Aliasing problem? gcc-3.3 miscompiles mozilla-1.4

[Franz Sirl <Franz dot Sirl-kernel at lauterbach dot com>]

Hi,

this little routine is miscompiled with -O2 with gcc-3.3 on ppc-linux:

nsresult imgContainerGIF::DecodingComplete(void)
{
  mDoneDecoding = 1;


  PRUint32 numFrames = inlinedGetNumFrames();
  if (numFrames == 1) {
    nsCOMPtr<gfxIImageFrame> currentFrame;
    inlinedGetFrameAt(0, getter_AddRefs(currentFrame));
    currentFrame->SetMutable(0);
  }
  return 0;
}

This is the assembly:

_ZN15imgContainerGIF16DecodingCompleteEv:
        stwu %r1,-48(%r1)
        mflr %r0
        stw %r0,52(%r1)
        li %r0,1
        stw %r0,104(%r3)         #  <variable>.mDoneDecoding
        stw %r31,44(%r1)
        addi %r31,%r3,16         #  this,  this
        lwz %r0,16(%r31)         #  <variable>.mCount
        stw %r30,40(%r1)
        addi %r30,%r1,16         #  this
        cmpwi %cr0,%r0,1
        stw %r29,36(%r1)
        mr %r3,%r30      #  this
        beq- %cr0,.L202
.L188:
        lwz %r0,52(%r1)
        li %r3,0
        lwz %r29,36(%r1)
        lwz %r30,40(%r1)
        mtlr %r0
        lwz %r31,44(%r1)
        addi %r1,%r1,48
        blr
.L202:
        li %r0,0         #  rawPtr
        stw %r0,16(%r1)  #  <variable>.mRawPtr,  rawPtr
        bl _ZN13nsCOMPtr_base16begin_assignmentEv@plt
        mr %r29,%r3      #  <anonymous>
        li %r4,0
        mr %r3,%r31      #  this
        bl _ZN15nsSupportsArray9ElementAtEj@plt
        lwz %r9,16(%r1)  #  <variable>.mRawPtr,  <anonymous> XXXXX
        cmpwi %cr0,%r3,0         #  _elem
        mr %r0,%r3       #  _elem
        li %r4,0
        mr %r3,%r9       #  <anonymous>
        beq- %cr0,.L196
        stw %r0,0(%r29)  # * <anonymous>,  _elem XXXXX
.L196:
        lwz %r9,0(%r9)   #  <variable>._vptr.nsISupports
        lwz %r9,20(%r9)
        mtctr %r9
        bctrl
        mr %r3,%r30      #  this
        bl _ZN13nsCOMPtr_baseD2Ev@plt
        b .L188


The problem are the marked lines, they are swapped ( 0(%r29) and 16(%r1) point 
to the same stack slot, r29 is the return value of 
_ZN13nsCOMPtr_base16begin_assignmentEv). Compiling with -O1 or 
-fno-strict-aliasing lets the code work.

I've attached the preprocessed source of the miscompiled file.

Franz.

Attachment: imgContainerGIF.ii.bz2
Description: BZip2 compressed data