Ok! Thanks; sorry for the misunderstanding on my side. --matt
On Sat, Aug 21, 2021 at 2:53 PM Stefan Kanthak <stefan.kant...@nexgo.de> wrote: > Matt Godbolt <m...@godbolt.org> wrote: > > > I believe your example doesn't take into account that the values can be > NaN > > which compares false in all situations. > > That's a misbelief! > Please notice the first if-clause, which rules out NaNs for both arguments. > Also notice that GCC did NOT generate JP after the 4 COMISD instructions > in question, i.e. it knew that NaNs had been ruled out. > I included the 3 initial if-clauses just to give GCC enough rope to hang > himself. > > > If you allow the compiler to > > optimize without supporting NaN (-ffast-math), I think it generates the > > code you want: https://godbolt.org/z/1ra7zcsnd > > Replace > if (isnan(argx) || isnan(argy)) return argx + argy; > with > if ((argx != argx) || (argy != argy)) return argx + argy; > then feed the changed snippet to compiler explorer again, with and without > -ffast-math > > Stefan > > > --matt > > > > On Sat, Aug 21, 2021 at 1:59 PM Stefan Kanthak <stefan.kant...@nexgo.de> > > wrote: > > > >> Hi, > >> > >> the following snippet is from the nextafter() function of > >> <http://www.netlib.no/netlib/toms/722> > >> > >> --- repro.c --- > >> #define Zero 0.0 > >> double nextafter(double argx, double argy) > >> { > >> double z = argx; > >> > >> if (isnan(argx) || isnan(argy)) return argx + argy; > >> > >> if (argx == argy) return argx; > >> > >> if (argx != Zero) > >> if (((argx < Zero) && (argx < argy)) > >> || ((argx > Zero) && (argx > argy))) > >> z += 1.0; > >> else > >> z -= 1.0; > >> return z; > >> } > >> --- EOF --- > >> > >> I expect that GCC knows DeMorgan's rules and is able to > >> simplify/optimize the last if-statement to > >> > >> if ((argx < Zero) == (argx < argy)) > >> > >> Unfortunately GCC fails to do so: see the lines from > >> label .L20: to label L7 > >> > >> $ gcc -m64 -O3 -o- -S > >> ... > >> nextafter: > >> ucomisd %xmm1, %xmm0 > >> jp .L19 > >> pxor %xmm2, %xmm2 > >> movl $1, %edx > >> ucomisd %xmm2, %xmm0 > >> setp %al > >> cmovne %edx, %eax > >> testb %al, %al > >> je .L3 > >> ucomisd %xmm1, %xmm0 > >> setp %al > >> cmove %eax, %edx > >> testb %dl, %dl > >> jne .L20 > >> .L3: > >> ret > >> .L20: > >> comisd %xmm0, %xmm2 > >> ja .L21 > >> .L4: > >> comisd %xmm2, %xmm0 > >> jbe .L7 > >> comisd %xmm1, %xmm0 > >> jbe .L7 > >> .L6: > >> addsd .LC1(%rip), %xmm0 > >> ret > >> .L21: > >> comisd %xmm0, %xmm1 > >> ja .L6 > >> jmp .L4 > >> .L7: > >> subsd .LC1(%rip), %xmm0 > >> ret > >> .L19: > >> addsd %xmm1, %xmm0 > >> ret > >> .LC1: > >> .long 0 > >> .long 1072693248 > >> > >> Stefan > >> > > > > > > -- > > Matt > > (he/him) > > > -- Matt (he/him)
