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authorEmilio G. Cota <cota@braap.org>2018-12-25 02:03:05 -0500
committerAlex Bennée <alex.bennee@linaro.org>2019-01-22 20:48:17 +0000
commitf6b3b108a808fe07f8a45f82bc88196f668cf627 (patch)
treec25d3a0c91ff44f25e6aa89313fb0da8c58da3f9 /fpu
parentb343eb462bcf825d60a29269054d54aff95b1722 (diff)
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softfloat: enforce softfloat if the host's FMA is broken
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>
Diffstat (limited to 'fpu')
-rw-r--r--fpu/softfloat.c33
1 files changed, 33 insertions, 0 deletions
diff --git a/fpu/softfloat.c b/fpu/softfloat.c
index 59eac97..9132d7a 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;
+ }
+}