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. Segher
