Re: where can I get gcc optimizations information

[Andrew Haley <aph at redhat dot com>]

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.