECK-NEXT: %y = lshr i32 %a, 5
; CHECK-NEXT: ret i32 %y
}
define i32 @test46(i32 %a) {
%y = ashr exact i32 %a, 3
%z = shl i32 %y, 1
ret i32 %z
; CHECK: @test46
; CHECK-NEXT: %z = ashr exact i32 %a, 2
; CHECK-NEXT: ret i32 %z
}
define i32 @test47(i32 %a) {
%y = lshr exact i32 %a, 3
%z = shl i32 %y, 1
ret i32 %z
; CHECK: @test47
; CHECK-NEXT: %z = lshr exact i32 %a, 2
; CHECK-NEXT: ret i32 %z
}