linux-kernel - Re: [PATCH 6/8] i386: bitops: Don't mark memory as clobbered unnecessarily

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Date:	Wed, 25 Jul 2007 18:18:14 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Trent Piepho <xyzzy@...akeasy.org>
cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	Satyam Sharma <ssatyam@....iitk.ac.in>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	David Howells <dhowells@...hat.com>, Andi Kleen <ak@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 6/8] i386: bitops: Don't mark memory as clobbered
 unnecessarily



On Wed, 25 Jul 2007, Trent Piepho wrote:
> 
> Specifically, check test6_memasm.s.  The C code looks like this:
> 
> extern int a; /* keep asm from being elided for having no used output */
> static inline void bar(void) { asm("call bar" : "=m"(a) : : "memory"); }
> /* float x can't alias asm's output int a */
> void foo(float *x) { x[20] = 1; bar(); x[20] = 2; }
> 
> The asm code ends up like this:
> foo:
>         call bar
>         movl    4(%esp), %eax   # x, x
>         movl    $0x40000000, 80(%eax)   #,
>         ret

Hmm. I really think you should take this up with the gcc people. That 
looks like a gcc bug - because there really is nothing that guarantees 
that the asm doesn't change the array that "x" points to, and the asm 
clearly talks about clobbering memory.

> Notice that the first write to x[20] was NOT done.  It's also not done for a
> volatile asm without a memory clobber.  But if you combine both volatile and a
> memory clobber, then it is!  How to explain that?

I can't explain it. I do think you've found a gcc bug.

That said, the kernel mostly uses "asm volatile()" _together_ with a 
memory clobber for these kinds of things, so it sounds like the kernel 
wouldn't be impacted. But you're definitely right - the above report makes 
me worry.

> The difference between test2_volasm.s and test2_normasm.s is hard to explain
> too.  It seems like some times gcc forgets that imull is commutative.  It will
> emit "imull %edx, %eax" in some cases, but change an asm slightly and it will
> decide it must do "imull %eax, %edx ; movl %edx, %eax" for no apparent reason.

Well, that's likely just a subtle register allocation issue, and 
understandable. Generating perfect code is impossible, you want to 
generate good code on average.

		Linus
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