Re: Table lookups - this is pretty definitive

[Denis Shelomovskij]

01.04.2014 22:35, Walter Bright пишет:
Try this benchmark comparing various classification schemes:
---------------------------------
import core.stdc.stdlib;
import core.stdc.string;
import std.algorithm;
import std.array;
import std.ascii;
import std.datetime;
import std.range;
import std.stdio;
import std.traits;

bool isIdentifierChar0(ubyte c)
{
     return isAlphaNum(c) || c == '_' || c == '$';
}

bool isIdentifierChar1(ubyte c)
{
     return ((c >= '0' || c == '$') &&
             (c <= '9' || c >= 'A')  &&
             (c <= 'Z' || c >= 'a' || c == '_') &&
             (c <= 'z'));
}

immutable bool[256] tab2;
static this()
{
     for (size_t u = 0; u < 0x100; ++u)
     {
         tab2[u] = isIdentifierChar0(cast(ubyte)u);
     }
}

bool isIdentifierChar2(ubyte c)
{
     return tab2[c];
}

/*********************************************/

int f0()
{
     int x;
     for (uint u = 0; u < 0x100; ++u)
     {
         x += isIdentifierChar0(cast(ubyte)u);
     }
     return x;
}

int f1()
{
     int x;
     for (uint u = 0; u < 0x100; ++u)
     {
         x += isIdentifierChar1(cast(ubyte)u);
     }
     return x;
}

int f2()
{
     int x;
     for (uint u = 0; u < 0x100; ++u)
     {
         x += isIdentifierChar2(cast(ubyte)u);
     }
     return x;
}

void main()
{
     auto r = benchmark!(f0, f1, f2)(10_000);
     writefln("Milliseconds %s %s %s", r[0].msecs, r[1].msecs, r[2].msecs);
}

Some regular benchmark notes:

1. The first one passed to `benchmark` is always slower (e.g. pass `f2` 
and see).

2. Unexpected program behaviour changes:

Let's use `benchmark!(f1, f1, f1)(1_000_000)`:
Milliseconds 928 889 888

Then copy `isAlphaNum` in file (benchmark still shows same results) and 
change `dchar c` to `ubyte c`, result changes:
Milliseconds 849 815 827
The difference is sufficient but `isAlphaNum` not called in benchmark.

Also `f0` is faster than `f1`, benchmark!(f0, f0, f0)(1_000_000):
Milliseconds 731 694 702

And `f2` wins, it's the only obvious thing, benchmark!(f2, f2, 
f2)(1_000_000):
Milliseconds 227 184 184


Compiler used: dmd -O -inline -release

--
Денис В. Шеломовский
Denis V. Shelomovskij