Can you shrink it further?

[Matthias Bentrup via Digitalmars-d]

On Tuesday, 11 October 2016 at 15:01:47 UTC, Andrei Alexandrescu 
wrote:
> On 10/11/2016 10:49 AM, Matthias Bentrup wrote:
>>
>> void popFrontAsmIntel(ref char[] s) @trusted pure nothrow {
>>   immutable c = s[0];
>>   if (c < 0x80) {
>>     s = s[1 .. $];
>>   } else {
>>     uint l = void;
>>     asm pure nothrow @nogc {
>>       mov EAX, 1;
>>       mov BL, 0xf8-1;
>>       sub BL, c;
>>       cmp BL, 0xf8-0xc0;
>>       adc EAX, 0;
>>       cmp BL, 0xf8-0xe0;
>>       adc EAX, 0;
>>       cmp BL, 0xf8-0xf0;
>>       adc EAX, 0;
>>       mov l, EAX;
>>     }
>>     s = s[l <= $ ? l : $ .. $];
>>   }
>> }
>
> Did you take a look at the codegen on http://ldc.acomirei.ru? 
> It's huge. -- Andrei

Here are three branch-less variants that use the sign instead of 
the carry bit.

The last one is the fastest on my machine, although it mixes the 
rare error case and the common 1-byte case into one branch.

void popFront1(ref char[] s) @trusted pure nothrow {
   immutable c = cast(byte)s[0];
   if (c >= 0) {
     s = s[1 .. $];
   } else if (c < -8) {
     uint i = 4 + (c + 64 >> 31) + (c + 32 >> 31) + (c + 16 >> 31);
     import std.algorithm;
     s = s[min(i, $) .. $];
   } else {
     s = s[1 .. $];
   }
}

void popFront1a(ref char[] s) @trusted pure nothrow {
   immutable c = cast(byte)s[0];
   if (c >= 0) {Three
     s = s[1 .. $];
   } else {
     uint i = 1 + ((3 + (c + 64 >> 31) + (c + 32 >> 31) + (c + 16 
 >> 31)) & (c + 8 >> 31));
     import std.algorithm;
     s = s[min(i, $) .. $];
   }
}

void popFront1b(ref char[] s) @trusted pure nothrow {
   immutable c = cast(byte)s[0];
   if (c >= -8) {
     s = s[1 .. $];
   } else {
     uint i = 4 + (c + 64 >> 31) + (c + 32 >> 31) + (c + 16 >> 31);
     import std.algorithm;
     s = s[min(i, $) .. $];
   }
}