J. Mayer wrote: [snip] > > > I can say both: > > > for most program, using floating point arithmetic ala "fast-math", it's > > > not necessary to maintain a precise FPU state, as those program will > > > never raise any FPU exception, never generate NaNs, infinites, ... > > > The other reason is that it would need to check every FPU insn arguments > > > and results at run time and treat all special cases following the actual > > > PowerPC implementations behavior if we want to get a precise emulation. > > > This behavior could be for example selected at compile time: then one > > > would have the choice to have a quick FPU emulation model or a precise > > > one. > > > > For mips I chose the middle ground: The emulation is architecturally > > correct but may not reflect FPU behaviour of the specific silicon. > > E.g. one effect is that in certain cases the emulation computes values > > close to underflow, while real hardware would throw the (mips FPU > > specific) unimplemented exception. > > > > For most cases this should be good enough, since only specialized > > software will rely on a specific implementation's oddities. > > Well, what you've done for Mips is exactly what I called the "precise > emulation" and is far slower than the "fast math" emulation I got for > PowerPC. I was wrong talking about "PowerPC implementations" when I > should have said "PowerPC specification"; but there should be no > difference between the two (or it's not a PowerPC CPU...) because the > POWER/PowerPC specification describes very precisely the behavior of the > FPU. > > The "fast math" model relies on the native-softmmu code and is suficient > for most applications. But there are a few instructions that should > always take care (or maybe at least reset) the FPSCR register, which is > not done in the current code.
My theory is that occasional FP users won't suffer from the performance impact, and heavy FP users are likely to expect IEEE conformance. Thus I gave priority to correctness. Implementing a R8000-style fast FP mode sounds like fun, but for the moment I have already enough unfinished bits and pieces in Qemu. :-) Thiemo
