Re: Fast binary IEEE to float

From:

=?UTF-8?B?UGF3ZcWC?= Sikora <root@atlantis.news.tpi.pl>

Newsgroups:

comp.lang.c++.moderated

Date:

4 May 2006 17:59:18 -0400

Message-ID:

<e3ckpv$rk4$1@atlantis.news.tpi.pl>

kanze wrote:

Actually, I think I'd write it using references :

    float QuickBinaryToFloat( uint32_t in )
    {
        return reinterpret_cast< float& >( in ) ;
    }

gcc-4.1.1-svn:

bin2float.cpp: In function 'float quickBinaryToFloat(unsigned int)':
bin2float.cpp:3: warning: dereferencing type-punned pointer will
                          break strict-aliasing rules

00000000 <quickBinaryToFloat(unsigned int)>:
   0: d9 44 24 04 flds 0x4(%esp)
   4: c3 ret

but...

float quickBinaryToFloat( unsigned& in )
{
        return reinterpret_cast< float& >( in ) ;
}

00000000 <quickBinaryToFloat(unsigned int&)>:
   0: 8b 44 24 04 mov 0x4(%esp),%eax
   4: d9 00 flds (%eax)
   6: c3 ret

2) Method two: The "union trick":

float QuickBinaryToFloat(LONG in)
{
union {
LONG i;
float o;
} u;

u.i = in;
return u.o;
}

Formally, that's undefined behavior.

could you explain this?

if it's true tell_endian_good() from [1] is also UB.

[1] http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26069#c2

[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.moderated. First time posters: Do this! ]