[Bug 77580] GCC memory starvation caused by flatten attribute with LTO

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=77580

--- Comment #3 from Martin Liska <mliska at suse.cz> ---
(In reply to Patrick McMunn from comment #2)
> I have wanted for a long time to be able to use LTO on xf86-video-intel, so
> I was very pleased when I found this patch submission. I tried it on the
> latest development version of the Intel driver from git. It applied
> successfully, and I was able to successfully compile the Intel driver using
> LTO instead of experiencing the seeming infinite compile time otherwise
> resulting from LTO.
> 
> However, despite compilation being successful, my tests which involved
> glxgears, monitoring CPU usage, and watching videos on Youtube showed
> significantly poorer video performance with the LTO-compiled driver than
> without LTO. Though glxgears showed no discernible difference, Youtube
> performance was incredibly slow such that the audio portion of the video
> continued at normal speed while the video lagged progressively further
> behind in slow motion.
> 
> I have no way of knowing if LTO is directly responsible for the poor
> performance or if the patch somehow led to poor optimization by LTO, but
> this should be investigated further. I used GCC 4.9.3 for the test. My Linux
> distro currently doesn't offer the 5.x branch of GCC, so I was unable to
> test with GCC 5.3.

Hi Patrick.

Well, it looks that xf86-video-intel driver needs flattened functions to
produce optimal code. It would be interesting, if you rebuild the driver with
the suggested patch applied (or is it part of mainline?) and try to generate
perf report that can provide comparison between LTO and non-LTO build.

Martin

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