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Re: where can I get gcc optimizations information
> Juan Castillo writes:
> > I need some help with gcc optimizations and I don't know where I
> > can get it.
> >
> > The objective of my work is to estimate how many machine
> > instructions I can save in a program execution employing compiler
> > optimizations. I am working with ARM architecture. Anyway, I have
> > understood that main gcc optimizations are
> > architecture-independent.
> >
> > In the gcc user's manual I have read that flags -O, -O1, -O2, -O3
> > and -Os activate different optimization flags (starting with "-f",
> > I think), but not all of them are available individually. If I
> > switch on all flags available individually for -O1 (-fdefer-pop,
> > -fmerge-constants...) they have no effect in obtained machine code,
> > but if I compile with -O1 option, there is a reduction of about 50%
> > in machine instructions number with respect to basic compilation.
> >
> > With these results, I suppose that -O1 performs more actions that
> > are not reported in any document I have found in The Internet. I
> > have downloaded the gcc source-code but it is too difficult for me
> > to understand it. My question is: where can I get full information
> > about gcc optimizations and how they are performed?
>
> Full information is in the source. As Euclid (or someone else!) may
> once have said, "There is no royal road to mathematics". If you are
> really to understand, you'll have to get in there some time.
>
> However, some things will help you. -fverbose-asm produces a list of
> optimization passes. The dumps "-da" and "-fdump-tree-all" produce a
> lot of files, one per optimization pass, so you can see what each pass
> did. Once you know which pass does what, you can start to look at the
> code.
>
Thanks for your answer.
When using "-fverbose-asm" flag in the commmand line to produce the list of
optimization passes, I suppose you mean adding the "-S" flag to see the
assemble file, don't you? I say this because using the flag in a common
compilation doesn't produce any change.
Anyway, taking a look at the assemble file, I can see the flags employed when
I compile with -O1 option, but if I introduce them all in the command line
instead of -O1 (it is supposed that the executable file would be the same),
optimizations are not performed. What am I doing wrong? I have read source
code and -O1 flag activates some other flags and it seems it does not any
more.
> > And another question: would be possible to get a benchmark or
> > similar for compiler optimizations? That is, some set of programs
> > that estimate the save that a compiler can achieve. I know it
> > depends on the algotihms compiled, employed architecture and more
> > (please, don't remember me that...). I just need an
> > estimation. That would be great for my work.
>
> SPEC is a standard, but it's unfree and therefore off-topic here.
>
> Andrew.
Thank you very much.
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Juan Castillo
Microelectronics Engineering Group. Dep. TEISA
University of Cantabria. Spain. E-mail: castillo@teisa.unican.es
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