Re: [PATCH][expr.c] PR 65358 Avoid clobbering partial argument during sibcall

[Kyrill Tkachov <kyrylo dot tkachov at arm dot com>]

Ping.
https://gcc.gnu.org/ml/gcc-patches/2015-03/msg01014.html
Thanks,
Kyrill

On 19/03/15 14:39, Kyrill Tkachov wrote:
> Hi all,
>
> This patch fixes PR 65358. For details look at the excellent write-up
> by Honggyu in bugzilla. The problem is that we're trying to pass a struct
> partially on the stack and partially in regs during a tail-call optimisation
> but the struct we're passing is also a partial incoming arg though the split
> between stack and regs is different from its outgoing usage.
>
> The emit_push_insn code ends up doing a block move for the on-stack part but
> ends up overwriting the part that needs to be loaded into regs.
> My first thought was to just load the regs part first and then do the stack
> part but that doesn't work as multiple comments in that function indicate
> (the block move being expanded to movmem or other functions being one of the
> reasons).
>
> My proposed solution is to detect when the overlap happens, find the
> overlapping region and load it before the stack pushing into pseudos and
> after the stack pushing is done move the overlapping values from the pseudos
> into the hard argument regs that they're supposed to go.
>
> That way this new functionality should only ever be triggered when there's
> the overlap in this PR (causing wrong-code) and shouldn't affect codegen
> anywhere else.
>
> Bootstrapped and tested on arm-none-linux-gnueabihf, aarch64-none-linux-gnu
> and x86_64-linux-gnu.
>
> According to the PR this appears at least as far back 4.6 so this isn't a
> regression on the release branches, but it is a wrong-code bug.
>
> I'll let Honggyu upstream the testcase separately
> (https://gcc.gnu.org/ml/gcc-patches/2015-03/msg00984.html)
After guidance from Jeff, I'll take this testcase in as well if this is 
approved.

> I'll be testing this on the 4.8 and 4.9 branches.
> Thoughts on this approach?
>
> Thanks,
> Kyrill
>
> 2015-03-19  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>
>
>       PR middle-end/65358
>       * expr.c (memory_load_overlap): New function.
>       (emit_push_insn): When pushing partial args to the stack would
>       clobber the register part load the overlapping part into a pseudo
>       and put it into the hard reg after pushing.