Challenge: write a really really small front() for UTF8

[Daniel N]

On Monday, 24 March 2014 at 12:21:55 UTC, Daniel N wrote:
> I'm currently too busy to submit a complete solution, but 
> please feel free to use my idea if you think it sounds 
> promising.

I now managed to dig up my old C source... but I'm still blocked 
by dmd not accepting the 'pext' instruction...

1) I know my solution is not directly comparable to the rest in 
this thread(for many reasons).
2) It's of course trivial to add a fast path for ascii... if 
desired.
3) It throws safety and standards out the window.

#include <x86intrin.h>

char32_t front(const char* inp)
{
   static const uint32_t mask[32] =
   {
       0b01111111'00000000'00000000'00000000,
       0b00000000'00000000'00000000'00000000,
       0b00011111'00111111'00000000'00000000,
       0b00001111'00111111'00111111'00000000,
       0b00000111'00111111'00111111'00111111,
   };
   uint32_t rev = __builtin_bswap32(reinterpret_cast<const 
uint32_t*>(inp)[0]);
   uint32_t len = __lzcnt32(~rev);

   return _pext_u32(rev, mask[len]);
}

This is what clang 3.4 generated:
## BB#0:
         pushq   %rbp
Ltmp2:
         .cfi_def_cfa_offset 16
Ltmp3:
         .cfi_offset %rbp, -16
         movq    %rsp, %rbp
Ltmp4:
         .cfi_def_cfa_register %rbp
         movl    (%rdi), %eax
         bswapl  %eax
         movl    %eax, %ecx
         notl    %ecx
         lzcntl  %ecx, %ecx
         leaq    __ZZ5frontPKcE4mask(%rip), %rdx
         pextl   (%rdx,%rcx,4), %eax, %eax
         popq    %rbp
         ret