Re: Can you shrink it further?

[Andrei Alexandrescu via Digitalmars-d]

That looks good. I'm just worried about the jump forward - ideally the 
case c < 127 would simply entail a quick return. I tried a fix, but it 
didn't do what I wanted in ldc. We shouldn't assert(0) if wrong - just 
skip one byte. Also, are we right to not worry about 5- and 6-byte 
sequences? The docs keep on threatening with it, and then immediately 
mention those are not valid.

void popFront3(ref char[] s) @trusted pure nothrow {
  immutable c = s[0];
  uint char_length = 1;
  if (c < 127)
  {
  Lend :
    s = s.ptr[char_length .. s.length];
  } else {
    if (c < 192)
    {
      char_length = 2;
      goto Lend;
    }
    if (c < 240) {
      char_length = 3;
      goto Lend;
    }
    if (c < 248) {
       char_length = 4;
    }
    goto Lend;
  }
}

Andrei

On 10/10/16 9:39 PM, Stefan Koch wrote:
This version has 24 instructions but these have a smaller encoding then
and are generally inexpensive
With inline asm and conditional moves it would be possible to reduce
this even further
to ~20 instructions.

void popFront1(ref char[] s) @trusted pure nothrow {
  immutable c = s[0];
  size_t char_length = 1;
  if (c < 127)
  {
    goto Lend;
  } else {
    if ((c & 0b1100_0000) == 0b1000_0000)
    {
      // This is invalid as a first char
      goto Lerror;
    }
    if (c < 192)
    {
      char_length = 2;
      goto Lend;
    }
    if (c < 240) {
      char_length = 3;
      goto Lend;
    }
    if (c < 248) {
       char_length = 4;
      goto Lend;
    }

    //These characters are also no longer valid
    Lerror : assert(0);
  }
  Lend :
  s = s.ptr[char_length .. s.length];
}