Peter Soetens wrote: > I was wondering why the above gcc parameter does not enable the use > of the fst/fld opcodes for pentium processors, while -march=i686 > does. The Intel manuals specifically say that they can be used across > all pentium processors.
There are 2 options to tell the compiler about your wanted processor. The -march=xyz option tells you the instruction set to use, while the -mcpu=xyz option tells you for which processor the program should run fastest. If you supply the -march option, but not the -mcpu option, then the compiler will assume you use the same processor for both. The difference in the code you see are actually due to the -mcpu option. For your first code example, you implicitly use -mcpu=586 and for the second example, you use -mcpu=686. So your first code is supposed to run fastest on a Pentium class processor while your second code is supposed to run fastest on a Pentium2 class processor. Now, an a Pentium processor, the FLD and FST instructions are (relatively) expensive. So the compiler decides it is faster to do load/store operations using integer registers. On Pentium2 class processors, the FLD and FST instructions are much faster, and now the compiler considers it worthwhile to use them. Now if you want to generate code that will be guaranteed to run on Pentium processors, but runs best on Pentium2 class processors, you have to use both the options -march=pentium and -mcpu=pentium2 (you can also use 586 and 686 which are aliases, but I would recommend you to use real processor names) Of course, as Pentium2 processors are not so common any more either, you can also tune your code for Pentium4 using -mcpu=pentium4, or for AMD Athlon processors using -mcpu=athlon or some specific athlon model. Note that on newer versions of GCC (starting with 3.4.0), the -mcpu option has been deprecated and replaced by the -mtune option to be consistent with other processor architectures supported by GCC. -- Marcel Cox (using XanaNews 1.17.2.4)
