On Mon, 8 Oct 2018, Richard Biener wrote: > So I think it would be fine if we'd have -fno-math-errno as documented > and then the C library would annotate their math functions according > to whether they will ever set errno or not. Once a math function is > const or pure it cannot ever set errno so -fno-math-errno shouldn't have > any effect.
Note that "will ever set errno" includes possibly setting it in the future, since code may be built with one libm version and used with another. So it wouldn't be correct to have a "never sets errno" attribute on glibc logb / lround / llround / lrint / llrint / fma / remquo (missing errno setting is a known bug). It also wouldn't be correct to have it on many complex.h functions, because while a requirement to set errno is never part of their interface, they may call real functions which may set errno (and may sometimes move from not setting it to setting it; e.g., they call the internal exp, sincos etc. functions, which for implementations using a wrapper do not set errno, but for implementations without a wrapper have errno setting integrated; whether this results in errno set by complex functions depends on details of exactly how they handle overflow etc. cases). (Also, directly setting const/pure on most libm functions runs into issues of what exactly that means regarding floating-point exceptions and rounding modes - and the need to avoid libm specifying a stricter attribute than the built-in function if that results in diagnostics from GCC. glibc specifies const on a few functions that don't raise exceptions or depend on the rounding mode, and even some of those are questionable if raising "invalid" for signaling NaNs is an issue.) > glibc has the ieee entry points but I'm not sure if it has any way > to use those by default. __ieee754_* have never been exported from shared libm. The old -lieee mechanism, no longer supported, involved libm function wrappers reading a writable global variable _LIB_VERSION to determine how to handle errors; depending on writable global data like that was itself obviously problematic. -- Joseph S. Myers jos...@codesourcery.com
