Can you shrink it further?

[Andrei Alexandrescu via Digitalmars-d]

On 10/11/2016 05:45 AM, Temtaime wrote:
> void popFront7(ref char[] s) @trusted pure nothrow
> {
>   import core.bitop;
>   auto v = 7 - bsr(~s[0] | 1);
>   s = s[v > 6 ? 1 : (v ? (v > s.length ? s.length : v) : 1)..$];
> }
>
> Please check this.

Thanks. This does a lot of work on the frequent path c < 0x80:

pure nothrow @trusted void example.popFront7(ref char[]):
         movq    8(%rdi), %rax
         movzbl  (%rax), %ecx
         xorq    $254, %rcx
         orq     $1, %rcx
         bsrq    %rcx, %rcx
         notl    %ecx
         addl    $8, %ecx
         cmpl    $6, %ecx
         jg      .LBB0_2
         testl   %ecx, %ecx
         je      .LBB0_2
         movslq  %ecx, %rdx
         movq    (%rdi), %rcx
         cmpq    %rcx, %rdx
         cmovaq  %rcx, %rdx
         jmp     .LBB0_3
.LBB0_2:
         movq    (%rdi), %rcx
         movl    $1, %edx
.LBB0_3:
         addq    %rdx, %rax
         subq    %rdx, %rcx
         movq    %rcx, (%rdi)
         movq    %rax, 8(%rdi)
         retq

So I changed it to:

void popFront7(ref char[] s) @trusted pure nothrow
{
   immutable c = s[0];
   if (c < 0x80) {
     s = s.ptr[1 .. s.length];
   } else {
     import core.bitop;
     uint v = 7 - bsr(~c | (c > 0xfd) << 6u);
     s = s.ptr[v > s.length ? s.length : v .. s.length];
   }
}

That's about as large as the baseline.


Andrei