Date: Mon, 18 Nov 2013 00:13:25 +0100 From: Szabolcs Nagy <nsz@...t70.net> To: musl@...ts.openwall.com Subject: Re: math_errhandling definition * Bobby Bingham <koorogi@...rogi.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
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