[Mesa-dev] [PATCH] mesa: Enable LTO compilation

[Eero Tamminen]

Hi,

On 30.05.2016 20:57, Rob Clark wrote:
> On Mon, May 30, 2016 at 1:39 PM, Matt Turner <mattst88 at gmail.com> wrote:
>> On Mon, May 30, 2016 at 10:28 AM, ⚛ <0xe2.0x9a.0x9b at gmail.com> wrote:
>>> This patch enables compilation with -flto.
>>>
>>> The performance benefits of LTO (GCC 4.9 & 6.1) are about 1% for glxgears.
>>> Performance changes in OpenGL games haven't been measured yet, my feeling is
>>> that they are negligible.
>>
>> Without a compelling reason, I don't think the build system should be
>> adding compiler flags like this.
>>
>> -flto makes debugging impossible, at least the last time I tried it
>> with gcc. I don't think that's something we would want enabled
>> whenever it's supported.
>
> It would be interesting to know what gains it brings in scenarios less
> synthetic than glxgears..  my suspicion is that we have been doing
> manual lto forever (ie. use of static-inline fxn's where it matters).
> If it turns out to be a significant gain, I wouldn't be against it.
> Although perhaps only for non-debug builds..

Martin did some testing with LTO last year for a real customer 3D 
benchmark that was partly CPU bound.  In that case there were actually 
several percent performance improvements.

How much, depends on:
- How CPU bound the 3D benchmark is and do those CPU bottlenecks hit 
cases where LTO can actually help
- What's the CPU/GPU balance on the given given HW
   (more visible on more CPU bound HW)
- What GCC version is used
   (newer GCC versions provide more benefit with LTO)

And it should not hurt performance (at least if one is using 
"performance" frequency policy, or fixed CPU frequency, instead of 
"powersave" one.  "Powersave" can drop CPU frequency dramatically when 
use-case drop its CPU usage and large enough CPU freq drop can make 
things slower).


	- Eero