[Mono-devel-list] String::GetHashCode speedup

[Ben Maurer]

Hey guys,

I transformed String.GetHashCode into a managed function. It works
fairly well, even for somewhat large strings:
Len +--- Old ---+--- New ---+
  2 |    .150 s |    .042 s |
  8 |    .223 s |    .091 s |
 14 |    .279 s |    .174 s |
 20 |    .309 s |    .226 s |
 26 |    .344 s |    .287 s |
 32 |    .376 s |    .333 s |
 38 |    .408 s |    .401 s |
 44 |    .449 s |    .471 s |
 50 |    .482 s |    .528 s |
 56 |    .514 s |    .605 s |
 62 |    .548 s |    .747 s |
 68 |    .580 s |    .645 s |
 74 |    .614 s |    .701 s |
 80 |    .648 s |    .749 s |
 86 |    .685 s |    .860 s |
 92 |    .715 s |    .937 s |
 98 |    .751 s |    .934 s |
----+-----------+-----------+

So it appears the break-even point here is at ~ 38 chars. Also, when I
ran the GetHashCode function on MS's runtime, it was always faster than
MS's String.GetHashCode function. So, it seems that with a little bit of
JIT work, we could push this break-even point much higher, or make it so
we always win.

In the process, I discovered that constructs such as:

fixed (char* foo = my_string) { ... }

Call a function that basically gets a fixed value. I have attached a
patch to mini that will embed the constant in the emitted code. This
gives a nice speedup to anyone using the construct. Before, we were
working around this in String.cs by doing

fixed (char* foo = &s.start_char) { ... }

However, that is absurdly messy ;-). Also, you obviously cant do it
outside of String.

-- Ben
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