Hi,
On 30.05.2016 20:57, Rob Clark wrote:
On Mon, May 30, 2016 at 1:39 PM, Matt Turner <matts...@gmail.com> wrote:
On Mon, May 30, 2016 at 10:28 AM, ⚛ <0xe2.0x9a.0...@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
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