[Issue 4489] std.array.insert is slow

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=4489



--- Comment #2 from Andrei Alexandrescu <andrei at metalanguage.com> 2012-08-01 16:46:38 PDT ---
Brought the code to date with:

----
import std.array;
import std.datetime;
import std.stdio;
import std.c.string;

enum COUNT = 100_000;


void myInsert(T)(ref T[] arr, size_t where, T value)
{
    assert(where <= arr.length);

    size_t oldLen = arr.length;
    arr.length = arr.length + 1;

    T* ptr = arr.ptr + where;
    memmove(ptr + 1, ptr, (oldLen - where) * T.sizeof);
    arr[where] = value;
}


void bench(alias insert)(ref int[] arr)
{
    auto sw = StopWatch(AutoStart.yes);
    foreach (_; 0..10) {
        arr.length = 0;
        foreach (i; 0..COUNT)
            insert(arr, i, i);
    }
    writeln(sw.peek.msecs);
}


void main()
{
    int[] arr = new int[COUNT];

    bench!(myInsert)(arr);
    bench!(std.array.insertInPlace)(arr);
}
----

I was able to measure a notable the performance mismatch. The culprit is:

----
void insertInPlace(T, U...)(ref T[] array, size_t pos, U stuff)
    if(!isSomeString!(T[]) && allSatisfy!(isInputRangeOrConvertible!T, U))
{
    T[staticConvertible!(T, U)] stackSpace = void;
    auto range = chain(makeRangeTuple(stackSpace[], stuff).expand);
    insertInPlaceImpl(array, pos, range);
}
----

I replaced that with:

----
void insertInPlace(T, U...)(ref T[] array, size_t pos, U stuff)
    if(!isSomeString!(T[]) && allSatisfy!(isInputRangeOrConvertible!T, U))
{
    immutable oldLen = array.length;
    array.length += stuff.length;
    auto ptr = array.ptr + pos;
    import core.stdc.string;
    memmove(ptr + stuff.length, ptr, (oldLen - pos) * T.sizeof);

    ptr = array.ptr + pos;
    foreach (i, Unused; U)
    {
        emplace(ptr + i, stuff[i]);
    }
}
----

and was able to measure similar performance.

Clearly the code once approved belongs to the entire team, not only to its
author, and I should be a better man than this, but I can't stop noticing the
irony of this given
http://forum.dlang.org/thread/mailman.811.1343564241.31962.digitalmars-d@puremagic.com

I'll make a pull request soon.

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