fenv_t should include architecture-specific floating-point modes and
status flags. i386 and x86_64 fesetenv limit which bits they use from
the x87 status and control words, when using saved state, and limit
which parts of the state they set to fixed values, when using
FE_DFL_ENV / FE_NOMASK_ENV. The following should be included but are
excluded in at least some cases: status and masking for the "denormal
operand" exception (which isn't part of FE_ALL_EXCEPT); precision
control (explicitly mentioned in Annex F as something that counts as
part of the floating-point environment); MXCSR FZ and DAZ bits (for
FE_DFL_ENV and FE_NOMASK_ENV). This patch arranges for this extra
state to be handled by fesetenv (and thereby by feupdateenv, which
calls fesetenv).
(Note that glibc functions using floating point are not generally
expected to work correctly with non-default values of this state,
especially precision control, but it is still logically part of the
floating-point environment and should be handled as such by fesetenv.
Changes to the state relating to subnormals ought generally to work
with libm functions when the arguments aren't subnormal and neither
are the expected results; that's a consequence of functions avoiding
spurious internal underflows.)
A question arising from this is whether FE_NOMASK_ENV should or should
not mask the "denormal operand" exception. I decided it should mask
that exception. This is the status quo - previously that exception
could only be unmasked by direct manipulation of control registers
(possibly via <fpu_control.h>). In addition, it means that use of
FE_NOMASK_ENV leaves a floating-point environment the same as could be
obtained by fesetenv (FE_DFL_ENV); feenableexcept (FE_ALL_EXCEPT);,
rather than an environment in which an exception is unmasked that
could only be masked again by using fesetenv with FE_DFL_ENV (or a
previously saved environment) - this exception not being usable with
other <fenv.h> functions because it's outside FE_ALL_EXCEPT.
Tested for x86_64 and x86.
[BZ #16068]
* sysdeps/i386/fpu/fesetenv.c: Include <fpu_control.h>.
(FE_ALL_EXCEPT_X86): New macro.
(__fesetenv): Use FE_ALL_EXCEPT_X86 in most places instead of
FE_ALL_EXCEPT. Ensure precision control is included in
floating-point state. Ensure that FE_DFL_ENV and FE_NOMASK_ENV
handle "denormal operand exception" and clear FZ and DAZ bits.
* sysdeps/x86_64/fpu/fesetenv.c: Include <fpu_control.h>.
(FE_ALL_EXCEPT_X86): New macro.
(__fesetenv): Use FE_ALL_EXCEPT_X86 in most places instead of
FE_ALL_EXCEPT. Ensure precision control is included in
floating-point state. Ensure that FE_DFL_ENV and FE_NOMASK_ENV
handle "denormal operand exception" and clear FZ and DAZ bits.
* sysdeps/x86/fpu/test-fenv-sse-2.c: New file.
* sysdeps/x86/fpu/test-fenv-x87.c: Likewise.
* sysdeps/x86/fpu/Makefile [$(subdir) = math] (tests): Add
test-fenv-x87 and test-fenv-sse-2.
[$(subdir) = math] (CFLAGS-test-fenv-sse-2.c): New variable.
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