Fast integer log10 in C

log10.c

#include <stdint.h>

/*
 
  Fast 64bit integer log10
  WARNING: calling ilog10c(0) yields undefined behaviour!

  On x64 this compiles down to:

  pushq	%rbp
  movq	%rsp, %rbp
  bsrq	%rdi, %rax
  xorq	$63, %rax
  movl	%eax, %ecx
  xorl	$63, %ecx
  leal	(%rcx,%rcx,2), %ecx
  movl	$3435973837, %edx
  imulq	%rcx, %rdx
  shrq	$35, %rdx
  leaq	_ilog10c.thr(%rip), %rcx
  cmpq	%rdi, (%rcx,%rax,8)
  setbe	%al
  movzbl	%al, %eax
  addl	%edx, %eax
  popq	%rbp
  ret
	
*/

uint32_t ilog10c(uint64_t v) {
  static const uint64_t thr[64] = {
    10000000000000000000ULL, 0, 0, 0, 1000000000000000000ULL, 0, 0, 100000000000000000ULL, 0, 0,
       10000000000000000ULL, 0, 0, 0,    1000000000000000ULL, 0, 0,    100000000000000ULL, 0, 0,
          10000000000000ULL, 0, 0, 0,       1000000000000ULL, 0, 0,       100000000000ULL, 0, 0,
             10000000000ULL, 0, 0, 0,          1000000000ULL, 0, 0,          100000000ULL, 0, 0,
                10000000ULL, 0, 0, 0,             1000000ULL, 0, 0,             100000ULL, 0, 0,
                   10000ULL, 0, 0, 0,                1000ULL, 0, 0,                100ULL, 0, 0,
                      10ULL, 0, 0, 0
  };
  uint32_t lz = __builtin_clzll(v);
  return (63 - lz) * 3 / 10 + (v >= thr[lz]);
}