Re: Improving math function wrappers

[Szabolcs Nagy <szabolcs dot nagy at arm dot com>]

On 16/03/17 18:24, Joseph Myers wrote:
> On Thu, 16 Mar 2017, Szabolcs Nagy wrote:
>> On 16/03/17 14:29, Joseph Myers wrote:
>>> On Thu, 16 Mar 2017, Wilco Dijkstra wrote:
>>>
>>>> Given other languages don't even have the concept of errno, C99 doesn't 
>>>> require it, and no application ever reads errno, it is inefficient to 
>>>> force the use of the wrappers in almost all cases.
>>>
>>> C99 not requiring it was an incompatible quiet change from C90.
>>
>> i think glibc only has to set errno when user code is
>> compiled with c89 compatibility and even then only for
>> math functions that were defined in c89 and then only
>> guarantee correct errno setting in default rounding mode
>> (c89 code cannot access the fenv).
> 
> Incompatible quiet change means a good idea to implement the compatible 
> mode ((math_errhandling & MATH_ERRNO) != 0) as far as possible.  Just like 
> it's a good idea to avoid size_t wider than unsigned long.

i think this would be a valid argument if

 (math_errhandling & MATH_ERRNO) != 0

was the only way to get backward compatibility,
but that's not the case: errno can be clobbered
any time and thus c89 backward compatibility can
be maintained with

 (math_errhandling & MATH_ERRNO) == 0

in which case there is no requirement for non-c89
math functions to set errno (or getting errno
right in non-default fenv) and thus the wrapper
can be removed from all single and long double
precision functions (and kept for double prec
c89 functions if glibc cares about compatibility).

> Compilers for non-C languages without the concept of these functions 
> setting errno can of course default to -fno-math-errno by default (and 
> built-in no-errno functions that can be used with -fmath-errno is 
> otherwise enabled would also be desirable for fma, sqrt, etc.).

then fortran can only use no-errno functions
when those can be inlined, since extern calls
use the standard libc symbols which set errno
in glibc.