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

[Bernd Edlinger <bernd.edlinger@hotmail.de>]

On 12/17/18 2:42 PM, Christophe Lyon wrote:
> On Mon, 17 Dec 2018 at 14:35, Richard Sandiford
> <richard.sandiford@arm.com> wrote:
>>
>> Christophe Lyon <christophe.lyon@linaro.org> writes:
>>> On Mon, 17 Dec 2018 at 12:47, Richard Sandiford
>>> <richard.sandiford@arm.com> 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'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.
>>>>
>>>
>>> If we leave the error, maybe a more explanatory message would be helpful?
>>> (along the lines of what I posted earlier in this thread, which may be
>>> too verbose)
>>
>> The message in that patch suggested removing the clobber and hoping for
>> the best, which IMO is bad advice.  If the current message doesn't make
>> it clear enough that changing the sp is not allowed, how about:
>>
>>      inline %<asm%> statements must not change the value of the stack pointer
>>
>> ?
>>
> 
> My understanding is that in some cases, some users still want to
> deliberately change SP,
> so "must not" may be confusing in such cases?
> 

What do they want to achieve, something like a longjmp ?

BTW: on arm, you can add r15 (PC) to the clobber list, and
that seems to be ignored as well.  If you want to jump to
another function, you might want to clobber "sp", "pc".
PS: I know that is invalid, just guessing why there is an itch.


Bernd.

>> Thanks,
>> Richard
>>
>>