Re: target triplet

[Steven G. Johnson]

Bob Rossi wrote:
> I know the target triplet is the cpu-vendor-os triplet where the
> executable runs. Does changing the cpu from i386-pc-mingw32 to
> i686-pc-mingw32 actually change executable that is created? 

No (not unless your configure script does something unusual).

> If not, does anyone know where the correct place to ask about cpu
> specific gcc optimizations would be?

For some scientific/numerical code that I work on, I created a macro 
that tries fairly hard to guess the "native" architecture flag for gcc. 
  See:

   http://autoconf-archive.cryp.to/ax_gcc_archflag.html

The target triplet is not nearly enough information, unfortunately, at 
least not on x86.  On x86, the only reasonably comprehensive and 
portable thing I could find was to look at the output of the CPUID 
instruction (which doesn't work when cross-compiling, of course). 
Unfortunately, I found that information about the CPUID output is rather 
scattered (you'd think that there would be a web page or something that 
just gives a list of all the CPUID codes for the different Intel and AMD 
processors, but I haven't found anything even close to this).

By default, it uses the architecture type (which can be overridden by a 
--with-gcc-arch flag) only for an -mtune option so that the executable 
is still portable.  I usually add configure flag 
--enable/disable-portable-binary to switch this to -march if the user 
only wants to run the executable on a single target.

If you are not writing your own code, but are just compiling someone 
else's code, then you can usually override the compiler flags by setting 
CFLAGS (for a C program) or CXXFLAGS (for a C++ program), e.g.
   ./configure CFLAGS="-march=pentium4 -O3 -fomit-frame-pointer"
or whatever flags you want.

Steven G. Johnson