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DAGCombine: Make sure we only eliminate trunc/extend when the scales of truncation and extension match.

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This fixes PR33368.

Reviewer: rksimon

Differential Revision:  https://reviews.llvm.org/D34069

llvm-svn: 306345
This commit is contained in:
Wolfgang Pieb 2017-06-26 23:05:51 +00:00
parent 57e415b733
commit 038c037dfc
2 changed files with 44 additions and 5 deletions
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14
lib/CodeGen/SelectionDAG/DAGCombiner.cpp
View File

@ -15013,6 +15013,11 @@ static SDValue combineTruncationShuffle(ShuffleVectorSDNode *SVN,
unsigned NumElts = VT.getVectorNumElements();
unsigned EltSizeInBits = VT.getScalarSizeInBits();
unsigned ExtSrcSizeInBits = N00.getScalarValueSizeInBits();
unsigned ExtDstSizeInBits = N0.getScalarValueSizeInBits();
if (ExtDstSizeInBits % ExtSrcSizeInBits != 0)
return SDValue();
unsigned ExtScale = ExtDstSizeInBits / ExtSrcSizeInBits;
// (v4i32 truncate_vector_inreg(v2i64)) == shuffle<0,2-1,-1>
// (v8i16 truncate_vector_inreg(v4i32)) == shuffle<0,2,4,6,-1,-1,-1,-1>
@ -15034,11 +15039,10 @@ static SDValue combineTruncationShuffle(ShuffleVectorSDNode *SVN,
if (EltSizeInBits != ExtSrcSizeInBits)
return SDValue();
// Attempt to match a 'truncate_vector_inreg' shuffle, we just search for
// power-of-2 truncations as they are the most likely.
for (unsigned Scale = 2; Scale < NumElts; Scale *= 2)
if (isTruncate(Scale))
return DAG.getBitcast(VT, N00);
// We can remove *extend_vector_inreg only if the truncation happens at
// the same scale as the extension.
if (isTruncate(ExtScale))
return DAG.getBitcast(VT, N00);
return SDValue();
}

35
test/CodeGen/X86/vector-truncate-combine.ll Normal file
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@ -0,0 +1,35 @@
; RUN: llc -mtriple=x86_64-- -O2 -start-after=stack-protector -stop-before=loops %s -o - | FileCheck %s
; This test verifies the fix for PR33368.
;
; The expected outcome of the operation is to store bytes 0 and 2 of the incoming
; parameter into c2 (a 2 x i8 vector). DAGCombine converts shuffles into a
; sequence of extend and subsequent truncate operations. The bug was that an extension
; by 4 followed by a truncation by 8 was completely eliminated.
; The test checks for the correct sequence of operations that results from the
; preservation of the extend/truncate operations mentioned above (2 extend and
; 3 truncate instructions).
;
; NOTE: This operation could be collapsed in to a single truncate. Once that is done
; this test will have to be adjusted.
; CHECK: PUNPCKLBWrr
; CHECK: PUNPCKLWDrr
; CHECK: PACKUSWBrr
; CHECK: PACKUSWBrr
; CHECK: PACKUSWBrr
define void @test(double %vec.coerce) local_unnamed_addr {
entry:
%c2 = alloca <2 x i8>, align 2
%0 = bitcast double %vec.coerce to <8 x i8>
%1 = shufflevector <8 x i8> %0, <8 x i8> undef, <4 x i32> <i32 2, i32 undef, i32 undef, i32 0>
%2 = shufflevector <4 x i8> %1, <4 x i8> undef, <2 x i32> <i32 3, i32 0>
store volatile <2 x i8> %2, <2 x i8>* %c2, align 2
br label %if.end
if.end:
%3 = bitcast <2 x i8> %2 to i16
ret void
}