On 12/18/18 3:16 PM, Bernd Edlinger wrote:
> Hi,
> 
> while I looked closely at the asm statement in the gdb,
> I realized that the SP clobber forces the function to use
> the frame pointer, and prevents the red zone.  That
> makes the push / pop sequence in the asm statement safe
> to use, as long as the stack is restored to the original
> value.  That can be a quite useful feature.  And that might
> have been the reason why the rsp clobber was chosen in the
> first place.
> 
> This seems to work for all targets, but it started to work
> this way with gcc-6, all versions before that do ignore
> this clobber stmt (as confirmed by godbolt).
> 
> The clobber stmt make the LRA register allocator switch
> frame_pointer_needed to 1, and therefore in all likelihood,
> all targets should use that consistently.
> 
> On 12/17/18 12:47 PM, Richard Sandiford wrote:
>> Dimitar Dimitrov <dimitar@dinux.eu> writes:
>>> On Sun, Dec 16 2018 at 14:36:26 EET Bernd Edlinger wrote:
>>>> Hi,
>>>>
>>>> if I understood that right, then clobbering sp is and has always been
>>>> ignored.
>>
>> PR77904 was about the clobber not being ignored, so the behaviour
>> hasn't been consistent.
>>
> 
> I think 77904 was a fall-out from the change in the LRA register allocator.
> The patch referenced in the PR does simply honor frame_pointer_needed,
> which changed with gcc-6, and caused a regression on arm.
> 
>> I'm also not sure it was always ignored in recent sources.  The clobber
>> does get added to the associated rtl insn, and it'd be surprising if
>> that never had an effect.
>>
>>>> If that is right, then I would much prefer a warning, that says exactly
>>>> that, because that would also help to understand why removing that clobber
>>>> statement is safe even for old gcc versions.
>>
>> If the asm does leave sp with a different value, then it's never been safe,
>> regardless of the gcc version.  That's why an error seems more appropriate.
>>
>>> Thank you. Looks like general consensus is to have a warning. See attached
>>> patch that switches the error to a warning.
>>
>> I don't think there's a good reason to treat this differently from the
>> preexisting PIC register error.  If the argument for making it a warning
>> rather than an error is that the asm might happen to work by accident,
>> then the same is true for the PIC register.
>>
> 
> In the light of my findings, I believe with a good warning message that
> explains that the SP needs to be restored to the previous value, that
> is a useful feature, that enables the asm statement to push temporary
> values on the stack which would not be safe otherwise.
> 
> Therefore I propose not to rip it out at this time.
> See my proposed patch.  What do you think?
> 
> Is it OK?
> 
> 

Oops, previous version missed the fix of the PR77904 test case, which is
currently broken too.


Bernd-