gcc -O flags (was GLIBC Coredump)

Mattias Axelsson mattias at barbanet.com
Fri Jul 14 14:12:38 PDT 2000


Is the advice from FreeBSD to _never_ use -O2 or -O3 on intel machines?
>From what I can understand, -O3 among other things makes loop unrolling
and function inlining, which I thought were safe performance boosters.

glibc seems to fail compiling with -O3 (I'm not ready with -O2 yet) but
how is it with other packages? Are there any more packages that will fail
to compile, or worse, compile but fail to work, when using -O2 or -O3 then
I would be glad to hear about it. Or should I just give up and use just -O
all the time?

/Mattias

"Thomas T. Veldhouse" wrote:
> 
> I use:  -O -pipe -march=pentiumpro
> 
> That is it. I use it for both CFLAGS and CXXFLAGS.
> 
> I am going to rebuild all my C++ packages with
> 
> CXXFLAGS= -O -pipe -march=pentiumpro -fmemoize-lookups -fsave-memoized
> 
> -O2 and above is known to cause some problems in rare cases on i386 - and
> many cases on alpha machines.  I did run into a problem with -O2 once (for
> gcc-2.95.2), but I don't recall what package it was (it was before I started
> building my own system).
> 
> Here is a quote from the /etc/make.conf in FreeBSD 4.0-STABLE.
> 
> # CFLAGS controls the compiler settings used when compiling C code.
> # Note that optimisation settings above -O (-O2, ...) are not recommended
> # or supported for compiling the world or the kernel - please revert any
> # nonstandard optimisation settings to "-O" before submitting bug reports
> # to the developers.
> # Note also that at this time the -O2 setting is known to produce BROKEN
> # CODE on the Alpha platform.
> 
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