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Re: [Qemu-devel] [RFC PATCH] fpu: add compile time check for old glibc/l
From: |
Emilio G. Cota |
Subject: |
Re: [Qemu-devel] [RFC PATCH] fpu: add compile time check for old glibc/libm and fma |
Date: |
Fri, 21 Dec 2018 14:30:37 -0500 |
User-agent: |
Mutt/1.9.4 (2018-02-28) |
On Thu, Dec 20, 2018 at 11:10:08 +0000, Alex Bennée wrote:
(snip)
> +#if defined(__GLIBC__) && (__GLIBC__ == 2 && __GLIBC_MINOR__ <= 12)
> +#define QEMU_HARDFLOAT_USE_FMA 0
> +#else
> +#define QEMU_HARDFLOAT_USE_FMA 1
> +#endif
> +#else
> +#define QEMU_HARDFLOAT_USE_FMA 1
> +#endif
> +
> /*
> * QEMU_HARDFLOAT_USE_ISINF chooses whether to use isinf() over
> * float{32,64}_is_infinity when !USE_FP.
> @@ -1551,6 +1570,9 @@ float32_muladd(float32 xa, float32 xb, float32 xc, int
> flags, float_status *s)
> ub.s = xb;
> uc.s = xc;
>
> + if (!QEMU_HARDFLOAT_USE_FMA) {
> + goto soft;
> + }
I don't think this should be a compile-time check; if the QEMU binary
is run on a system with a newer, fixed glibc (or any other libc), then
we'll have disabled fma hardfloat unnecessarily.
What do you think about the following?
Laurent: if you want to test the below, you can pull it from
https://github.com/cota/qemu/tree/fma-fix
Thanks,
Emilio
---
commit ddeec29a2c33550c5d018aeea05d45a23579ae1b
Author: Emilio G. Cota <address@hidden>
Date: Fri Dec 21 14:08:57 2018 -0500
softfloat: enforce softfloat if the host's FMA is broken
The added branch is marked as unlikely and therefore its impact
on performance (measured with fp-bench) is within the noise range
when measured on an Intel(R) Xeon(R) Gold 6142 CPU @ 2.60GHz.
Laurent Desnogues <address@hidden>
Signed-off-by: Emilio G. Cota <address@hidden>
diff --git a/fpu/softfloat.c b/fpu/softfloat.c
index 59eac97d10..8b3670ca9d 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 host_fma_is_broken;
+
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(host_fma_is_broken)) {
+ 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(host_fma_is_broken)) {
+ 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 +7986,25 @@ 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 = 0x0020000000000001;
+ ub.s = 0x3ca0000000000000;
+ uc.s = 0x0020000000000000;
+ ur.h = fma(ua.h, ub.h, uc.h);
+ if (ur.s != 0x0020000000000001) {
+ host_fma_is_broken = true;
+ }
+}