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
