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change the preferred canonical form for a sign extension to be

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lshr+ashr instead of trunc+sext.  We want to avoid type 
conversions whenever possible, it is easier to codegen expressions
without truncates and extensions.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@93107 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2010-01-10 07:08:30 +00:00
parent abff82d99e
commit f4fb91181c
3 changed files with 8 additions and 30 deletions
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11
lib/Transforms/InstCombine/InstCombineCasts.cpp
View File

@ -680,6 +680,8 @@ Instruction *InstCombiner::visitZExt(ZExtInst &CI) {
// types and if the sizes are just right we can convert this into a logical
// 'and' which will be much cheaper than the pair of casts.
if (TruncInst *CSrc = dyn_cast<TruncInst>(Src)) { // A->B->C cast
// TODO: Subsume this into EvaluateInDifferentType.
// Get the sizes of the types involved. We know that the intermediate type
// will be smaller than A or C, but don't know the relation between A and C.
Value *A = CSrc->getOperand(0);
@ -707,7 +709,7 @@ Instruction *InstCombiner::visitZExt(ZExtInst &CI) {
APInt AndValue(APInt::getLowBitsSet(DstSize, MidSize));
return BinaryOperator::CreateAnd(Trunc,
ConstantInt::get(Trunc->getType(),
AndValue));
AndValue));
}
}
@ -927,6 +929,7 @@ Instruction *InstCombiner::visitSExt(SExtInst &CI) {
// always do the transformation.
if (NumCastsRemoved >= 2 ||
NumBitsSExt > DestBitSize-SrcBitSize) {
// Okay, we can transform this! Insert the new expression now.
DEBUG(dbgs() << "ICE: EvaluateInDifferentType converting expression type"
" to avoid sign extend: " << CI);
@ -939,8 +942,10 @@ Instruction *InstCombiner::visitSExt(SExtInst &CI) {
ComputeNumSignBits(Res) > DestBitSize - SrcBitSize)
return ReplaceInstUsesWith(CI, Res);
// We need to emit a cast to truncate, then a cast to sext.
return new SExtInst(Builder->CreateTrunc(Res, Src->getType()), DestTy);
// We need to emit a shl + ashr to do the sign extend.
Value *ShAmt = ConstantInt::get(DestTy, DestBitSize-SrcBitSize);
return BinaryOperator::CreateAShr(Builder->CreateShl(Res, ShAmt, "sext"),
ShAmt);
}
}

20
lib/Transforms/InstCombine/InstCombineShifts.cpp
View File

@ -325,26 +325,6 @@ Instruction *InstCombiner::FoldShiftByConstant(Value *Op0, ConstantInt *Op1,
return BinaryOperator::CreateAnd(X,
ConstantInt::get(I.getContext(), Mask));
}
// We can simplify ((X << C) >>s C) into a trunc + sext.
// NOTE: we could do this for any C, but that would make 'unusual' integer
// types. For now, just stick to ones well-supported by the code
// generators.
const Type *SExtType = 0;
switch (Ty->getBitWidth() - ShiftAmt1) {
case 1 :
case 8 :
case 16 :
case 32 :
case 64 :
case 128:
SExtType = IntegerType::get(I.getContext(),
Ty->getBitWidth() - ShiftAmt1);
break;
default: break;
}
if (SExtType)
return new SExtInst(Builder->CreateTrunc(X, SExtType, "sext"), Ty);
// Otherwise, we can't handle it yet.
} else if (ShiftAmt1 < ShiftAmt2) {
uint32_t ShiftDiff = ShiftAmt2-ShiftAmt1;

7
test/Transforms/InstCombine/apint-shift.ll
View File

@ -168,13 +168,6 @@ define i11 @test23(i44 %A) {
ret i11 %D
}
define i17 @test24(i17 %X) {
%Y = and i17 %X, -5 ; <i17> [#uses=1]
%Z = shl i17 %Y, 9 ; <i17> [#uses=1]
%Q = ashr i17 %Z, 9 ; <i17> [#uses=1]
ret i17 %Q
}
define i37 @test25(i37 %tmp.2, i37 %AA) {
%x = lshr i37 %AA, 17 ; <i37> [#uses=1]
%tmp.3 = lshr i37 %tmp.2, 17 ; <i37> [#uses=1]