On Thu, 11 Oct 2018, Wilco Dijkstra wrote: > Hi, > > > 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). > > If these functions should set errno, there are many issues both in GCC and > GLIBC. GCC treats fma and remquo always as const/pure, so inlines fma > always even with -fmath-errno. GLIBC targets which support fma as a > single instruction always inline it as a single instruction, as expected. > > GCC incorrectly treats many other math functions as const/pure, for > example GCC "knows" that the C89 sin function will never set errno with > -fmath-errno:
That (for sin) is bug 80042. (While bug 37073 is enabling for FreeBSD the same optimizations as on Darwin and bug 64101 deals with a case where glibc's rules for errno setting on underflow do not require errno to be set, ISO C leaving that as implementation-defined, but some implementations, including the one used by the bug reporter, follow different rules that do involve setting errno.) I suspect the current settings in GCC are based on someone's observations of what one particular implementation did around 2003; they aren't based on the standard. > So if anything this is good evidence that nobody is using errno in actual > applications, otherwise these bugs would have been found many years ago. People *did* find such bugs, referenced above (but many uses likely would be such that the code movement in question does not occur and so the errno tests work anyway). I think it would be reasonable to move to a -fno-math-errno default that actually matches the standard semantics (various functions may set errno but aren't required to, so replacing with single instructions is OK - without requiring a fix for the existing functions wrongly marked as never setting errno). A stronger default (assume the functions never set errno) is appropriate on Darwin and FreeBSD (and potentially for some functions on some other platforms, if those platforms define no-errno-setting semantics for those functions rather than simply not setting errno by accident for them), and an explicit option could be used to enable such a stronger version on other platforms, and we could provide a way for libm implementations to declare that a function variant will never set errno. -- Joseph S. Myers jos...@codesourcery.com
