Re: [Sheflug] CFLAGS and USE Question

[Seb James <seb [at] xxxxxxxxxxxxxxxxxxxxxx>]

On Wed, 2004-07-07 at 10:13, Marcus D. Hanwell wrote:
> Brett Royles wrote:
> >I agree , those flags seem good to me , I personally use -O6 , but
> >thats just me!!!
> >  
> I saw an article somewhere on the net about optimisation flags and how 
> anything above -O3 is aliased to -O3 anyway - with the relevant bit of 
> the gcc source shown to prove it! As far as I read there isn't anything 
> above -O3 but I could be wrong...

Marcus is correct, nothing beyond -O3.

Each of the -O flags turns on a set of optimizations; for example -O
sets these:

          -fdefer-pop 
          -fmerge-constants 
          -fthread-jumps 
          -floop-optimize 
          -fif-conversion 
          -fif-conversion2 
          -fdelayed-branch 
          -fguess-branch-probability 
          -fcprop-registers

-O2 sets these, plus a bunch more.

-O3 sets all those set by -O2 plus yet more . With -O3 you get a bit
more speed, but quite a lot more binary, from flags like
-finline-functions. 

-Os optimizes for size to reduce the space required for the resultant
binary. It sets the -O2 flags, but missing a few that cause the binary
to inflate.

-O0 switches off optimization and is the default.

Seb
-- 
Sebastian James - Director, Embedded Systems.
Hypercube Systems Ltd 'Embedded Linux Solutions'
35 Walkley Crescent Road, Sheffield, S6 5BA
Tel: 0845 4580277  Web: www.hypercubesystems.co.uk

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