"Maciej W. Rozycki" <ma...@codesourcery.com> writes: > On Tue, 8 Jan 2013, Richard Sandiford wrote: >> >> > I disabled 64 bit FPU instructions by "-msoft-float". This works, but >> >> > using "-msingle-float" fails. This would be the better >> >> > configuration. There are still 64 bit FPU instructions used (e.g. "dmfc1 >> >> > $2,$f0" when using "long double" multiplication). So "-msingle-float" >> >> > doesn't seem to work on generic mips64-linux-gnu. >> >> >> >> Right. That combination hasn't really been defined. What happens >> >> for plain doubles? Do you pass those in FPRs or GPRs? >> > >> > IIUC the R5900 has an FPU that is functionally the same as that of the >> > R4640/R4650. If that is the case, then there is no way to pass doubles in >> > FPRs -- there is no room to store the upper halves. >> >> My point was that you could pass them in consecutive FPRs, like n32 does >> for long double. There's no architectural support for long double either, >> but the decision was still to pass them in FPRs rather than GPRs. > > You mean using a pair of FPRs (e.g. $f0/$f2) as a sum of two values of > different exponents for extra precision? That would make sense, but would > not match the way the double type has been defined in the ISO C standard > for IEEE-754 targets -- please note that contrariwise the standard > provides more freedom as to how the long double type can be implemented on > IEEE-754 targets.
No, I mean passing the two 32-bit halves in two FPRs, like we pass the two 64-bit halves of long doubles in two FPRs. Like I say... >> I'm not saying that that's a sensible precendent to copy. I was just >> using it as one example of why an ABI has to be defined. > > Not necessarily, the double type may simply be banned or alias to the > single type. Especially the latter -- such an arrangement is allowed by > ISO C as long as the target does not claim IEEE-754 compliance (we'd have > a problem with the Java frontend though) and I think such a compilation > mode might be permitted as long as it is useful to someone. But that's the point: we have to define what the rules are. The definition includes what isn't allowed. Richard
