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softfloat: enforce softfloat if the host's FMA is broken

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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.

Reported-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Signed-off-by: Emilio G. Cota <cota@braap.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
This commit is contained in:
Emilio G. Cota 2018-12-25 02:03:05 -05:00 committed by Alex Bennée
parent b343eb462b
commit f6b3b108a8
1 changed files with 33 additions and 0 deletions
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33
fpu/softfloat.c
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@ -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;
}
}