Re: binary compiled with -O1 and w/ individual optimization flags are not the same

[Andrew Haley <aph at redhat dot com>]

CSights wrote:
> Hi,
> 	I'm trying to debug some mismatching results from a program compiled with 
> O1,2,3) and without (-O0 or nothing) optimization flags.
> 	My thought was to individually turn on optimization flags and see which one 
> changes the program's output.
> 	Unfortunately for this plan, the binaries produced using -O1 and those flags 
> said to be turned on by g++ in the manual (-fdefer-pop -fdelayed-branch
> -fguess-branch-probability -fcprop-registers -fif-conversion -fif-conversion2 
> -ftree-ccp -ftree-dce -ftree-dominator-opts -ftree-dse -ftree-ter -ftree-lrs 
> -ftree-sra -ftree-copyrename -ftree-fre -ftree-ch -funit-at-a-time -fmerge-constants) 
> do not match.

No, they wouldn't: without -O you get no optimizations.  Doesn't matter
what individual opt flags you put on the command line.

> I also tried without -fdelayed-branch b/c it is not supported 
> on my architectures (i386: athlon xp, core duo) and with -fomit-frame-pointer 
> in case that was the difference.
> 	I've tried this with g++ versions
> g++ (GCC) 4.2.3 (Debian 4.2.3-1) 
> and
> i686-apple-darwin8-g++-4.0.1 (GCC) 4.0.1 (Apple Computer, Inc. build 5363)
> 	Is there a way to have g++ tell me which flags it is actually using when 
> compiling a program.  E.g. expand -O1 to the individual optimization flags at 
> run time?

-save-temps -fverbose-asm puts all the optimizations into the .s file.

> 	Anyone have any other suggestions?

Being a bit more scientific, what is the difference between runs?

Andrew.