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now that the cost model has changed, we can always consider

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elimination of a sign extend to be a win, which simplifies 
the client of CanEvaluateSExtd, and allows us to eliminate
more casts (examples taken from real code).

llvm-svn: 93109
This commit is contained in:
Chris Lattner 2010-01-10 07:40:50 +00:00
parent f4272b50d6
commit e68d6e61b1
2 changed files with 60 additions and 27 deletions
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41
lib/Transforms/InstCombine/InstCombineCasts.cpp
View File

@ -919,34 +919,25 @@ Instruction *InstCombiner::visitSExt(SExtInst &CI) {
if (NumBitsSExt == 0)
return 0;
// Okay, we can transform this! Insert the new expression now.
DEBUG(dbgs() << "ICE: EvaluateInDifferentType converting expression type"
" to avoid sign extend: " << CI);
Value *Res = EvaluateInDifferentType(Src, DestTy, true);
assert(Res->getType() == DestTy);
uint32_t SrcBitSize = SrcTy->getScalarSizeInBits();
uint32_t DestBitSize = DestTy->getScalarSizeInBits();
// If the high bits are already filled with sign bit, just replace this
// cast with the result.
if (NumBitsSExt > DestBitSize - SrcBitSize ||
ComputeNumSignBits(Res) > DestBitSize - SrcBitSize)
return ReplaceInstUsesWith(CI, Res);
// Because this is a sign extension, we can always transform it by inserting
// two new shifts (to do the extension). However, this is only profitable
// if we've eliminated two or more casts from the input. If we know the
// result will be sign-extended enough to not require these shifts, we can
// 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);
Value *Res = EvaluateInDifferentType(Src, DestTy, true);
assert(Res->getType() == DestTy);
// If the high bits are already filled with sign bit, just replace this
// cast with the result.
if (NumBitsSExt > DestBitSize - SrcBitSize ||
ComputeNumSignBits(Res) > DestBitSize - SrcBitSize)
return ReplaceInstUsesWith(CI, Res);
// 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);
}
// 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);
}
// If the input is a shl/ashr pair of a same constant, then this is a sign

46
test/Transforms/InstCombine/cast.ll
View File

@ -185,8 +185,8 @@ define i32 @test22(i32 %X) {
%c2 = sext i8 %c1 to i32 ; <i32> [#uses=1]
%RV = shl i32 %c2, 24 ; <i32> [#uses=1]
ret i32 %RV
; CHECK: %RV = shl i32 %X, 24
; CHECK: ret i32 %RV
; CHECK: shl i32 %X, 24
; CHECK-NEXT: ret i32
}
define i32 @test23(i32 %X) {
@ -457,3 +457,45 @@ define i64 @test48(i8 %A, i8 %a) {
; CHECK-NEXT: ret i64 %D
}
define i64 @test49(i64 %A) {
%B = trunc i64 %A to i32
%C = or i32 %B, 1
%D = sext i32 %C to i64
ret i64 %D
; CHECK: @test49
; CHECK-NEXT: %C = shl i64 %A, 32
; CHECK-NEXT: ashr i64 %C, 32
; CHECK-NEXT: %D = or i64 {{.*}}, 1
; CHECK-NEXT: ret i64 %D
}
define i64 @test50(i64 %A) {
%a = lshr i64 %A, 2
%B = trunc i64 %a to i32
%D = add i32 %B, -1
%E = sext i32 %D to i64
ret i64 %E
; CHECK: @test50
; CHECK-NEXT: shl i64 %A, 30
; CHECK-NEXT: add i64 {{.*}}, -4294967296
; CHECK-NEXT: %E = ashr i64 {{.*}}, 32
; CHECK-NEXT: ret i64 %E
}
define i64 @test51(i64 %A, i1 %cond) {
%B = trunc i64 %A to i32
%C = and i32 %B, -2
%D = or i32 %B, 1
%E = select i1 %cond, i32 %C, i32 %D
%F = sext i32 %E to i64
ret i64 %F
; CHECK: @test51
; CHECK-NEXT: %C = and i64 %A, 4294967294
; CHECK-NEXT: %D = or i64 %A, 1
; CHECK-NEXT: %E = select i1 %cond, i64 %C, i64 %D
; CHECK-NEXT: %sext = shl i64 %E, 32
; CHECK-NEXT: %F = ashr i64 %sext, 32
; CHECK-NEXT: ret i64 %F
}