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

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Message-ID: <55942D8A.7060105@redhat.com>
Date:	Wed, 01 Jul 2015 14:12:26 -0400
From:	Vladimir Makarov <vmakarov@...hat.com>
To:	Jakub Jelinek <jakub@...hat.com>
CC:	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 07/01/2015 01:43 PM, Jakub Jelinek 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.
>
>
Making attributes a part of type probably creates a lot issues too.

Although I am not a front-end developer, still I think it is hard to 
implement in front-end.  Sticking fully to this approach, it would be 
logical to describe this as a debug info (I am not sure it is even 
possible).

Portability would be an issue too.  It is hard to prevent for a regular 
C developer to assign such function to variable because it is ok on his 
system while the compilation of such code may fail on another system.

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