Re: some questions about GCC's options

[Ian Lance Taylor <iant at google dot com>]

ohav chochmah <philomath868@gmail.com> writes:

> first, is momit-leaf-frame-pointer bad for debugging (in the way
> fomit-frame-pointer can be)?

It's not as bad.  It may still trip up some debuggers.

> if not, why is it disabled by default
> even when optimizing (as -Q --help=target | grep omit reveals)?

When optimizing on x86 or x86_64, -fomit-frame-pointer is the default
anyhow.  -fomit-frame-pointer essentially implies
-momit-leaf-frame-pointer.

> next, the manual mentions that fno-fat-lto-objects improves
> compilation time over plain LTO, but requires the whole toolchain to
> be aware of LTO and support plugins, which is why it's not (yet) the
> default.  how can I know for certain if the toolchain I'm using meets
> the criteria (seems to be the case)?

Use the most recent GNU binutils release.

> similarly, mtls-dialect=gnu2 is better then the default gnu, "but it
> may add compile- and run-time requirements that cannot be satisfied on
> all systems."  how can I test for them?

Build a program with it and see.  Basically your GNU binutils need to be
fairly new and your glibc needs to be 2.9 or later.

> is it true that mfpmath=sse can result in poor code when using glibc?
> (maybe I shouldn't ask that here...)

For x86_64 -mfpmath=sse is the default.

> why is msse disabled by default even after march=native, while
> msse[234], msse4.[12] and mssse3 are all enabled?

Not sure--what makes you think this is the case?

> isn't the CRC32 instruction part of sse4.2?, why is mcrc32 disabled by
> default even when sse4.2 is enabled?

It's not.  Don't be confused by the fact that -msse4.2 does not
automatically turn on -mcrc32.  Everything that is available when using
-mcrc32 is also available when using -msse4.2.

Ian