[Bug c/47742] New: Large Values Increment By 1 When Casting Long to Double

[dogbreath69 at hotmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47742

   Summary: Large Values Increment By 1 When Casting Long to
Double
   Product: gcc
   Version: 4.1.2
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: c
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: dogbreat...@hotmail.com


When casting some large long values to a double the resultant double values is
1 larger than the long. This can easily be seen with the value for LONG_MAX,
but will even appear with long values as low as 23372036854775807 (and possibly
lower). It is not consistent and does not affect all large longs, but does
appear in a great number of them.

ratscat> gcc -v
Using built-in specs.
Target: x86_64-redhat-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man
--infodir=/usr/share/info --enable-shared --enable-threads=posix
--enable-checking=release --with-system-zlib --enable-__cxa_atexit
--disable-libunwind-exceptions --enable-libgcj-multifile
--enable-languages=c,c++,objc,obj-c++,java,fortran,ada --enable-java-awt=gtk
--disable-dssi --enable-plugin
--with-java-home=/usr/lib/jvm/java-1.4.2-gcj-1.4.2.0/jre --with-cpu=generic
--host=x86_64-redhat-linux
Thread model: posix
gcc version 4.1.2 20080704 (Red Hat 4.1.2-48)

Program is:
#include 
#include 
#include 
#include 

int main( int argc, char **argv ) {

   long infc = LONG_MAX;
   double real_size = (double) infc;

   printf( "infc  %ld\nreal_size %lf\n", infc, real_size );

   exit( 0 );
}


ratscat> gcc -Wall truncfc.c
ratscat> a.out
infc  9223372036854775807
real_size 9223372036854775808.00

[Bug c/47742] Large Values Increment By 1 When Casting Long to Double

[dogbreath69 at hotmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47742

--- Comment #4 from dogbreath69 at hotmail dot com 2011-02-16 05:08:06 UTC ---
I haven't tried it with anything greater than GCC 4.1.2. Therefore I do not
know the answer.

> From: gcc-bugzi...@gcc.gnu.org
> To: dogbreat...@hotmail.com
> Subject: [Bug c/47742] Large Values Increment By 1 When Casting Long to Double
> Date: Tue, 15 Feb 2011 23:16:17 +
> 
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47742
> 
> --- Comment #3 from Manuel López-Ibáñez  2011-02-15 
> 23:16:09 UTC ---
> Let me try again:
> 
> I think GCC (>= 4.4) should catch this using -Wconversion if you drop the
> explicit
> cast like in:
> 
> long infc = LONG_MAX;
> double real_size = infc;
> 
> Doesn't it?
> 
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