loop optimized in 4.7.2, not in 4.8.1?

[Ángel González]

On 11/08/13 02:54, Oliver Kullmann wrote:
> For general enligthenment, could you quickly explain what "reduce the
> loop" means? One can of course just get rid off the loop, precomputing
> sum, but I guess that's not what you mean? (I don't understand the assembler code.)
>
> With gcc-4.7.1 I get for the following (with a bigger loop):
>
> #include<iostream>
> #include<cassert>
> template<typename T>
> inline T const&  max (T const&  a, T const&  b) { return a<b?b:a; }
> int main() {
>    typedef long long unsigned UInt;
>    typedef long long Index;
>    UInt sum = 0;
>    constexpr UInt n = 100000000000;
>    for (Index x = 0; x<  Index(n); ++x) sum+=max(x,x+1);
>    assert(sum == (n * (n+1))/2);
>    std::cout<<  sum<<  "\n"; // prints "932356074711512064" for 64-bit long long
> }
>
>> g++ --std=c++11 -Ofast -funroll-loops -Wall Example.cpp
>> time ./a.out
> 932356074711512064
>
> real    0m56.278s
> user    0m56.126s
> sys     0m0.001s
>
> Interestingly, when changing Index to UInt I get
>
> real    1m7.261s
> user    1m7.076s
> sys     0m0.018s
>
> which is, at least for me, an unexpected difference? (I would have assumed that,
> if there is a difference, then Index=UInt should be faster.)
>
> Oliver
Differences between signed and unsigned behavior because with unsigned 
there's wraparound,
while in signed the result of an overflow is undefined (and thus the 
compiler can make more
assumptions)

However, I do not see that difference when compiling your code (g++ 4.8.1)
$ time ./UInt
932356074711512064

real    1m39.374s
user    1m15.413s
sys    0m0.157s

$ time ./normal
932356074711512064

real    1m46.876s
user    1m4.443s
sys    0m0.207s

$ time ./normal
932356074711512064

real    1m10.107s
user    1m4.360s
sys    0m0.023s

$ time ./UInt
932356074711512064

real    1m29.006s
user    1m15.437s
sys    0m0.067s


$ time ./normal
932356074711512064

real    1m19.602s
user    1m4.383s
sys    0m0.033s

$ time ./UInt
932356074711512064

real    1m17.318s
user    1m15.467s
sys    0m0.010s