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Re: where can I get gcc optimizations information
Juan Castillo writes:
> > 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.
You have to use -save-temps to get the assembly file. The dumps are
probably the most interesting part.
> 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)
It won't be. Without -O, you won't get any optimization.
> 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.
Andrew.