[llvm] r216523 - InstCombine: Optimize GEP's involving ptrtointbetter

[David Majnemer]

On Thu, Aug 28, 2014 at 3:44 PM, Renato Golin <renato.golin at linaro.org>
wrote:

> So, the problem is indeed the transformation, as it changes:
>
>    0x0002d6c8 <+276>: mov r4, r7
>    0x0002d6cc <+280>: ldr r0, [r4, #4]
>    0x0002d6d0 <+284>: ldr r1, [r8]
>    0x0002d6d4 <+288>: rsb r0, r0, #0
>    0x0002d6d8 <+292>: asr r0, r0, r6
>    0x0002d6dc <+296>: ldr r0, [r1, r0, lsl #2]
>    0x0002d6e0 <+300>: ldr r1, [r5]
>    0x0002d6e4 <+304>: ldr r0, [r0]
>
> into
>
>    0x0002d61c <+264>: mov r4, r5
>    0x0002d620 <+268>: ldr r0, [r4, #4]
>    0x0002d624 <+272>: ldr r1, [r8]
>    0x0002d628 <+276>: ldr r0, [r1, -r0]
>    0x0002d62c <+280>: ldr r1, [r7]
>    0x0002d630 <+284>: ldr r0, [r0]
>
> Note the -r0 versus (^r0>>r6)<<2, only that r0 is already negative
> when it gets to the load with offset. That's from the line:
>
>   fputs(S->name, stdout)
>
> with r0 as S->name, and r0 being 0x0 at the final load.
>
> Given that this transformation has also broken MIPS bots, I think we
> should revert it for now until better investigations are done.
>

I'd be all for reverting it if we had LLVM IR that we can commit with the
revert which shows the problem.
Better yet, I could fix the bug if I had the problematic IR.


>
> cheers,
> --renato
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20140828/d85ad664/attachment.html>