Unrolling the loop and using Anonymonk's lookup trick extracts another 5%:

int l[] = { 0, 2, 4, 6, 8, 1, 3, 5, 7, 9 };
int c_luhn2( char *s ) {
    int total = 0;

    total += l[ s[ 0] - '0' ];
    total +=    s[ 1] - '0';
    total += l[ s[ 2] - '0' ];
    total +=    s[ 3] - '0';
    total += l[ s[ 4] - '0' ];
    total +=    s[ 5] - '0';
    total += l[ s[ 6] - '0' ];
    total +=    s[ 7] - '0';
    total += l[ s[ 8] - '0' ];
    total +=    s[ 9] - '0';
    total += l[ s[10] - '0' ];
    total +=    s[11] - '0';
    total += l[ s[12] - '0' ];
    total +=    s[13] - '0';
    total += l[ s[14] - '0' ];

    total *= 9;
    return total % 10;
}
[download]

I doubt there is much fat left in there :)

Looks fat to me ( just kidding :)

Is this any faster? Or does the compiler optimize away the differences?

int l[] = {
  0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0,
  0, 2, 4, 6, 8, 1, 3, 5, 7, 9 };

int
c_luhn2( char *s ) {
    return ((
        l[ (int)s[ 0] ]
      +    s[ 1]
      + l[ (int)s[ 2] ]
      +    s[ 3]
      + l[ (int)s[ 4] ]
      +    s[ 5]
      + l[ (int)s[ 6] ]
      +    s[ 7]
      + l[ (int)s[ 8] ]
      +    s[ 9]
      + l[ (int)s[10] ]
      +    s[11]
      + l[ (int)s[12] ]
      +    s[13]
      + l[ (int)s[14] ] - 7 * '0' ) * 9) % 10;
}
[download]

The optomiser eliminates any differences. Yours (c_luhn3 below) can be slightly faster one run and slightly slower on the next; but the difference is less that 0.5% either way:

#! perl -slw
use strict;
use Inline C => Config => BUILD_NOISY => 1;
use Inline C => <<'END_C',  NAME => '_luhn', CLEAN_AFTER_BUILD =>0;

int c_luhn( char *s ) {
    int i, total = 0;

    for( i=0; i < 15; ++i ) {
        int d = s[ i ] - '0';
        if( !( i  & 1 ) ) {
            d *= 2;
            if( d > 9 ) d -= 9;
        }
        total += d;
    }
    total *= 9;
    return total % 10;
}

int l[] = { 0, 2, 4, 6, 8, 1, 3, 5, 7, 9 };
int c_luhn2( char *s ) {
    int total = 0;

    total += l[ s[ 0] - '0' ];
    total +=    s[ 1] - '0';
    total += l[ s[ 2] - '0' ];
    total +=    s[ 3] - '0';
    total += l[ s[ 4] - '0' ];
    total +=    s[ 5] - '0';
    total += l[ s[ 6] - '0' ];
    total +=    s[ 7] - '0';
    total += l[ s[ 8] - '0' ];
    total +=    s[ 9] - '0';
    total += l[ s[10] - '0' ];
    total +=    s[11] - '0';
    total += l[ s[12] - '0' ];
    total +=    s[13] - '0';
    total += l[ s[14] - '0' ];

    total *= 9;
    return total % 10;
}

int ll[] = {
  0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0,
  0, 2, 4, 6, 8, 1, 3, 5, 7, 9 };

int
c_luhn3( char *s ) {
    return ((
        ll[ (int)s[ 0] ]
      +    s[ 1]
      + ll[ (int)s[ 2] ]
      +    s[ 3]
      + ll[ (int)s[ 4] ]
      +    s[ 5]
      + ll[ (int)s[ 6] ]
      +    s[ 7]
      + ll[ (int)s[ 8] ]
      +    s[ 9]
      + ll[ (int)s[10] ]
      +    s[11]
      + ll[ (int)s[12] ]
      +    s[13]
      + ll[ (int)s[14] ] - 7 * '0' ) * 9) % 10;
}

END_C
use Time::HiRes qw[ time ];

my @samples = qw[
    4011350000000008
    4011350000000016
    4011350000000024
    4011350000000032
    4011350000000040
    4011350000000057
    4011350000000065
    4011350000000073
    4011350000000081
    4011350000000099
];

sub luhn {
    use integer;
    my $s = $_[ 0 ];
    my $total = 0;
    for my $i ( 0 .. 14 ) {
        my $d = substr( $s, $i, 1 );
        unless( $i & 1 ) {
            $d *= 2;
            $d -= 9 if $d > 9;
        }
        $total += $d;
    }
    $total *= 9;
    return chop $total;
}

for ( @samples ) {
    print "$_: ", luhn( substr $_, 0, 15 );
}

my $start = time;
#for ( 401135000000000..401135000999999 ) {
#    my $chk = luhn( $_ );
#}
#printf "Took %.9f seconds.\n", time() - $start;

#for ( @samples ) {
#    print "$_: ", c_luhn( $_ );
#}

$start = time;
for ( 401135000000000..401135000999999 ) {
    my $chk = c_luhn( $_ );
}
printf "Took %.9f seconds.\n", time() - $start;

for ( @samples ) {
    print "$_: ", c_luhn2( $_ );
}

$start = time;
for ( 401135000000000..401135000999999 ) {
    my $chk = c_luhn2( $_ );
}
printf "Took %.9f seconds.\n", time() - $start;

for ( @samples ) {
    print "$_: ", c_luhn3( $_ );
}

$start = time;
for ( 401135000000000..401135000999999 ) {
    my $chk = c_luhn3( $_ );
}
printf "Took %.9f seconds.\n", time() - $start;
[download]

C:\test>luhn
4011350000000008: 8
4011350000000016: 6
4011350000000024: 4
4011350000000032: 2
4011350000000040: 0
4011350000000057: 7
4011350000000065: 5
4011350000000073: 3
4011350000000081: 1
4011350000000099: 9
Took 0.751899958 seconds.
4011350000000008: 8
4011350000000016: 6
4011350000000024: 4
4011350000000032: 2
4011350000000040: 0
4011350000000057: 7
4011350000000065: 5
4011350000000073: 3
4011350000000081: 1
4011350000000099: 9
Took 0.695394039 seconds.
4011350000000008: 8
4011350000000016: 6
4011350000000024: 4
4011350000000032: 2
4011350000000040: 0
4011350000000057: 7
4011350000000065: 5
4011350000000073: 3
4011350000000081: 1
4011350000000099: 9
Took 0.693738222 seconds.


C:\test>luhn
4011350000000008: 8
4011350000000016: 6
4011350000000024: 4
4011350000000032: 2
4011350000000040: 0
4011350000000057: 7
4011350000000065: 5
4011350000000073: 3
4011350000000081: 1
4011350000000099: 9
Took 0.751657963 seconds.
4011350000000008: 8
4011350000000016: 6
4011350000000024: 4
4011350000000032: 2
4011350000000040: 0
4011350000000057: 7
4011350000000065: 5
4011350000000073: 3
4011350000000081: 1
4011350000000099: 9
Took 0.694734097 seconds.
4011350000000008: 8
4011350000000016: 6
4011350000000024: 4
4011350000000032: 2
4011350000000040: 0
4011350000000057: 7
4011350000000065: 5
4011350000000073: 3
4011350000000081: 1
4011350000000099: 9
Took 0.697062969 seconds.
[download]

---

With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority". The enemy of (IT) success is complexity.

In the absence of evidence, opinion is indistinguishable from prejudice. Suck that fhit

Update:Ignore this! It doesn't stand up to syphilis's scrutiny. (It only works for the OPs limited test range by luck!)

Looks fat to me ( just kidding :)

If its skinny you want, try this for (its lack of) size :)

Correct results and 35% faster to boot:

int lookup[] = { 8, 6, 4, 2, 0, 7, 5, 3, 1, 9, 7, 5, 3, 1, 9, 6, 4, 2,
+ 0 };
int c_fluhn( int n ) {
    return lookup[ n % 20 ];
}
[download]

---

With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority". The enemy of (IT) success is complexity.

In the absence of evidence, opinion is indistinguishable from prejudice. Suck that fhit