[Mesa-dev] [PATCH] mesa: Fix build to properly check for supported compiler flags

[Chad Versace]

On 12/19/2013 01:50 AM, Lauri Kasanen wrote:
> On Wed, 18 Dec 2013 17:57:16 -0800
> Matt Turner <mattst88 at gmail.com> wrote:
>
>> On Tue, Dec 17, 2013 at 7:15 AM, Lauri Kasanen <cand at gmx.com>

>>> It seems to me that the Intel code that uses this SSE4.1 function
>>> is still buggy, as it has no runtime check - would it not crash
>>> if built on a SSE4-capable system but used with a lower-class cpu?
>>
>> You haven't thought this through. :)
>>
>> The SSE 4.1 code is in i965 #ifdef __SSE4_1__. __SSE4_1__ is defined
>> if the code is compiled with the appropriate CFLAGS, e.g., -msse4.1 or
>> -march=native and your CPU support SSE 4.1.
>>
>> That is, your Mesa would have to be compiled with flags incompatible
>> with your CPU for this to break, and in that case gcc would already
>> have used instructions your CPU doesn't have.
>
> Yes, you're right that there is no crash risk on a normal build.
> However, it is practically useless for distro builds like that, as they
> can't ship a normal build and get the speedup at the same time.

I want to clarify how this is currently useful. Product-specific builds,
such as Chrome OS builds, do benefit from this SSE code, because they
are compiled with the appropriate CFLAGS.

Your point still stands though for regular distros.