I currently use LTO in production code.
I and my team have been using LTO since gcc 4.7, although as you say it's
more mature in 4.9. We are a small games development house so performance
is critical and LTO makes a big difference to our release builds. However,
I have also seen a few spurious bugs that I was not able to isolate with
4.9.0 that were not present with 4.8; especially one case where perfectly
valid and bug-free code was segfaulting when compiled with LTO; unless
"remedied" by std::cout-ing one of the variables involved before it was
used in the function call producing the segfault.
We were later able to avoid the bug by not returning a container as we were
doing before, but passing it by reference to the same function. I would
have liked to be able to report it but i was never able to isolate a clean
test-case, as changing other code in other compile units made it disappear
or reappear arbitrarily (further confirming that it was specifically an
issue with LTO).
However, this was an isolated incident and we haven't had more of those
since; also 4.9.1 is now out which may fix a lot of these issues. I should
also note that we also optimise our release builds with -Ofast which
enables unsafe math optimisations, which may have been a contributing
factor in that case. It's hard to debug potential LTO issues because it
doesn't play well with debugging symbols; in fact I recall reading
somewhere that using both LTO and -g can produce undefined compiler
behaviour, although I cannot now find this quote, so you may want to take
that with a pinch of salt.
TL;DR: LTO is mature enough to use in production code, and the benefit is
large, but keep an eye out for any remaining bugs, and test your release
build properly rather than assuming it'll work because your debug build
works.
SlowRiot
On 25 June 2014 15:14, <m...@morous.org> wrote:
>
> Hello,
>
> when I tried LTO (link time optiomization) the last time (gcc 4.8 or 4.7,
> I do not remember precisely), it did not work much for me (frequent
> crashes during my project build with LTO enabled).
>
> I know gcc dev team spent a lot of time on improving LTO in 4.9.
>
> So, what is LTO status in mingw-w64 now? Do you consider it already "a
> standard feature" or is it still experimental thing and there may be lions
> hidden on the way? Anyone out there who already uses mingw-w64 with LTO in
> production?
>
> Thanks in advance for any feedback,
> Martin
>
>
>
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Turn processes into business applications with Bonita BPM Community Edition
Quickly connect people, data, and systems into organized workflows
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http://p.sf.net/sfu/Bonitasoft
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