Hi Emilio, On 12/25/18 8:03 AM, Emilio G. Cota wrote: > The added branch to the FMA ops is marked as unlikely and therefore > its impact on performance (measured with fp-bench) is within noise range > when measured on an Intel(R) Xeon(R) Gold 6142 CPU @ 2.60GHz.
Did you forget to write here the result of your measurements? > > Reported-by: Laurent Desnogues <[email protected]> Suggested-by: Richard Henderson <[email protected]> > Signed-off-by: Emilio G. Cota <[email protected]> Reviewed-by: Philippe Mathieu-Daudé <[email protected]> > --- > v2: drop the cpuid bits, since that work (1) is only a performance > optimization, not a correctness fix and (2) requires additional > consolidation work that I do not have time to do right now. > > fpu/softfloat.c | 33 +++++++++++++++++++++++++++++++++ > 1 file changed, 33 insertions(+) > > diff --git a/fpu/softfloat.c b/fpu/softfloat.c > index 59eac97d10..9132d7a0b0 100644 > --- a/fpu/softfloat.c > +++ b/fpu/softfloat.c > @@ -1542,6 +1542,8 @@ soft_f64_muladd(float64 a, float64 b, float64 c, int > flags, > return float64_round_pack_canonical(pr, status); > } > > +static bool force_soft_fma; > + > float32 QEMU_FLATTEN > float32_muladd(float32 xa, float32 xb, float32 xc, int flags, float_status > *s) > { > @@ -1562,6 +1564,11 @@ float32_muladd(float32 xa, float32 xb, float32 xc, int > flags, float_status *s) > if (unlikely(!f32_is_zon3(ua, ub, uc))) { > goto soft; > } > + > + if (unlikely(force_soft_fma)) { > + goto soft; > + } > + > /* > * When (a || b) == 0, there's no need to check for under/over flow, > * since we know the addend is (normal || 0) and the product is 0. > @@ -1623,6 +1630,11 @@ float64_muladd(float64 xa, float64 xb, float64 xc, int > flags, float_status *s) > if (unlikely(!f64_is_zon3(ua, ub, uc))) { > goto soft; > } > + > + if (unlikely(force_soft_fma)) { > + goto soft; > + } > + > /* > * When (a || b) == 0, there's no need to check for under/over flow, > * since we know the addend is (normal || 0) and the product is 0. > @@ -7974,3 +7986,24 @@ float128 float128_scalbn(float128 a, int n, > float_status *status) > , status); > > } > + > +static void __attribute__((constructor)) softfloat_init(void) > +{ > + union_float64 ua, ub, uc, ur; > + > + if (QEMU_NO_HARDFLOAT) { > + return; > + } > + /* > + * Test that the host's FMA is not obviously broken. For example, > + * glibc < 2.23 can perform an incorrect FMA on certain hosts; see > + * https://sourceware.org/bugzilla/show_bug.cgi?id=13304 > + */ > + ua.s = 0x0020000000000001ULL; > + ub.s = 0x3ca0000000000000ULL; > + uc.s = 0x0020000000000000ULL; > + ur.h = fma(ua.h, ub.h, uc.h); > + if (ur.s != 0x0020000000000001ULL) { > + force_soft_fma = true; > + } > +} >
