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From: Paolo Bonzini <pbonzini@redhat.com>
To: Peter Maydell <peter.maydell@linaro.org>,
	Liang Li <liang.z.li@intel.com>
Cc: Amit Shah <amit.shah@redhat.com>,
	Juan Quintela <quintela@redhat.com>,
	qemu list <qemu-devel@nongnu.org>,
	"Dr. David Alan Gilbert" <dgilbert@redhat.com>
Subject: Re: [Qemu-devel] [PULL 0/5] migration pull
Date: Tue, 23 Feb 2016 15:04:46 +0100	[thread overview]
Message-ID: <56CC66FE.30702@redhat.com> (raw)
In-Reply-To: <CAFEAcA9+xoLBtFFv+wPpz7jvXj+NOi4KsmS_9Cvzx1FG=-LWXg@mail.gmail.com>



On 23/02/2016 12:25, Peter Maydell wrote:
> On 23 February 2016 at 10:43, Peter Maydell <peter.maydell@linaro.org> 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

  reply	other threads:[~2016-02-23 14:05 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-02-23  7:30 [Qemu-devel] [PULL 0/5] migration pull Amit Shah
2016-02-23  7:30 ` [Qemu-devel] [PULL 1/5] migration: move bdrv_invalidate_cache_all of of coroutine context Amit Shah
2016-02-23  7:30 ` [Qemu-devel] [PULL 2/5] " Amit Shah
2016-03-07 12:49   ` Dr. David Alan Gilbert
2016-03-07 13:30     ` Paolo Bonzini
2016-03-07 18:06       ` Dr. David Alan Gilbert
2016-03-07 18:58     ` Denis V. Lunev
2016-03-08 10:45       ` Dr. David Alan Gilbert
2016-03-08 10:54         ` Denis V. Lunev
2016-02-23  7:30 ` [Qemu-devel] [PULL 3/5] migration: reorder code to make it symmetric Amit Shah
2016-02-23  7:30 ` [Qemu-devel] [PULL 4/5] configure: detect ifunc and avx2 attribute Amit Shah
2016-02-23  7:30 ` [Qemu-devel] [PULL 5/5] cutils: add avx2 instruction optimization Amit Shah
2016-02-23  9:09 ` [Qemu-devel] [PULL 0/5] migration pull Peter Maydell
2016-02-23  9:38   ` Amit Shah
2016-02-23  9:48   ` Paolo Bonzini
2016-02-23 10:43     ` Peter Maydell
2016-02-23 11:18       ` Li, Liang Z
2016-02-23 11:25       ` Peter Maydell
2016-02-23 14:04         ` Paolo Bonzini [this message]
2016-02-24  9:27           ` Li, Liang Z
2016-03-08  4:23             ` Amit Shah
2016-03-08  4:28               ` Li, Liang Z
2016-02-23  9:55   ` Li, Liang Z

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