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Re: Compiler speed (vanilla vs. LTO, PGO and LTO+PGO)
- From: Richard Biener <richard dot guenther at gmail dot com>
- To: Markus Trippelsdorf <markus at trippelsdorf dot de>
- Cc: gcc_mailinglist at abwesend dot de, gcc-help at gcc dot gnu dot org, gcc at gcc dot gnu dot org
- Date: Mon, 25 Mar 2013 14:11:26 +0100
- Subject: Re: Compiler speed (vanilla vs. LTO, PGO and LTO+PGO)
- References: <trinity-46d93944-181a-4a69-836d-b1cd693c2d5c-1364154786327 at msvc024> <20130325070650 dot GA12504 at x4> <20130325125652 dot GB12504 at x4>
On Mon, Mar 25, 2013 at 1:56 PM, Markus Trippelsdorf
<markus@trippelsdorf.de> wrote:
> On 2013.03.25 at 08:06 +0100, Markus Trippelsdorf wrote:
>> On 2013.03.24 at 20:53 +0100, gcc_mailinglist@abwesend.de wrote:
>> >
>> > is it useful to compile gcc 4.8.0 with the lto option?
>>
>> If you want a (slightly) faster compiler then yes.
>> Simply add "--with-build-config=bootstrap-lto" to your configuration.
>> You can combine this with profile feedback: "make profiledbootstrap".
>
> To qualify "(slightly) faster" in the statement above, I build gcc with
> four different configurations on my AMD64 4-core machine (vanilla, LTO
> only, PGO only, LTO+PGO). Then I measured how much time it takes to
> build the Linux kernel and Firefox. Here are the results:
>
> Firefox:
> vanilla: 5143.27s user 267.27s system 346% cpu 26:02.03 total
> PGO : 4590.37s user 270.21s system 344% cpu 23:28.89 total
> LTO : 5056.11s user 268.04s system 348% cpu 25:28.73 total
> LTO+PGO: 4598.79s user 269.01s system 347% cpu 23:22.13 total
>
> kernel (measured three times):
> vanilla: 382.34s user 23.74s system 334% cpu 2:01.41 total 382.08s user 24.05s system 333% cpu 2:01.93 total 385.20s user 23.63s system 330% cpu 2:03.73 total
> PGO : 341.18s user 23.25s system 323% cpu 1:52.71 total 341.72s user 23.66s system 323% cpu 1:52.93 total 340.32s user 23.42s system 326% cpu 1:51.38 total
> LTO : 381.23s user 23.55s system 328% cpu 2:03.05 total 380.41s user 24.35s system 328% cpu 2:03.24 total 379.47s user 23.98s system 331% cpu 2:01.82 total
> LTO+PGO: 347.12s user 25.11s system 317% cpu 1:57.34 total 344.38s user 24.05s system 326% cpu 1:52.99 total 344.74s user 24.61s system 323% cpu 1:54.03 total
>
> To summarize:
> * GCC build with PGO is ~10% faster than a vanilla bootstrapped compiler.
> * GCC build with LTO only is only ~2% faster when building Firefox. The
> kernel build time difference is in the noise.
> * A LTO+PGO build is almost exactly as fast as a pure PGO build.
>
> So it appears, contrary to the advice given above, that it is not useful
> to build gcc 4.8.0 with the lto option at the moment.
Probably Honza did a too good job in making sure optimizations LTO does
can be done without LTO as well by fixing up GCC sources ;)
Did you compare binary sizes of the compiler itself (w/o debuginfo)?
Richard.
> --
> Markus