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optimize a common idiom generated by clang for bitfield access, PR2638.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@54408 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2008-08-06 07:35:52 +00:00
parent bbe51362d5
commit 46bbad217b
2 changed files with 41 additions and 1 deletions
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32
lib/Transforms/Scalar/InstructionCombining.cpp
View File

@ -7666,7 +7666,37 @@ Instruction *InstCombiner::visitSExt(SExtInst &CI) {
return new TruncInst(Op, CI.getType(), "tmp");
}
}
// If the input is a shl/ashr pair of a same constant, then this is a sign
// extension from a smaller value. If we could trust arbitrary bitwidth
// integers, we could turn this into a truncate to the smaller bit and then
// use a sext for the whole extension. Since we don't, look deeper and check
// for a truncate. If the source and dest are the same type, eliminate the
// trunc and extend and just do shifts. For example, turn:
// %a = trunc i32 %i to i8
// %b = shl i8 %a, 6
// %c = ashr i8 %b, 6
// %d = sext i8 %c to i32
// into:
// %a = shl i32 %i, 30
// %d = ashr i32 %a, 30
Value *A = 0;
ConstantInt *BA = 0, *CA = 0;
if (match(Src, m_AShr(m_Shl(m_Value(A), m_ConstantInt(BA)),
m_ConstantInt(CA))) &&
BA == CA && isa<TruncInst>(A)) {
Value *I = cast<TruncInst>(A)->getOperand(0);
if (I->getType() == CI.getType()) {
unsigned MidSize = Src->getType()->getPrimitiveSizeInBits();
unsigned SrcDstSize = CI.getType()->getPrimitiveSizeInBits();
unsigned ShAmt = CA->getZExtValue()+SrcDstSize-MidSize;
Constant *ShAmtV = ConstantInt::get(CI.getType(), ShAmt);
I = InsertNewInstBefore(BinaryOperator::CreateShl(I, ShAmtV,
CI.getName()), CI);
return BinaryOperator::CreateAShr(I, ShAmtV);
}
}
return 0;
}

10
test/Transforms/InstCombine/sext-misc.ll
View File

@ -63,3 +63,13 @@ F:
ret i16 %W
}
; PR2638
define i32 @test2(i32 %i) nounwind {
entry:
%tmp12 = trunc i32 %i to i8 ; <i8> [#uses=1]
%tmp16 = shl i8 %tmp12, 6 ; <i8> [#uses=1]
%a = ashr i8 %tmp16, 6 ; <i8> [#uses=1]
%b = sext i8 %a to i32 ; <i32> [#uses=1]
ret i32 %b
}