If you have RAM to spare, (you are using perl, of course you do) you could build a 2048-entry lookup table, mask arg with 2047, shift once with the value from the table and then compare with 9007199254740993 once.

Something like: (untested)

static uint8_t ctz_table_11bit[2048] = { /* elided */ };

int uv_fits_double(UV arg) {
  arg >>= ctz_table_11bit[arg & 2047];
  return arg < 9007199254740993;
}

[download]

The relative elegance of this solution is probably a matter of opinion, but it should run much faster, since it is theoretically a single basic block and should be a candidate for inlining. This trades considerable computation for a single memory read, which will have relative performance depending on your hardware.

Here is some Perl code to generate that table: (I think this is the first time I have used formats; output spot checked for plausibility)

#!/usr/bin/perl

use strict;
use warnings;

use constant BITS => 11;

our @pos; our @bits; our @val;

format STDOUT_TOP=
.
$= = 1<<(BITS-1);
format STDOUT =
  /* @### -> @0######### */ @#, /* @### -> @0######### */ @#,
     $pos[0], $bits[0], $val[0],    $pos[1], $bits[1], $val[1],
.

print "static uint8_t ctz_table_",BITS,"bit[",1<<BITS,"] = {\n";

for (my $pos = 0; $pos < (1<<BITS); $pos += 2) {
  @pos = ($pos, $pos+1);
  @bits = map substr(unpack('B*', pack 'n', $_), -(BITS)), @pos;
  @val = map length((m/(0*?)\z/)[0]), @bits;
  write
}

print "};\n";

# EOF

[download]
Update: change two magic numbers to use symbolic constant.

In reply to Re: Detecting whether UV fits into an NV by jcb
in thread Detecting whether UV fits into an NV by syphilis

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