Date: Wed, 30 Oct 2013 14:33:01 +0100 From: Tijl Coosemans <tijl@coosemans.org> To: Bruce Cran <bruce@cran.org.uk> Cc: FreeBSD Questions <freebsd-questions@freebsd.org> Subject: Re: Code segfaults with clang -O1, works with gcc49 and clang -O2 Message-ID: <20131030143301.623be910@kalimero.tijl.coosemans.org> In-Reply-To: <5270D99C.6070901@cran.org.uk> References: <5270D99C.6070901@cran.org.uk>
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On Wed, 30 Oct 2013 10:04:12 +0000 Bruce Cran wrote:
> The following code works (on FreeBSD 11-CURRENT) with gcc49 or clang -O2
> or higher, but segfaults in do_cpuid() with clang -O1 or lower. Is there
> a bug in the asm statement, or is clang doing something wrong?
>
> #include <stdio.h>
>
> static inline void do_cpuid(unsigned int *eax, unsigned int *ebx,
> unsigned int *ecx, unsigned int *edx)
> {
> asm volatile("cpuid"
> : "=a" (*eax), "=b" (*ebx), "=r" (*ecx), "=d" (*edx)
Should be: "=c" (*ecx)
But you can also use the '+' modifier and remove the input operands:
: "+a" (*eax), "=b" (*ebx), "+c" (*ecx), "=d" (*edx)
:
: "memory"
> : "0" (*eax), "2" (*ecx)
> : "memory");
> }
>
> int main(int argc, char **argv)
> {
> unsigned int a = 0, b = 0, c = 0, d = 0;
> do_cpuid(&a, &b, &c, &d);
> printf("%d %d %d %d\n", a, b, c, d);
> return 0;
> }
>
>
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