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

[Jeff Law <law@redhat.com>]

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