Am 28.06.2012 01:00, schrieb Catalin Patulea: > Hey guys, > > I've been hacking up the FPREM and FPREM1 i386 instructions (without KVM) to > be implemented using SoftFloat only. This was motivated by some correctness > issues > that we noticed with the current implementation which follows the AMD > datasheet.I believe the datasheet explains the operation as if intermediate > results had "infinite" precision, but in this case intermediate rounding > causes a loss of precision at the final output. > > My reference was TestFloat [1], specifically the floatx80_rem suite, which > tests the FPREM instruction quite thoroughly (FPREM1 is technically untested > but very similar). The current code fails about half the suite; with the > patch, all tests pass. > > There are still lots of dark corners - the code originates from Bochs' copy > of SoftFloat and I tried to port the Bochs exception handling logic as much > as I could. Many details still elude me though (see comments in the code). > TestFloat does test some of the exception logic but not as thoroughly as I > would have liked. If anyone can offer some guidance here, I am happy to fix > up the patch. > > That's about it.. let me know what you think. > > Catalin > > [1] http://www.jhauser.us/arithmetic/TestFloat.html > > --- > fpu/softfloat.c | 182 > +++++++++++++++++++++++++++++++++++++++++++++++ > fpu/softfloat.h | 3 + > target-i386/op_helper.c | 163 +++++++++++++++++------------------------- > 3 files changed, 249 insertions(+), 99 deletions(-) > > diff --git a/fpu/softfloat.c b/fpu/softfloat.c > index b29256a..9c6e4a3 100644 > --- a/fpu/softfloat.c > +++ b/fpu/softfloat.c > @@ -5234,6 +5234,16 @@ int floatx80_unordered_quiet( floatx80 a, floatx80 b > STATUS_PARAM ) > } > > > /*---------------------------------------------------------------------------- > +| Returns 1 if the extended double-precision floating-point value `a' is an > +| unsupported; otherwise returns 0. > +*----------------------------------------------------------------------------*/ > +int floatx80_is_unsupported(floatx80 a) > +{ > + return (extractFloatx80Exp(a) && > + !(extractFloatx80Frac(a) & LIT64(0x8000000000000000))); > +}
Disclaimer: It's been a while that I had to learn IEEE 754. Unsupported for some i386 instruction? Or unsupported in the SoftFloat library due to some limitation? Please run scripts/checkpatch.pl for CODING_STYLE issues, I spotted one slightly misplaced if brace, and one empty line at the bottom seemed to have indentation (git-am complains about that, too). The patch description will need to be cleaned up (not be letter-style). I'm not too thrilled to introduce more uses of int32 (we started converting int16 to int_fast16_t) but I won't veto. It would be nice if you could review for any potential int32 vs. int32_t breakages though, to not make things worse than they are. Regards, Andreas -- SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg, Germany GF: Jeff Hawn, Jennifer Guild, Felix Imendörffer; HRB 16746 AG Nürnberg
