linux-kernel - Re: [BUG FIX] Make x86_32 uni-processor Atomic ops, Atomic

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Date:	Mon, 25 May 2009 14:18:37 -0500
From:	"Michael S. Zick" <lkml@...ethan.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Harald Welte <HaraldWelte@...tech.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [BUG FIX] Make x86_32 uni-processor Atomic ops, Atomic

On Mon May 25 2009, Michael S. Zick wrote:
> On Sun May 24 2009, H. Peter Anvin wrote:
> > Michael S. Zick wrote:
> > > 
> > > Note: I have seem to recall that newer gcc's optimizer presume 
> > > that the flags register is preserved across asm - 
> > > It didn't use to do that - but there is now a "cc" to deal with
> > > that - Have not yet audited for that, but it is high on my list.
> > > 
> > 
> > I am pretty sure that's false... if it was true we'd have failures all
> > over the kernel.
> > 
> 
> No information on the above (yet) - but you gotta love this one: ;)
> 
> Programmer authors code specifying that the subtraction be done
> prior to the addition to avoid over-flow conditions;
> 
> GCC's optimizer, in its great wisdom, codes in the overflow case:
> ( the case of finding the characters used/free in a ring buffer )
> 
> extern int diff_umask(int mask, int *cnt1, int *cnt2) 
> { return (((mask - *cnt1) + *cnt2) & mask); }
> 
> /**
>  * gcc -O2 -S -fomit-frame-pointer difftest.c
>  *
>          .file   "difftest.c"
>         .text
>         .p2align 4,,15
> .globl diff_umask
>         .type   diff_umask, @function
> diff_umask:
>         movl    12(%esp), %eax
>         movl    4(%esp), %ecx
>         movl    (%eax), %edx
>         leal    (%ecx,%edx), %eax
>         movl    8(%esp), %edx
>         subl    (%edx), %eax
>         andl    %ecx, %eax
>         ret
>         .size   diff_umask, .-diff_umask
>         .ident  "GCC: (Debian 4.3.2-1.1) 4.3.2"
>         .section        .note.GNU-stack,"",@progbits
> */
> 
> Note: That is not the compiler version I am building my kernels with.
> 

The compiler I am using (Gentoo 4.1.2) gets it correct:

        .file   "difftest.c"
        .text
        .p2align 4,,15
.globl diff_umask
        .type   diff_umask, @function
diff_umask:
        movl    4(%esp), %eax
        movl    8(%esp), %edx
        movl    %eax, %ecx
        subl    (%edx), %ecx
        movl    %ecx, %edx
        movl    12(%esp), %ecx
        addl    (%ecx), %edx
        andl    %edx, %eax
        ret
        .size   diff_umask, .-diff_umask
        .ident  "GCC: (GNU) 4.1.2 (Gentoo 4.1.2 p1.1)"
        .section        .note.GNU-stack,"",@progbits

Mike
> Don't blame me, I didn't write the compiler. ;)
> 
> Mike
> > 	-hpa
> > 
> 
> 
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