On Fri, May 8, 2020 at 4:38 PM Richard Biener <rguent...@suse.de> wrote: > > On May 8, 2020 4:28:24 PM GMT+02:00, Alexander Monakov <amona...@ispras.ru> > wrote: > >On Fri, 8 May 2020, Uros Bizjak wrote: > > > >> > Am I missing something? > >> > >> Is the above enough to declare min/max as IEEE compliant? > > > >No. SSE min/max instructions semantics match C expression x < y ? x : > >y. > >IEEE min/max operations are commutative when exactly one operand is a > >NaN, > >and so are C fmin/fmax functions: > > > > fmin(x, NaN) == fmin(NaN, x) == x // x is not a NaN > > > >In contrast, (x < y ? x : y) always returns y when x or y is a NaN, and > >likewise the corresponding SSE instructions are not commutative. > > > >Therefore they are explicitly non-compliant in presence of NaNs. > > > >I don't know how GCC defines the semantics of GIMPLE min/max IFNs. > > The IFNs are supposed to match fmin and fmax from the C standard which IIRC > have IEEE semantics. > > Note the ISA likely behaves this way because it matches open coded C > semantics. > > Arm folks added the IFNs so I have to dig up what exactly they were after... > > I'd hate to add a third variant here...
So, I found [1], that tries to explain this issue. [1] https://2pi.dk/2016/05/ieee-min-max Uros.
