Aliasing problem? gcc-3.3 miscompiles mozilla-1.4

[Franz Sirl]

On Wednesday 18 June 2003 20:48, Franz Sirl wrote:
> 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.

I just found out that in addition to -O2, -fno-exceptions is a prerequisite to 
show the bug.

Franz.