From: Michael Hudson-Doyle <michael.hudson@canonical.com>
To: libc-alpha@sourceware.org
Subject: [PATCH] Ensure calculations happen with desired rounding mode in y1lf128
Date: Fri, 12 Aug 2022 12:05:10 +1200 [thread overview]
Message-ID: <20220812000510.1595861-1-michael.hudson@canonical.com> (raw)
math/test-float128-y1 fails on x86_64 and ppc64el with gcc 12 and -O3,
because code inside a block guarded by SET_RESTORE_ROUNDL is being moved
after the rounding mode has been restored. Use math_force_eval to
prevent this (and insert some math_opt_barrier calls to prevent code
from being moved before the rounding mode is set).
Fixes #29463
---
I don't know if this patch should be committed as is, although it does
fix an observed failure for me.
My read of the discussion in the gcc bug I filed about this
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106574 is that every single
use of a SET_RESTORE_ROUND macro is vulnerable to this. I can't think of
a generic way to fix this -- I guess you could have a macro to call an
always_inline function with a rounding mode set? But that would uglify
the control flow in the code quite a bit.
---
sysdeps/ieee754/ldbl-128/e_j1l.c | 3 +++
1 file changed, 3 insertions(+)
diff --git a/sysdeps/ieee754/ldbl-128/e_j1l.c b/sysdeps/ieee754/ldbl-128/e_j1l.c
index 54c457681a..9a9c5c6f00 100644
--- a/sysdeps/ieee754/ldbl-128/e_j1l.c
+++ b/sysdeps/ieee754/ldbl-128/e_j1l.c
@@ -869,10 +869,13 @@ __ieee754_y1l (_Float128 x)
{
/* 0 <= x <= 2 */
SET_RESTORE_ROUNDL (FE_TONEAREST);
+ xx = math_opt_barrier (xx);
+ x = math_opt_barrier (x);
z = xx * xx;
p = xx * neval (z, Y0_2N, NY0_2N) / deval (z, Y0_2D, NY0_2D);
p = -TWOOPI / xx + p;
p = TWOOPI * __ieee754_logl (x) * __ieee754_j1l (x) + p;
+ math_force_eval (p);
return p;
}
--
2.34.1
next reply other threads:[~2022-08-12 0:05 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-08-12 0:05 Michael Hudson-Doyle [this message]
2022-08-12 12:28 Wilco Dijkstra
2022-08-15 20:59 ` Joseph Myers
2022-08-17 4:23 ` Michael Hudson-Doyle
2022-08-17 11:53 ` Wilco Dijkstra
2022-08-17 16:52 ` Joseph Myers
2022-08-22 12:28 ` Paul Zimmermann
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