Thanks to all who have responded so far. Thanks to Martin for the -mfloat-abi=softfp option as that could be useful to us.
On Jan 16, 2010, at 6:23 AM, David Edelsohn wrote: > On Fri, Jan 15, 2010 at 6:42 PM, Robert Grimm <rfgr...@gmail.com> wrote: >> Greetings all, >> >> I'm working with the powerpc-eabi architecture (specifically, the MPC563 >> processor). For some time we have been using GCC 3.4.3 and I noticed gcc >> generating code that makes use of the floating point registers for 64-bit >> integer loads and whatnot... which we don't want it to do as there are times >> when we disable the FPU. I'm aware of the -msoft-float option but we didn't >> want to use that as we only wanted it to not use FP registers in certain >> functions and not entire files. I was under the impression that a newer GCC >> wouldn't do this based on some old mailing posts I've read. I've just tried >> out GCC 4.4.2 and notice that the FP registers are still being used even >> though there is no explicit FPU usage going on in this particular part of >> the code. >> >> Is there a way to get GCC to only use the FPU when we explicitly want to use >> it (i.e. when we use doubles/floats)? Is -msoft-float my only option here? >> Is there any sort of #pragma that could do the same thing as -msoft-float (I >> didn't see one)? > > Recent versions of GCC greatly decrease the use of FPRs for > non-floating point code, but do not prevent it. > > To absolutely prevent use of FPRs, one must use -msoft-float. The > hard-float and soft-float ABIs are incompatible and one cannot mix > object files. > I get why one cannot mix hard/soft object files if you have functions whose interfaces use floats/doubles/uint64_ts, but what about those that don't. In other words, if I had one hard-float object file that depends on a soft-float object file but no floats, doubles or 64-bit values are passed in between, is that okay? One thing we've tried is to compile only certain libraries as soft-float as long as those libraries don't contain any float/double/64-bit interfaces. It seems to work but now I'm wondering if we're playing with fire and that we've just been lucky. > GCC is a compiler, not an assembler. It tries to optimize programs, > not read programmers' minds. If the person invoking the compiler does > not deny the resource to the compiler, it will try to use it when > profitable, which is what it should do. > > David I'm not asking it to read my mind. I'm asking for a "-fonly-use-the-FPU-when-you-see-a-float-or-a-double-and-not-when-you-see-a-uint64_t" flag. This would be very useful for those of us, as Joel Sherill pointed out, leave the FPU disabled at times and use uint64_t's inside interrupt service routines. Actually, I saw some old posts that talked about a -fno-implicit-fp option which seemed to be just what I want. Any reason why that was abandoned or not considered? Thanks again to all for your help, Rob
