https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112449
--- Comment #5 from post+gcc at ralfj dot de --- > See > https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#index-fsignaling-nans That's unrelated. That's about whether operation on signaling NaNs can trap. I am asking when operations can output a signaling NaN. So, for code like float x = y <op> z; return is_signaling_nan(x); when can that code return `true`? Normal IEEE semantics would say "never". And yet if "z" is the constant 1, <op> is `*`, and "y" is a signaling NaN, then this evidently can output a signaling NaN. I would hope the answer is "this can output a signaling NaN only if one of the inputs is a signaling NaN", but is that documented anywhere? > Note mips and sh and a few other targets have the quiet bit meaning the > opposite. I know. LLVM is currently buggy on those targets. > GCC does document some of this on > https://gcc.gnu.org/onlinedocs/gcc-13.2.0/gcc/Floating-point-implementation.html > but not the signaling nan part. This seems to list a bunch of implementation-defined aspects of C? To my knowledge, my question is not implementation defined. C (with the annex for floating-point arithmetic) requires the above operations to always return "false". GCC violates the C spec here (since it defines __STDC_IEC_559__, declaring support for the annex), and it'd be good to know how far it is going in that violation.
