| Message ID | 53C701C0.70503@twiddle.net |
|---|---|
| State | New |
| Headers | show |
On Wed, 16 Jul 2014, Richard Henderson wrote: > On 07/16/2014 01:56 PM, Joseph S. Myers wrote: > > On Thu, 3 Jul 2014, Richard Henderson wrote: > > > >> It seems to me that there's a typo on that exact zero test: a2 should be used, > >> not m2. Correct, or have I mis-read the code? > > > > The existing exact zero test seems correct to me. The conditions for such > > an exact zero are that the result of the multiplication is exactly > > representable in 53 bits (i.e., m2 == 0, with m1 being the exact result of > > the multiplication), and that the result of the addition (of z to the high > > part of the multipliation result) is an exact zero (i.e. a1 == 0, which > > implies a2 == 0). > > > > Thanks. This second version, then, does not s/m2/a2/, but merely adds the > appropriate barriers. It also makes sure that m2 is complete before clearing > inexact, even though I saw no evidence of it being scheduled after the call. > > Ok? It seems ldbl-96/s_fma.c, ldbl-96/s_fmal.c and ldbl-128/s_fmal.c could all have the same issue and so should have corresponding changes made.
diff --git a/sysdeps/ieee754/dbl-64/s_fma.c b/sysdeps/ieee754/dbl-64/s_fma.c index 389acd4..77065aa 100644 --- a/sysdeps/ieee754/dbl-64/s_fma.c +++ b/sysdeps/ieee754/dbl-64/s_fma.c @@ -198,16 +198,17 @@ __fma (double x, double y, double z) t1 = m1 - t1; t2 = z - t2; double a2 = t1 + t2; + /* Ensure the arithmetic is not scheduled after feclearexcept call. */ + math_force_eval (m2); + math_force_eval (a2); feclearexcept (FE_INEXACT); - /* If the result is an exact zero, ensure it has the correct - sign. */ + /* If the result is an exact zero, ensure it has the correct sign. */ if (a1 == 0 && m2 == 0) { libc_feupdateenv (&env); - /* Ensure that round-to-nearest value of z + m1 is not - reused. */ - asm volatile ("" : "=m" (z) : "m" (z)); + /* Ensure that round-to-nearest value of z + m1 is not reused. */ + z = math_opt_barrier (z); return z + m1; }