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.
