linux-kernel - Re: gcc feature request / RFC: extra clobbered regs

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Date:	Wed, 1 Jul 2015 13:09:06 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Jakub Jelinek <jakub@...hat.com>
Cc:	Vladimir Makarov <vmakarov@...hat.com>,
	Andy Lutomirski <luto@...nel.org>, gcc@....gnu.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: gcc feature request / RFC: extra clobbered regs

On Wed, Jul 1, 2015 at 10:43 AM, Jakub Jelinek <jakub@...hat.com> wrote:
> On Wed, Jul 01, 2015 at 01:35:16PM -0400, Vladimir Makarov wrote:
>> Actually it raise a question for me.  If we describe that a function
>> clobbers more than calling convention and then use it as a value (assigning
>> a variable or passing as an argument) and loosing a track of it and than
>> call it.  How can RA know what the call clobbers actually.  So for the
>> function with the attributes we should prohibit use it as a value or make
>> the attributes as a part of the function type, or at least say it is unsafe.
>> So now I see this as a *bigger problem* with this extension.  Although I
>> guess it already exists as we have description of different ABI as an
>> extension.
>
> Unfortunately target attribute is function decl attribute rather than
> function type.  And having more attributes affect switchable targets will be
> non-fun.

Just to make sure we're on the same page here, if I write:

extern void normal_func(void);

void weird_func(void) __attribute__((used_regs("r12")))
{
  // do something
  normal_func();
  // do something
}

I'd want the code that calls normal_func() to be understand that
normal_func() *will* preserve r12 despite the fact that weird_func is
allowed to clobber r12.  I think this means that the attribute would
have to be an attribute of a function, not of the RA while compiling
the function.

--Andy
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