Re: math_errhandling definition

[Szabolcs Nagy <nsz@port70.net>]

* Bobby Bingham <koorogi@koorogi.info> [2013-11-17 15:54:54 -0600]:
> I think the definition of math_errhandling is wrong on a couple
> architectures.  C99 (7.12) says:
> 
>     If the expression math_errhandling & MATH_ERREXCEPT can be nonzero,
>     the implementation shall define the macros FE_DIVBYZERO, FE_INVALID,
>     and FE_OVERFLOW in <fenv.h>.
> 
> math.h always defines math_errhandling as 2 (MATH_ERREXCEPT), but
> whether those FE_* macros are defined or not is architecture-dependent.
> In particular, ARM only defines them if __ARM_PCS_VFP is defined, and
> microblaze does not define them.
> 
> It looks like if the architecture doesn't support floating point
> exceptions, math_errhandling must be MATH_ERRNO. This in turn requires
> that the various math functions set errno appropriately, which it
> doesn't look like musl's do.

yes, but..

musl aims to support annex f

annex f requires math_errhandling & MATH_ERREXCEPT to be true
ie math errors are reported as floating-point exceptions
according to ieee 754 semantics

annex f also requires all five ieee status flags to be defined
FE_OVERFLOW, FE_UNDERFLOW, FE_DIVBYZERO, FE_INVALID, FE_INEXACT

(c99 requires that unsupported flags are not defined and has
weaker requirements without annex f as you quoted)

unfortunately c99 made it (nearly) impossible to implement
floating-point with ieee exception semantics in software and
indeed gcc does not provide exceptions for fpu-less archs
(unless the underlying arch has hw registers for it)

this is fixed in c11 which changed signal handler semantics wrt
the floating-point environment and introduced thread storage
duration (however libgcc does not implement fp status flags with
thread storage duration just yet)

musl can only support annex f if the underlying hardware
(or on fpu-less archs the compiler or kernel fp emulation)
support ieee754

in short this should be reported against libgcc

in theory we could support MATH_ERRNO on such platforms, but
it would be a significant effort and still would not give useful
fp semantics other than slightly stricter c99 conformance