Segher Boessenkool <seg...@kernel.crashing.org> writes: > On Wed, Nov 20, 2019 at 05:30:48PM +0100, Jakub Jelinek wrote: >> On Wed, Nov 20, 2019 at 10:17:38AM -0600, Segher Boessenkool wrote: >> > On Wed, Nov 20, 2019 at 03:46:29PM +0000, Richard Sandiford wrote: >> > > Segher Boessenkool <seg...@kernel.crashing.org> writes: >> > > > UNLT & ORDERED is always LT. When would it not be true? >> > > >> > > LT traps on quiet NaNs for -ftrapping-math, UNLT and ORDERED don't. >> > >> > No? -ftrapping-math makes nothing trap. The only thing it does is to >> > not do optimisations that are not valid if traps are considered to be >> > a user-visible thing. >> > >> > Almost nothing ever traps on quiet NaNs. >> >> A lot traps even with quiet NaNs, assuming exceptions are enabled. >> E.g. for x86 https://www.felixcloutier.com/x86/cmppd#tbl-3-1 lists the >> details which operations rise FE_INVALID and which don't if it is enabled. > > That is what ordered comparisons (aka signaling comparisons) do, sure. > This is part of "almost nothing" in my count ;-) > > Ordered comparisons should trap both with and without -ftrapping-math. > The difference is that with -fno-trapping math GCC can ignore that and > just optimise code how it wants to.
Sure, no-one was disputing that. I think you're arguing against a strawman here. Richard
