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 <[email protected]
> <javascript:>> 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
>
>
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