Use the Assembly Luke. https://godbolt.org/g/nGFMbf
It looks like clang manages to compute a table of powers of x. Which, is very very impressive. Which is roughly https://play.golang.org/p/CZkiJKfe7s -- except clang, also does inner loop unrolling. + Egon On Friday, 4 August 2017 11:16:10 UTC+3, Dorival Pedroso wrote: > > wait, what?! > > the same code: > > #include "stdio.h" > #include "math.h" > int main() { > double res = 0.0; > double x = 2.5; > int Nmax = 10000000; > for (int N=0; N<Nmax; N++) { > for (int i=0; i<20; i++) { > res += pow(x, i); > } > } > printf("res = %g\n", res); > } > > compiled with Clang is much faster? > > yes, I get: > > gcc -O2 c-code-sum.c -o c-code-sum_GCC -lm > time ./c-code-sum_GCC > res = 6.0633e+14 > > real 0m8.210s > user 0m8.208s > sys 0m0.000s > > and: > > clang -O2 c-code-sum.c -o c-code-sum_CLANG -lm > time ./c-code-sum_CLANG > res = 6.0633e+14 > > real 0m0.157s > user 0m0.156s > sys 0m0.004s > > So the question might be: what is CLANG doing? (and how to do this with > Go?...) > > > On Friday, August 4, 2017 at 6:04:01 PM UTC+10, Sebastien Binet wrote: >> >> >> >> On Fri, Aug 4, 2017 at 9:51 AM, Henrik Johansson <dahan...@gmail.com> >> wrote: >> >>> Actually I get the same as the original program on my mac. >>> >>> time ./ccode >>> sum=606329794183272.375000 >>> ./ccode 0.17s user 0.00s system 98% cpu 0.170 total >>> >>> The Go version -O2 -Wall >>> time ./pow >>> sum=6.063297941832724e+14./pow 5.47s user 0.01s system 99% cpu 5.490 >>> total >>> >> >> interesting :) >> >> I have: >> >> $> gcc --version >> gcc (GCC) 7.1.1 20170630 >> Copyright (C) 2017 Free Software Foundation, Inc. >> This is free software; see the source for copying conditions. There is NO >> warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR >> PURPOSE. >> >> also interestingly, with clang: >> $> clang --version >> clang version 4.0.1 (tags/RELEASE_401/final) >> Target: x86_64-unknown-linux-gnu >> Thread model: posix >> InstalledDir: /usr/bin >> >> $> time ./c-code >> >> real 0m0.001s >> user 0m0.001s >> sys 0m0.000s >> >> $> time ./c-code-sum >> sum=606329794183272.375000 >> >> real 0m0.228s >> user 0m0.227s >> sys 0m0.000s >> >> I haven't looked at the assembly to see what's really going on for the >> GCC version... >> >> -s >> >> -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
