Open64 5.0 released [LWN.net]

Open64 5.0 released

Posted Nov 12, 2011 15:16 UTC (Sat) by endecotp (guest, #36428)
In reply to: Open64 5.0 released by Cyberax
Parent article: Open64 5.0 released

> -O2 is a generic flag which means "turn on all sane
> optimizations" in all three compilers.

"Citation required", as they say. That certainly isn't what "man gcc" says, for example.

Surely it is pointless to compare the performance of compilers based on anything other than their maximum optimisation level.

BTW, I have always found it strange that binary distributions compile by default at -O2. Do they not have the capacity in their build farms to use a higher level?

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There are no such thing...

Posted Nov 12, 2011 15:57 UTC (Sat) by khim (subscriber, #9252) [Link] (4 responses)

Surely it is pointless to compare the performance of compilers based on anything other than their maximum optimisation level.

This is most pointless comparison of them all. Nobody uses -O3. Either people use -O2 (as "generally safe" optimization level) or they carefully tune flags and test the result.

BTW, I have always found it strange that binary distributions compile by default at -O2.

If you don't know why and when people use different options then perhaps you should not proclaim that you unjustified beliefs have are relation to real world?

Do they not have the capacity in their build farms to use a higher level?

This is not about build farms. This is about Q&A. Because "normal" optimization level is -O2 upstream developers often close bugs related to -O3 level as "WONTFIX" or "INVALID". This makes it absolutely unpractical to use -O3 for most packages. Only handful of packages where upstream actually test everything as -O3 (plus some hand-picked additional options) are compiled as -O3. This is the only realistic possibility. Gentoo allows you to recompile everything with -O3 but even there bugreports from such people are viewed as suspicious.

There are no such thing...

Posted Nov 12, 2011 17:07 UTC (Sat) by endecotp (guest, #36428) [Link] (2 responses)

> Nobody uses -O3.

Well, I use it as my normal setting for "release" builds.

Again, "citation required". Maybe you're right and (almost) everyone uses -O2 - but I would like to know where that info comes from.

> Because "normal" optimization level is -O2 upstream developers
> often close bugs related to -O3 level as "WONTFIX" or "INVALID"

Could you point me to a couple of examples of that? Thanks.

There are no such thing...

Posted Nov 14, 2011 2:10 UTC (Mon) by rsidd (guest, #2582) [Link]

It's not that -O3 is buggy (it can be but usually isn't) - it's that -O3's additional optimisations are not always of benefit. Source - gcc docs. On my code I find no significant speed difference in -O2 and -O3.

There are no such thing...

Posted Nov 16, 2011 19:32 UTC (Wed) by tetromino (subscriber, #33846) [Link]

> Maybe you're right and (almost) everyone uses -O2 - but I would like to know where that info comes from.

Take a look at *any* distribution's default optimization flags. For rpm-based distros, the default /usr/lib/rpm/rpmrc says 'optflags: amd64 -O2 -g'. In Debian, dpkg-buildflags returns 'CFLAGS=-g -O2' by default. Even in Gentoo, /usr/portage/profiles/arch/amd64/make.defaults says 'CFLAGS="-O2 -pipe"'.

There are no such thing...

Posted Nov 12, 2011 17:57 UTC (Sat) by cowboy (guest, #2231) [Link]

> Nobody uses -O3.

Wrong.

I use -O3 on Linux, and -O4 on AIX (xlc) for my product builds.

To be fair, there a few modules that don't fair well at -O4, but only because I've not had time to find out what is wrong with them (usually uninitialized variables).

My next step on AIX is IPA at link time, it will do inlining and other code movement that couldn't be done at compile time, due to the plethora of little pieces, all compiled separately.

Open64 5.0 released

Posted Nov 12, 2011 20:10 UTC (Sat) by nix (subscriber, #2304) [Link]

Given that -O3 under GCC means 'carry out optimizations that should reduce cycle count regardless of effects on code size', it fairly often leads to icache bloat or register pressure not compensated for at later stages and corresponding excessive stack spilling which end up slowing the code down.

So you end up with larger and/or slower code than under -O2.

Open64 5.0 released

Posted Nov 14, 2011 20:46 UTC (Mon) by daglwn (guest, #65432) [Link] (1 responses)

> Surely it is pointless to compare the performance of compilers based on
> anything other than their maximum optimisation level.

That would be no better and quite possibly much worse.

The "maximum" optimization level is very code-dependent. It takes a long time to tune the optimization flags to make your code run as fast as possible. Most Free Software projects don't bother. But in places like HPC where performance is everything, people spend a lot of time trying various flags and directives to squeeze out performance.

The only reasonable way to compare compilers is to tune the flags to get the best result for each and then see where you're at.

Open64 5.0 released

Posted Nov 22, 2011 17:54 UTC (Tue) by JanC_ (subscriber, #34940) [Link]

The only reasonable way to compare compilers is to tune the flags to get the best result for each and then see where you're at.

I think the only reasonable way to compare compilers is to test them the way you intend to use them:

In case of a linux distro, that means testing them with some default options that were chosen because they work well with all (or at least most) packages. You don't want to tune compiler options for 30000 packages one by one...
In case of a HPC environment, that means you tune each compiler to give its best result for the type(s) of code you need to run as fast as possible.