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) >
