On Friday, 20 September 2013 at 16:28:49 UTC, Joseph Rushton
Wakeling wrote:
[...]
The canonical example would be something like,
foreach (i; iota(10)) { ... }
which in theory shouldn't be any slower than,
foreach (i; 0 .. 10) { ... }
but in practice is, no matter what the compiler.
Some compilers are unexpectedly smart in this case. I just
benchmarked somewhat similar code with two version of the same
function: one using a straight for loop, the other mixture of
iota and "..". Plain for loop should be the fastest, right?
Almost right. Here are the functions:
int e28_0(int N = 1002) {
int diagNumber = 1;
int sum = diagNumber;
for (int width = 2; width < N; width += 2)
for (int j = 0; j < 4; ++j) {
diagNumber += width;
sum += diagNumber;
}
return sum;
}
int e28_1(int N = 1002) {
int diagNumber = 1;
int sum = diagNumber;
foreach (width; iota(2, N, 2))
foreach (_; 0..4) {
diagNumber += width;
sum += diagNumber;
}
return sum;
}
Here are the results:
GDC 4.8.1: gdc-4.8 -m64 -march=native -fno-bounds-check
-frename-registers -frelease -O3
669171001 830ns e28_0
669171001 830ns e28_1
DMD64 2.063.2: dmd -O -noboundscheck -inline -release
669171001 1115ns e28_0
669171001 1958ns e28_1
LDC 0.11.0: ldmd2 -m64 -O -noboundscheck -inline -release
669171001 454ns e28_0
669171001 395ns e28_1
