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Date: Thu, 13 Dec 2018 14:49:00 -0000
From: Segher Boessenkool <segher@kernel.crashing.org>
To: Dimitar Dimitrov <dimitar@dinux.eu>
Cc: Christophe Lyon <christophe.lyon@linaro.org>,        Thomas Preudhomme <thomas.preudhomme@linaro.org>,        gcc Patches <gcc-patches@gcc.gnu.org>,        Richard Sandiford <richard.sandiford@arm.com>,        "Thomas Preud'homme" <thomas.preudhomme@arm.com>
Subject: Re: [PATCH] [RFC] PR target/52813 and target/11807
Message-ID: <20181213144837.GD3803@gate.crashing.org>
References: <20181209100856.14051-1-dimitar@dinux.eu> <CAKnkMGvqbnVU-Gap5X55mbY8f8cG1ZiVMitczgYVyPvK6EEvRA@mail.gmail.com> <CAKdteOaqBa4Cf8kTOEEWMeJgRxMaq45sXGTrjdq=4G7SWBw8Fg@mail.gmail.com> <4591520.WeSID5Fo3g@tpdeb>
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On Wed, Dec 12, 2018 at 06:26:10PM +0200, Dimitar Dimitrov wrote:
> I expect that if I mark a HW register as "clobber", compiler would save its 
> contents before executing the asm statement, and after that it would restore 
> its contents. This is the GCC behaviour for all but the SP and PIC registers. 
> That is why I believe that PR52813 is a valid bug. 

It won't do it for *any* fixed registers.  But you do not want to error
or even warn for some fixed registers, for example the "flags" register
on x86 is *always* written to by asm.

But you never want to warn for non-fixed registers, and e.g.
PIC_OFFSET_TABLE_REGNUM isn't always a fixed register (when flag_pic is 0
for example).

> I'm not sure how GCC could recover if SP is clobbered. If SP is clobbered in 
> such a way that GCC will not notice (e.g. thread switching), then why should 
> GCC know about it in the first place?

Up until today, GCC has always just ignored it if you claimed to clobber
the stack pointer.


Segher