Re: What are the best gcc optimization options for Pentium 200 MMX

[Matthew Dillon]

:On Sat, 8 Apr 2000, Alexey N. Dokuchaev wrote:
:
:> AFAIK, Linux Mandrake has it's kernel and userland highly optimized for
:> Pentium architecture.  However, they have additional gcc optimization 
:> flags turned on by default, including -O3 and -mfast_math.
:
:Can you say "gimmick"? :-) gcc often produces demonstrably broken code for
:optimisation levels higher than -O.
:
:Probably the only useful and safe option apart from -O is the
:-march=pentium/pentiumpro/pentiumii/etc option for using
:processor-specific opcodes and instruction scheduling.
:Kris

    I use -Os for everything.  I wouldn't bother with anything else.  Someone
    ran a bunch of benchmarks with various gcc/egcs options a while back
    and, frankly, the top half dozen combinations were so close to each
    other performance-wise that it just didn't matter.  -Os was in that
    group, but also produced significantly smaller binaries.

    I wouldn't touch the -march stuff at all, nor would I use -O3 (which
    tries to inline standard static functions verses -O2) - that's useless
    on IA32 because call/returns are very fast (I had an argument with John
    Dyson about call/return overhead verses an L1 cache miss and
    we ran a bunch of timings.  I lost the argument :-) call/return won the
    race handily).


                                        -Matt
                                        Matthew Dillon 
                                        <[EMAIL PROTECTED]>


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