Re: [Qemu-trivial] [Qemu-devel] [PATCH] qemu-timer: Don't use RDTSC on 386s and 486s

[Jamie Lokier]

Peter Maydell wrote:
> On 23 November 2012 15:15, Paolo Bonzini <address@hidden> wrote:
> > Il 23/11/2012 16:12, Peter Maydell ha scritto:
> >> Adjust the conditional which guards the implementation of
> >>
> >> -#elif defined(__i386__)
> >> +#elif defined(__i586__)
> >>
> >>  static inline int64_t cpu_get_real_ticks(void)
> >>  {
> >>
> >
> > You should at least test __i686__ too:
> >
> > $ gcc -m32 -dM -E -x c /dev/null |grep __i
> > #define __i686 1
> > #define __i686__ 1
> > #define __i386 1
> > #define __i386__ 1
> 
> Yuck. I had assumed gcc would define everything from i386
> on up when building for later cores.

No, and it doesn't define __i686__ on all x86-32 targets after i686 either:

    $ gcc -march=core2 -dM -E -x c /dev/null | grep __[0-9a-z] | sort
    #define __core2 1
    #define __core2__ 1
    #define __gnu_linux__ 1
    #define __i386 1
    #define __i386__ 1
    #define __linux 1
    #define __linux__ 1
    #define __tune_core2__ 1
    #define __unix 1
    #define __unix__ 1

x86 instruction sets haven't followed a linear progression of features
for quite a while, especially including non-Intel chips, so it stopped
making sense for GCC to indicate the instruction set in that way.

GCC 4.6.3 defines __i586__ only when the target arch is set by -march
(or default) to i586, pentium or pentium-mmx.

And it defines __i686__ only when -march= is set (or default) to c3-2,
i686, pentiumpro, pentium2, pentium3, pentium3m or pentium-m.

Otherwise it's just things like __athlon__, __corei7__, etc.

The only one that's consistent is __i386__ (and __i386).

-- Jamie