Re: i386 flags register clober in inline assembly

[H. Peter Anvin]

Followup to:  <[e-mail]>
By author:    Linus Torvalds <[e-mail]>
In newsgroup: linux.dev.kernel
>
> In article <[e-mail]> you write:
> >
> >They don't need to be. On i386, the flags are (partly for historical reasons) clobbered
> >by default.
> 
> However, this is one area where I would just be tickled pink if gcc were
> to allow asm's to return status in eflags, even if that means that we
> need to fix all our existing asms.
> 
> We have some really _horrid_ code where we use operations that
> intrinsically set the flag bits, and we actually want to use them.
> Using things like cmpxchg, and atomic decrement-and-test-with-zero have
> these horrid asm statements that have to move the eflags value (usually
> just one bit) into a register, so that we can tell gcc where it is.
> 

The clean way to do that would be for gcc to implement _Bool, the C99
boolean data type, and add a new kind of register for the flags, i.e.

_Bool c;

asm volatile(LOCK "subl %2,%0"
    : "=m" (v->counter), "=zf" (c)
    : "ir" (i), "0" (v->counter) : "memory", "cc");

	-hpa
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