> 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. -------------------------------------------------------------------- Juan Castillo Microelectronics Engineering Group. Dep. TEISA University of Cantabria. Spain. E-mail: castillo@xxxxxxxxxxxxxxx --------------------------------------------------------------------
