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

From: Michael S. Zick
Date: Mon May 25 2009 - 15:03:24 EST

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"
.p2align 4,,15
.globl diff_umask
.type diff_umask, @function
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
.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.

Don't blame me, I didn't write the compiler. ;)

> -hpa

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