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Fix a miscompile in the DAG combiner. Previously, we would incorrectly

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try to reduce the width of this load, and would end up transforming:

  (truncate (lshr (sextload i48 <ptr> as i64), 32) to i32)
to
  (truncate (zextload i32 <ptr+4> as i64) to i32)

We lost the sext attached to the load while building the narrower i32
load, and replaced it with a zext because lshr always zext's the
results. Instead, bail out of this combine when there is a conflict
between a sextload and a zext narrowing. The rest of the DAG combiner
still optimize the code down to the proper single instruction:

  movswl 6(...),%eax

Which is exactly what we wanted. Previously we read past the end *and*
missed the sign extension:

  movl 6(...), %eax

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@169802 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chandler Carruth 2012-12-11 00:36:57 +00:00
parent 0a0990af1c
commit 1c49fda408
2 changed files with 29 additions and 4 deletions
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8
lib/CodeGen/SelectionDAG/DAGCombiner.cpp
View File

@ -5068,11 +5068,15 @@ SDValue DAGCombiner::ReduceLoadWidth(SDNode *N) {
// At this point, we must have a load or else we can't do the transform.
if (!isa<LoadSDNode>(N0)) return SDValue();
// Because a SRL must be assumed to *need* to zero-extend the high bits
// (as opposed to anyext the high bits), we can't combine the zextload
// lowering of SRL and an sextload.
if (cast<LoadSDNode>(N0)->getExtensionType() == ISD::SEXTLOAD)
return SDValue();
// If the shift amount is larger than the input type then we're not
// accessing any of the loaded bytes. If the load was a zextload/extload
// then the result of the shift+trunc is zero/undef (handled elsewhere).
// If the load was a sextload then the result is a splat of the sign bit
// of the extended byte. This is not worth optimizing for.
if (ShAmt >= cast<LoadSDNode>(N0)->getMemoryVT().getSizeInBits())
return SDValue();
}

25
test/CodeGen/X86/sext-load.ll
View File

@ -1,9 +1,30 @@
; RUN: llc < %s -march=x86 | grep movsbl
; RUN: llc < %s -march=x86 | FileCheck %s
define i32 @foo(i32 %X) nounwind {
; When doing sign extension, use the sext-load lowering to take advantage of
; x86's sign extension during loads.
;
; CHECK: test1:
; CHECK: movsbl {{.*}}, %eax
; CHECK-NEXT: ret
define i32 @test1(i32 %X) nounwind {
entry:
%tmp12 = trunc i32 %X to i8 ; <i8> [#uses=1]
%tmp123 = sext i8 %tmp12 to i32 ; <i32> [#uses=1]
ret i32 %tmp123
}
; When using a sextload representation, ensure that the sign extension is
; preserved even when removing shifted-out low bits.
;
; CHECK: test2:
; CHECK: movswl {{.*}}, %eax
; CHECK-NEXT: ret
define i32 @test2({i16, [6 x i8]}* %this) {
entry:
%b48 = getelementptr inbounds { i16, [6 x i8] }* %this, i32 0, i32 1
%cast = bitcast [6 x i8]* %b48 to i48*
%bf.load = load i48* %cast, align 2
%bf.ashr = ashr i48 %bf.load, 32
%bf.cast = trunc i48 %bf.ashr to i32
ret i32 %bf.cast
}