Re: [PATCH] [RFC] PR target/52813 and target/11807

[Richard Sandiford <richard.sandiford@arm.com>]

Christophe Lyon <christophe.lyon@linaro.org> writes:
> On Fri, 11 Jan 2019 at 23:59, Jeff Law <law@redhat.com> wrote:
>>
>> On 1/8/19 5:03 AM, Richard Sandiford wrote:
>> > Bernd Edlinger <bernd.edlinger@hotmail.de> writes:
>> >> On 1/7/19 10:23 AM, Jakub Jelinek wrote:
>> >>> On Sun, Dec 16, 2018 at 06:13:57PM +0200, Dimitar Dimitrov wrote:
>> >>>> -  /* Clobbering the STACK POINTER register is an error.  */
>> >>>> +  /* Clobbered STACK POINTER register is not saved/restored by GCC,
>> >>>> +     which is often unexpected by users.  See PR52813.  */
>> >>>>    if (overlaps_hard_reg_set_p (regset, Pmode, STACK_POINTER_REGNUM))
>> >>>>      {
>> >>>> -      error ("Stack Pointer register clobbered by %qs in %<asm%>", regname);
>> >>>> +      warning (0, "Stack Pointer register clobbered by %qs in %<asm%>",
>> >>>> +         regname);
>> >>>> +      warning (0, "GCC has always ignored Stack Pointer %<asm%> clobbers");
>> >>>
>> >>> Why do we write Stack Pointer rather than stack pointer?  That is really
>> >>> weird.  The second warning would be a note based on the first one, i.e.
>> >>> if (warning ()) note ();
>> >>> and better have some -W* option to silence the warning.
>> >>>
>> >>
>> >> Yes, thanks for this suggestion.
>> >>
>> >> Meanwhile I found out, that the stack clobber has only been ignored up to
>> >> gcc-5 (at least with lra targets, not really sure about reload targets).
>> >> From gcc-6 on, with the exception of PR arm/77904 which was a regression due
>> >> to the underlying lra change, but fixed later, and back-ported to gcc-6.3.0,
>> >> this works for all targets I tried so far.
>> >>
>> >> To me, it starts to look like a rather unique and useful feature, that I would
>> >> like to keep working.
>> >
>> > Not sure what you mean by "unique".  But forcing a frame is a bit of
>> > a slippery concept.  Force it where?  For the asm only, or the whole
>> > function?  This depends on optimisation and hasn't been consistent
>> > across GCC versions, since it depends on the shrink-wrapping
>> > optimisation.  (There was a similar controversy a while ago about
>> > to what extent -fno-omit-frame-pointer should "force a frame".)
>> >
>> > The effect on the redzone seems like something that should be specified
>> > explicitly rather than as an (accidental?) side effect of listing the
>> > sp in the clobber list.  Maybe this would be another use for the "asm
>> > attributes" proposal.  "noreturn" was another attribute suggested on
>> > IRC yesterday.
>> >
>> > But either way, the general feeling seems to be that going straight to a
>> > hard error is too harsh, since there's quite a bit of existing code that
>> > has the clobber.  This patch implements the compromise discussed on IRC
>> > yesterday of making it a -Wdeprecated warning instead.
>> >
>> > Tested on x86_64-linux-gnu and aarch64-linux-gnu.  OK to install?
>> >
>> > Richard
>> >
>> > Dimitar: sorry the run-around on this patch, and thanks for the
>> > submission.  It looks from all the controversy like it was a
>> > long-festering PR for a reason. :-/
>> >
>> >
>> > 2019-01-07  Richard Sandiford  <richard.sandiford@arm.com>
>> >
>> > gcc/
>> >       PR inline-asm/52813
>> >       * doc/extend.texi: Document that listing the stack pointer in the
>> >       clobber list of an asm is a deprecated feature.
>> >       * common.opt (Wdeprecated): Moved from c-family/c.opt.
>> >       * cfgexpand.c (asm_clobber_reg_is_valid): Issue a -Wdeprecated
>> >       warning instead of an error for clobbers of the stack pointer.
>> >       Add a note explaining why.
>> >
>> > gcc/c-family/
>> >       PR inline-asm/52813
>> >       * c.opt (Wdeprecated): Move documentation and variable to common.opt.
>> >
>> > gcc/d/
>> >       PR inline-asm/52813
>> >       * lang.opt (Wdeprecated): Reference common.opt instead of c.opt.
>> >
>> > gcc/testsuite/
>> >       PR inline-asm/52813
>> >       * gcc.target/i386/pr52813.c (test1): Turn the diagnostic into a
>> >       -Wdeprecated warning and expect a following note:.
>> OK.
>>
>> FWIW the number of packages affected in Fedora was in single digits,
>> some of which have already been fixed.
>>
>> But if folks want to go with a deprecated warning instead of straight to
>> an error, I won't complain.
>>
>> jeff
>
>
> Hi,
>
> I originally complained because the arm test for pr77904.c was failing.
> Since Richard's change that test emits a warning rather than an error,
> but still fails. This small patch adds the missing dg-warning.
>
> OK?
>
> Thanks,
>
> Christophe
>
> 2019-01-17  Christophe Lyon  <christophe.lyon@linaro.org>
>
> 	* gcc.target/arm/pr77904.c: Add dg-warning for sp clobber.

OK, thanks.

Richard