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Re: [PATCH] improves exp() and expf() performance on Sparc.
On 9/1/2017 6:13 PM, Joseph Myers wrote:
You're defining ifuncs for exp and expf (rather than __ieee754_exp,
__exp_finite etc. as on x86_64). But you're not doing anything to stop
the w_exp_compat / w_expf_compat wrappers from being built that also
define exp and expf, so I don't see how that can work without ending up
with multiple definitions of exp and expf; I'd expect you to need to
override the wrappers with empty files in such a case of a function
implementation with all the error handling integrated.
The sysdeps/ieee754/dbl-64/w_exp_compat.c
declares __exp (double x)
and then adds:
hidden_def (__exp)
weak_alias (__exp, exp)
I believe the weak_alias in w_exp_compat.c is overriden by the
sparc_libm_ifunc in e_exp-generic.c. At least, I am not seeing any
link time errors about double exp declarations and I am seeing the new
code being executed (as proved by the speed and accuracy changes).
I'm also concerned that you have local matherr handling which is not
compatible with all the cases in __kernel_standard (which are not well
tested). If you need to have your own integrated error handling for
performance reasons, matherr handling should be bug-compatible with the
existing code, for both overflow and underflow. (Or define a new symbol
version, make the existing exp and expf into compat symbols for SPARC and
then your new version only needs to handle errno setting, not matherr.)
As for error handling, I believe the extra level of indirection on
return from exp provided by the sysdeps/ieee754/dbl-64/w_exp_compat.c
routine is an anti-performance design. Every normal return from e_exp
requires an extra level of indirection and several tests/branches that
should be bypassed unless an error is detected. The e_exp code already
checks for underflow/overflow/NAN/etc. Those unnecessary tests for the
non-exceptional cases add overhead for the most frequent uses of exp().
On the other hand, once an error occurs that triggers__set_err_exp,
extra overhead is not a significant performance issue. I will add a
call __kernel_standard in that routine when _LIB_VERSION is not _IEEE_
or _POSIX_. As you point out, that will provide more consistent error
handling behavior. I'll make that change in my next exp patch
submission.
- patrick