------- Comment #14 from jakub at gcc dot gnu dot org 2008-10-15 09:08 -------
The problem is that thread_across_edge figures out that the fabs (al[0] -
al[1])
< 1.e-5 test is unnecessary, always yields false for +-1.0, by substituting the
values in record_temporary_equivalences_from_stmts_at_dest, but doesn't
actually optimize all the computations to constants. If the threading
duplicated block is used just for one constant, not two, then fab optimizes
the __builtin_pow call into a constant and dom2 optimizes the rest into a
constant. But as we have two different constants leading to the same block,
nothing in GCC optimizes it out, and given -fno-signed-zeros and the testcase
very much depending on the sign of zeros, the outcome is different from what
the compiler expected. While GCC perhaps could optimize:
# cn_43 = PHI <-1.0e+0(3), 1.0e+0(9)>
D.1267_44 = __builtin_pow (cn_43, 2.0e+0);
D.1268_46 = 1.0e+0 - D.1267_44;
D.1269_47 = __builtin_sqrt (D.1268_46);
into a constant with some smarter fab or dom hack for multiple constants, it
can't already optimize the following:
D.1270_49 = __builtin_atan2 (D.1269_47, cn_43);
or
D.1270_49 = __builtin_atan2 (0.0, cn_43);
because that yields different values for -1 and 1. While even the partial
optimization would cure this testcase and is perhaps an useful enhancement,
I believe it is just wrong to compile this part of calculix with -ffast-math
and you get what you deserve.
`-fno-signed-zeros'
Allow optimizations for floating point arithmetic that ignore the
signedness of zero. IEEE arithmetic specifies the behavior of
distinct +0.0 and -0.0 values, which then prohibits simplification
of expressions such as x+0.0 or 0.0*x (even with
`-ffinite-math-only'). This option implies that the sign of a
zero result isn't significant.
is quite clear and this testcase definitely relies on +0 vs. -0 difference
heavily.
So I'd say this should be closed as INVALID.
--
jakub at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |law at gcc dot gnu dot org
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37449
