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

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

On 1/10/19 10:23 PM, Richard Sandiford wrote:
> Segher Boessenkool <segher@kernel.crashing.org> writes:
>> On Tue, Jan 08, 2019 at 12:03:06PM +0000, Richard Sandiford wrote:
>>> Bernd Edlinger <bernd.edlinger@hotmail.de> writes:
>>>> 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".)
>>
>> It's not forcing a frame currently: it's just setting frame_pointer_needed.
>> Whatever happens from that is the target's business.
> 
> Do you mean the asm clobber or -fno-omit-frame-pointer?  If the option,
> then yeah, and that was exactly what was controversial :-)
> 

Yes, what I meant is the asm clobber sets frame_pointer_needed,
on the function where this asm is used, that sounded to me like
it would have an impact on the frame pointer.

What I also expected, is that if an asm is accessing a local
via "m" then the a SP+x reference will be elimitated to a FP+x,
reference, which would allow the asm to push something on the
stack, and the memory references would remain valid,
as long as the stack is _restored_, again in the same asm.
I mean in case of register shortage.  I was not thinking about
noreturn at all.

But if -fno-omit-frame-pointer does the same, and that is not sufficient
to for forcing a frame pointer, because it is a target dependent, then I
wonder how ASAN is supposed to work on such a target.

But anyway I guess, your patch is fine.


Thanks
Bernd.