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            /* inputs */
             : [input] "mr"  (input)
            /* clobbers: we are messing with these registers: */
             : "eax"
            );
    
            // return an arrayref of the two outputs
            AV* ret = newAV();
            sv_2mortal((SV*)ret);
            av_push(ret, newSViv(num_leading_zeros));
            av_push(ret, newSViv(mssb));
    
            return ret;
        }

    You can also inline assembly in your Perl code with Inline::ASM

    Be advised that GCC builtins are also calling assembly code. In fact
    the above assembly code is how GCC implements clz(). So, inline
    assembly and GCC::Builtins should yield, more-or-less, the same
    performance gain.

TESTING

    For each exported sub there is a corresponding auto-generated test
    file. The test goes as far as loading the library and calling the
    function from Perl.

    However, there may be errors in the expected results because that was
    done without verifying with a C test program.

BENCHMARKS

    Counting leading zeros (clz) will be used to benchmark the GCC builtin
    __builtin_clz() and a pure Perl implementation as suggested by Perl
    Monk coldr3ality <https://perlmonks.org/?node_id=1232041> in this
    discussion <https://perlmonks.org/?node_id=11158279>

    clz() operating on the binary representation of a number counts the
    zeros starting from the most significant end until it finds the first
    bit set (to 1). Which essentially gives the zero-based index of the MSB
    set to 1.

    The benchmarks favour the GCC builtin __builtin_clz() which is about
    twice as fast as the pure Perl implementation.

    The benchmarks can be run with make benchmarks An easy way to let Perl
    fetch and unpack the distribution for you is to use cpanm to open a
    shell

       cpanm --look GCC::Builtins

    and then

       perl Makefile.PL && make all && make test && make benchmarks

    The following benchamrk results indicate that the use of GCC::Builtins
    (clz() in this case) yields more than 100% performance gain over
    equivalent pure perl code:

        Benchmark: timing 50000000 iterations of  clz/xs, clz/pp-ugly...
            clz/xs: 3.92331 wallclock secs ( 3.92 usr +  0.00 sys =  3.92 CPU) @ 12755102.04/s (n=50000000)
        clz/pp-ugly: 8.24574 wallclock secs ( 8.23 usr +  0.00 sys =  8.23 CPU) @ 6075334.14/s (n=50000000)
                          Rate clz/pp-ugly      clz/xs
        clz/pp-ugly  6075334/s          --        -52%
         clz/xs     12755102/s        110%          --
        KEY:
         clz/xs : calling GCC builtin clz() via XS from Perl
         clz/pp-ugly : as suggested by coldr3ality (see https://perlmonks.org/?node_id=11158279)
    
        Benchmark: timing 50000000 iterations of  clzl/xs, clzl/pp-ugly...
           clzl/xs: 3.84597 wallclock secs ( 3.84 usr +  0.00 sys =  3.84 CPU) @ 13020833.33/s (n=50000000)
        clzl/pp-ugly: 8.44006 wallclock secs ( 8.43 usr +  0.00 sys =  8.43 CPU) @ 5931198.10/s (n=50000000)
                           Rate clzl/pp-ugly      clzl/xs
        clzl/pp-ugly  5931198/s           --         -54%
         clzl/xs     13020833/s         120%           --
        KEY:
         clzl/xs : calling GCC builtin clzl() via XS from Perl
         clzl/pp-ugly : as suggested by coldr3ality (see https://perlmonks.org/?node_id=11158279)

    So, it pays to use this module if performance is an issue.

CAVEATS

    If you observe weird return results or core-dumps it is very likely
    that the fault is mine while compiling the XS typemap. The file in the
    distribution typemap was compiled by me to translate C's data types
    into Perls. And for some of this I am not sure what the right type is.
    For example, is C's uint_fast16_t equivalent to Perl's T_UV? How about
    C's long double mapping to Perl's T_DOUBLE and unsigned long long to
    T_U_LONG?

    Please report
    <https://rt.cpan.org/NoAuth/ReportBug.html?Queue=GCC-Builtins> any
    corrections.

    Note that lib/GCC/Builtins.pm, lib/GCC/Builtins.xs and typemap are
    auto-generated by above scripts. Do not edit them. Edit
    sbin/build-gcc-builtins-package.pl instead.

AUTHOR

    Andreas Hadjiprocopis, <bliako ta cpan.org / andreashad2 ta gmail.com>

BUGS

    Please report any bugs or feature requests to bug-gcc-builtins at
    rt.cpan.org, or through the web interface at
    https://rt.cpan.org/NoAuth/ReportBug.html?Queue=GCC-Builtins. I will be
    notified, and then you'll automatically be notified of progress on your
    bug as I make changes.

SUPPORT

    You can find documentation for this module with the perldoc command.

        perldoc GCC::Builtins

    You can also look for information at:

      * RT: CPAN's request tracker (report bugs here)

      https://rt.cpan.org/NoAuth/Bugs.html?Dist=GCC-Builtins

      * Review this module at PerlMonks

      https://www.perlmonks.org/?node_id=21144

      * Search CPAN

      https://metacpan.org/release/GCC-Builtins