Difference between GCC and Clang optimization options [closed]

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Precondition: I invoke gcc with -O2 optimization flag and clang with -O2 optimization flag.
Does it means the options list passed to the compilers will be the same?
Do clang understand standard GCC options like -funroll-loops?

I did not find anything useful about this question in manual:
http://llvm.org/releases/3.3/tools/clang/docs/UsersManual.html
I can not even find a list of possible GCC compatible options that can be used with clang.

Do clang even do anything with GCC options (or it just ignore them?)
I just need some clarifications on how it work.

Check clang manual for that. It understands that option but I'm not sure it'll understand ALL of them. — Adriano Repetti
– Adriano Repetti, Commented Dec 12, 2013 at 10:52
There is no guarantee that -O2 will mean the same in clang in gcc. Given that they have different optimizers, such a guarantee would not be possible anyway. — user4815162342
– user4815162342, Commented Dec 12, 2013 at 10:55
@Adriano The clang manual is surprisingly silent on what optimization options it actually supports, and their meaning. — user4815162342
– user4815162342, Commented Dec 12, 2013 at 10:55
@user4815162342 it's just an entry point for all documentation you need. For example this list of optimization flags. — Adriano Repetti
– Adriano Repetti, Commented Dec 12, 2013 at 11:01
@MikhailKalashnikov just read link I posted in previous comment. Manual is an entry point for other (more exhaustive) articles. — Adriano Repetti
– Adriano Repetti, Commented Dec 12, 2013 at 11:04

Community · Accepted Answer · 2017-05-23 12:08:30Z

5

Although in general Clang attempts to copy the command-line options from GCC, there are different optimization passes in GCC and LLVM, so specific optimization flags are not shared. In particular, it means that -O2 does not behave the same in the two compilers.

If you want to see the list of optimizations that GCC applies under -O2, check its documentation; if you want to see the list Clang applies, check this related Stackoverflow question about how to see the list.

edited May 23, 2017 at 12:08

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answered Dec 12, 2013 at 12:00

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