Re: Counter-intuitive io vs no-io time readings
On 09-04-2014 17:26, Ian Collins wrote:
Ney Andr?? de Mello Zunino wrote:
[...]
class vec_seq_access : public timed_task {
public:
vec_seq_access(const std::vector<unsigned int>& data) : data(data) {
}
protected:
virtual void do_run() const {
int odd_count = 0;
int even_count = 0;
for (const auto& i : data) {
if (i % 2 != 0) ++odd_count; else ++even_count;
}
std::cout << odd_count << " odd numbers and " << even_count <<
" even
numbers.\n";
If this is the line you refer to later, I would expect an optimiser to
remove the body of this function if you remove this line.
That's a reasonable assumption. Upon realizing that none of the values
produced in this function are actually read/referenced, the optimizer
could decide that no code needed to be generated. However, that would
make it all even more counter-intuitive, since the program output showed
that the time readings were greater for the no-output case, when
compared to the other case, where the output line was kept (and the code
not optimized away, according to the assumption at hand).
auto generate_random_data(int count) {
This auto declaration is missing the return type.
Luca Risolia's got it right. I've been away from C++ for a long time and
decided to try out some of its new features.
[...]
Surprisingly, the average run time has actually increased to about 2.14
seconds. Surely, I must be missing something or maybe there's some weird
combination of factors leading to this outcome. Can anybody see what's
going on?
Probably the latter! Did you look at the generated assembly code?
I did. I've created a repository on BitBucket and pushed the source
code, assembly outputs and a screenshot of the relevant part of the
comparison between them. I've used '-io' and '-no-io' suffixes to refer
to the instances with the 'cout' line enabled and suppressed, respectively.
https://bitbucket.org/Zunino/ds-performance/src
Compiler: g++ 4.8.1
Compilation command: g++ --std=c++1y -o ds-perf ds-perf.cpp
--std=c++1y ?
As Luca Risolia's said, it's g++'s option to enable support for the
latest language features.
P.S.: Just one last note to make matters a bit more contrived: I've run
the same tests here in my laptop at home and, guess what? The results
were more like what I expected, that is, the version without the
IO/stream call did perform better. Not by as much as I'd thought, but it
did. Oh, well...
Thank you all for your time.
Regards,
--
Zunino