Re: [ACPI] Re: Linux 2.4.26-rc1 (cmpxchg vs 80386 build)

From: Willy Tarreau
Date: Tue Mar 30 2004 - 11:18:39 EST

> > That's not what I meant. I only meant to declare the cmpxchg() function.
> It's not a function. It is actual op-codes.

I know it's an opcode (I even wrote an emulator for it). But it's used
as an inline function in linux. Check include/asm-i386/system.h.

> If you compile with '486 or higher, the C compiler is free to spew
> out these op-codes any time it thinks it's a viable instruction
> sequence. Since it basically replaces two other op-codes, gcc might
> certainly use if for optimization.

Yes, only if you compile with -m486 or higher. When Linux is compiled for
386 target, the -march=i386 is correctly appended, which prevents gcc from
using this instruction (as well as bswap and xadd BTW).

In what I described, a 386 target would be compiled with -march=i386,
but the cmpxchg() FUNCTION will still reference the cmpxchg op-code
in the __asm__ statement, and this is perfectly valid. In this case,
only callers of the cmpxchg() FUNCTION will have a chance to use it.
And at the moment, the only client seems to be ACPI.

Anyway, I think that basically we understand ourselves, and it's just
a matter of words.


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