Re: [Qemu-devel] [PULL 0/5] migration pull

[Li, Liang Z]

> 
> 
> On 23/02/2016 12:25, Peter Maydell wrote:
> > On 23 February 2016 at 10:43, Peter Maydell <address@hidden>
> wrote:
> >> That code fragment you suggest compiles fine normally, but not if I
> >> add -save-temps:
> >>
> >> $ cat /tmp/zz9.c
> >> #pragma GCC target("avx2")
> >> #include <immintrin.h>
> >> __m256i foo;
> >> $ gcc -g -Wall -o /tmp/zz9.o -c /tmp/zz9.c $ echo $?
> >> 0
> >> $ gcc -g -Wall -o /tmp/zz9.o -c /tmp/zz9.c -save-temps
> >> /tmp/zz9.c:4:1: error: unknown type name ‘__m256i’
> >>  __m256i foo;
> >>  ^
> >> /tmp/zz9.c: In function ‘bar’:
> >> /tmp/zz9.c:7:19: error: ‘__m256i’ undeclared (first use in this function)
> >>      return sizeof(__m256i);
> >>                    ^
> >> /tmp/zz9.c:7:19: note: each undeclared identifier is reported only
> >> once for each function it appears in
> >> /tmp/zz9.c:8:1: warning: control reaches end of non-void function
> >> [-Wreturn-type]  }  ^
> >>
> >> This seems to be because -save-temps causes the #pragma not to
> >> actually #define __AVX__.
> >
> > This is because -save-temps causes gcc to invoke the preprocessor and
> > the compiler as separate passes, and the standalone preprocessor
> > doesn't know that the target pragma should result in a new #define, so
> > the result is that the immintrin.h doesn't pull in what it should.
> >
> > This is also the reason why my build failed -- I use ccache, which is
> > another tool that results in the preprocessor being done as a
> > standalone pass rather than in the same pass as compilation proper.
> >
> > Arguably it's a gcc bug that the target pragma doesn't cause the
> > standalone preprocessor to define the same #defines that you get if
> > it's all in one pass, but regardless I don't think we can break ccache
> > builds, so you'll need to find a different way to do this, I'm afraid.
> 
> It's a bug in the header file, it was fixed in 4.9.
> 
> https://gcc.gnu.org/ml/gcc-patches/2013-06/txtvBBiTsFs8g.txt
> 
> Amit or Liang, can you restrict the new optimization to GCC 4.9+?
> 
> Paolo

Of course, how about using '#if QEMU_GNUC_PREREQ(4, 9) && defined 
CONFIG_AVX2_OPT' instead of '#ifdef CONFIG_AVX2_OPT'  in cutils.c?

Liang