Re: 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