Ensure calculations happen with desired rounding mode in y1lf128

Message ID 20220812000510.1595861-1-michael.hudson@canonical.com
State Committed, archived
Series Ensure calculations happen with desired rounding mode in y1lf128 |


Context Check Description
dj/TryBot-apply_patch success Patch applied to master at the time it was sent

Commit Message

Michael Hudson-Doyle Aug. 12, 2022, 12:05 a.m. UTC
  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 */
+      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;