[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 #2 from Patrick McMunn <doctorwhoguy at gmail.com> ---
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.

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