Re: [Valgrind-developers] Memcheck optimisation level; Makefile.am includes

[Julian S. <js...@ac...>]

> Is there any particular reason why Memcheck is built at -O2, but all the
> other tools are built at -O?

Is -O / -O2 the only distinction, or is it -O vs -O2 -fomit-frame-pointer?

I seem to remember trying to max out performance for the helper
routines for memcheck (obviously) and hence have 
-O2 -fomit-frame-pointer -malign-somthing-or-other=4 iirc.

The last flag says that a 4-byte aligned stack is OK and stopped some
clueless versions of gcc from adding/subtracting constants from the
stack pointer to get it aligned, which turned out to be a significant
time waster for the memcheck memory access helpers.  Such alignment is
particularly useless given that in those helpers gcc never generates
any %esp-relative accesses anyway.  Duh.

I don't have a problem with -O2 per se, but my general feel over N
years of hacking is that it doesn't give much performance improvment
over -O and exposes one to more gcc bugs, and makes compilation takes
longer, hence it wasn't worth it.  That's just religion, though, so
I'd say -O2ifiy it if you like.

-fomit-frame-pointer generally makes debugging difficult, so that
is probably not a good thing to plumb everywhere.  In fact there are
some places (where we want to get stack snapshots of, eg
vg_libpthread.c) which positively have to say -fno-omit-frame-pointer,
I believe.

J