My current favorites:
void popFront(ref char[] s) @trusted pure nothrow {
immutable byte c = s[0];
if (c >= -2) {
s = s.ptr[1 .. s.length];
} else {
import core.bitop;
size_t i = 7u - bsr(~c);
import std.algorithm;
s = s.ptr[min(i, s.length) .. s.length];
}
}
I also experimented with explicit speculation:
void popFront(ref char[] s) @trusted pure nothrow {
immutable byte c = s[0];
s = s.ptr[1 .. s.length];
if (c < -2) {
import core.bitop;
size_t i = 6u - bsr(~c);
import std.algorithm;
s = s.ptr[min(i, s.length) .. s.length];
}
}
LDC and GDC both compile these to 23 instructions.
DMD does worse than with my other code.
You can influence GDC's block layout with __builtin_expect.
I notice that many other snippets posted use uint instead of
size_t in the multi-byte branch. This generates extra
instructions for me.
