On Mon, 2008-05-12 at 08:49 -0700, caffeine wrote:
> I see that I was too slow on the "it's too slow" response. :-)
>
> Do you have any proof that a compiler will inline std::vector calls into raw
> C array accesses? If it were true, I would be ecstatic, but I don't believe
> it for a second.
Not "proof" as such---results will probably depend on context. But on my
machine, compiling with g++ 4.2.3 and with full optimization turned on
(i.e., -O3), the two following programs run in about the same time. It
seems pretty good evidence that, at least in this case, operator[] is
inlined.
However, if allocation is moved inside the outer loop, the vector
program runs slower. Either vector creation is slower, or adding the
allocation inside the loop changes the inlining policy---your guess is
as good as mine.
Luigi
-------------------------
test1.cpp
-------------------------
#include <vector>
int main() {
std::vector<double> v(10000);
for (int j=0; j<10000; ++j) {
for (int i=0; i<10000; ++i)
v[i] = 42.0;
double x;
for (int i=0; i<10000; ++i)
x = v[i];
}
return 0;
}
-------------------------
test2.cpp
-------------------------
int main() {
double* v = new double[10000];
for (int j=0; j<10000; ++j) {
for (int i=0; i<10000; ++i)
v[i] = 42.0;
double x;
for (int i=0; i<10000; ++i)
x = v[i];
}
delete[] v;
return 0;
}
--
There are no rules of architecture for a castle in the clouds.
-- Gilbert K. Chesterton
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