heh, unpack is actually faster than a custom XS solution (since it's not called via entersub???) 
unpack:      1.200000 1.400000 1.800000 12345678.901230 2147483648
custom:      1.200000 1.400000 1.800000 12345678.901230 2147483648
nonportable: 1.200000 1.400000 1.800000 12345678.901230 2147483648
unsafe:      1.200000 1.400000 1.800000 12345678.901230 2147483648
                Rate      custom      unsafe nonportable      unpack
custom      524225/s          --         -6%         -6%        -20%
unsafe      555156/s          6%          --         -1%        -16%
nonportable 559449/s          7%          1%          --        -15%
unpack      658398/s         26%         19%         18%          --

[download]

(Some variation, but this is a typical run)

You could save a itsy teeny bit by going to pure XS rather than using Inline::C, but I doubt it will be noticeable. 

use strict;
use warnings;

use Benchmark qw( cmpthese );

use Inline C => <<'__EOI__';

/* unpack("FFFFL", sv) */
void custom_unpack(SV* sv) {
   dXSARGS;

   NV  f1;
   NV  f2;
   NV  f3;
   NV  f4;
   U32 u;

   STRLEN len;
   char*  buf = SvPVbyte(sv, len);

   if (len != sizeof(NV)*4 + sizeof(U32))
      croak("Length mismatch");

   memcpy(&f1, buf, sizeof(NV));    buf += sizeof(NV);
   memcpy(&f2, buf, sizeof(NV));    buf += sizeof(NV);
   memcpy(&f3, buf, sizeof(NV));    buf += sizeof(NV);
   memcpy(&f4, buf, sizeof(NV));    buf += sizeof(NV);
   memcpy(&u,  buf, sizeof(U32));

   SP -= items;
   EXTEND(SP, 5);
   mPUSHn(f1);
   mPUSHn(f2);
   mPUSHn(f3);
   mPUSHn(f4);
   mPUSHu(u);
   XSRETURN(5);
}

/* unpack("FFFFL", sv)            */
/* Only works on machine where NV */
/*  and U32 can be byte-aligned   */
void custom_unpack_nonportable(SV* sv) {
   dXSARGS;

   STRLEN len;
   char*  buf = SvPVbyte(sv, len);

   if (len != sizeof(NV)*4 + sizeof(U32))
      croak("Length mismatch");

   SP -= items;
   EXTEND(SP, 5);
   mPUSHn(*(NV*)buf);
   mPUSHn(*(NV*)(buf + sizeof(NV)));
   mPUSHn(*(NV*)(buf + sizeof(NV)*2));
   mPUSHn(*(NV*)(buf + sizeof(NV)*3));
   mPUSHn(*(U32*)(buf + sizeof(NV)*4));
   XSRETURN(5);
}

/* unpack("FFFFL", sv)            */
/* Doesn't get magic              */
/* Doesn't stringify              */
/* Doesn't switch string format   */
/* Doesn't do size checks         */
/* Only works on machine where NV */
/*  and U32 can be byte-aligned   */
void custom_unpack_unsafe(SV* sv) {
   dXSARGS;
   char* buf = SvPVX(sv);
   SP -= items;
   EXTEND(SP, 5);
   mPUSHn(*(NV*)buf);
   mPUSHn(*(NV*)(buf + sizeof(NV)));
   mPUSHn(*(NV*)(buf + sizeof(NV)*2));
   mPUSHn(*(NV*)(buf + sizeof(NV)*3));
   mPUSHn(*(U32*)(buf + sizeof(NV)*4));
   XSRETURN(5);
}

__EOI__

my %tests = (
   'unpack'      => 'my @x = unpack("FFFFL", $buf);',
   'custom'      => 'my @x = custom_unpack($buf);',
   'nonportable' => 'my @x = custom_unpack_nonportable($buf);',
   'unsafe'      => 'my @x = custom_unpack_unsafe($buf);',
);

$_ = 'use strict; use warnings; our $buf; ' . $_
   for values(%tests);

local our $buf = pack('FFFFL', 1.2, 1.4, 1.8, 12345678.90123, 0x800000
+00);

printf("unpack:      %f %f %f %f %u\n", unpack("FFFFL", $buf));
printf("custom:      %f %f %f %f %u\n", custom_unpack($buf));
printf("nonportable: %f %f %f %f %u\n", custom_unpack_nonportable($buf
+));
printf("unsafe:      %f %f %f %f %u\n", custom_unpack_unsafe($buf));

cmpthese(-2, \%tests);

[download]

Update: Added custom_unpack_unsafe

In reply to Re: How do I unpack Perl floats in C by ikegami
in thread How do I unpack Perl floats in C by Anonymous Monk

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