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SubjectRe: [BUG FIX] Make x86_32 uni-processor Atomic ops, Atomic
Date
On Mon May 25 2009, Michael S. Zick wrote:

In actual application, this *should not* make a difference.

Mike
> On Mon May 25 2009, Michael S. Zick wrote:
> > 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
> >
>
> Gentoo's current 4.3 gets it wrong also:
>
> .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: (Gentoo 4.3.2-r3 p1.6, pie-10.1.5) 4.3.2"
> .section .note.GNU-stack,"",@progbits
>
> = = = =
>
> Might be time to put compiler version checking back into the
> build system and/or re-test the sources that do have version
> checking in them (hint: the boss's code).
>
> Mike
> > Mike
> > > Don't blame me, I didn't write the compiler. ;)
> > >
> > > Mike
> > > > -hpa
> > > >
> > >
> > >
> > > --
>
>
> --
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