On Monday, 2 March 2020 at 23:27:22 UTC, Steven Schveighoffer
wrote:
What I think is happening is that it determines nobody is using
the result, and the function is pure, so it doesn't bother
calling that function (probably not even the lambda, and then
probably removes the loop completely).
I'm assuming for some reason, the binary search is not flagged
pure, so it's not being skipped.
Apparently you're right:
https://github.com/dlang/phobos/blob/5e13653a6eb55c1188396ae064717a1a03fd7483/std/range/package.d#L11107
If I change to this to ensure side effects:
bool makeImpure; // TLS variable outside of main
...
auto results = benchmark!(
() => makeImpure = r1.canFind(max),
() => makeImpure = r2.contains(max),
() => makeImpure = r3.canFind(max),
)(5_000);
writefln("%(%s\n%)", results); // modified to help with the
comma confusion
I now get:
4 secs, 428 ms, and 3 hnsecs
221 μs and 9 hnsecs
4 secs, 49 ms, 982 μs, and 5 hnsecs
More like what I expected!
Ahhhh damn! And here I was thinking that branch prediction made a
HUGE difference! Ok, I'm taking my tail and slowly moving away
now :) Let us never speak of this again.
-Steve
