[Bug c/50347] New: unexpected -Wconversion error from gcc builtin

[Bug c/50347] New: unexpected -Wconversion error from gcc builtin

[gcc.hall at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50347

             Bug #: 50347
           Summary: unexpected -Wconversion error from gcc builtin
    Classification: Unclassified
           Product: gcc
           Version: 4.6.1
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: gcc.hall@gmail.com


This message is produced from the code example below.

bug.c: In function 'main':
bug.c:17:2: warning: conversion to 'long long int' from 'long long unsigned
int' may change the sign of the result [-Wsign-conversion]

builtin_ffsll is documented as:-
Built-in Function: int __builtin_ffsll (unsigned long long)

    Similar to __builtin_ffs, except the argument type is unsigned long long. 

Note this does not happen with the other similar builtin's (__ctzll for
example)

-----------------------------------------------------------------
#include <stdio.h>
#include <stdlib.h>

int
main( int argc, char *argv[] )
{
  if( argc > 1 )
  {
    char *tail;
    unsigned long long n = strtoull( argv[1], &tail, 0 );
    if( tail == argv[1] )
      return 1;

    int lsb = __builtin_ffsll( n );

    printf("lsb = %d\n", lsb );
  }
  return 0;
}
-------------------------------------------------------------------
* gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/local/libexec/gcc/i686-pc-linux-gnu/4.6.1/lto-wrapper
Target: i686-pc-linux-gnu
Configured with: ../gcc-4.6.1/configure
Thread model: posix
gcc version 4.6.1 (GCC)

[Bug c/50347] unexpected -Wconversion error from gcc builtin

[gcc.hall at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50347

--- Comment #1 from Jeremy <gcc.hall at gmail dot com> 2011-09-10 10:15:15 UTC ---
I see this builtin is presumably intended to implement the library function
ffsll() which takes a signed argument.  In which case this is just a
documentation issue.

[Bug c/50347] unexpected -Wconversion error from gcc builtin

[gcc.hall at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50347

--- Comment #2 from Jeremy <gcc.hall at gmail dot com> 2011-09-10 21:57:59 UTC ---
I think also the doc needs changing for __builtin_bswap64/32 as it looks like
they accept and return unsigned integers.  uint64_t instead of int64_t.

[Bug c/50347] unexpected -Wconversion error from gcc builtin

[manu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50347

Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |manu at gcc dot gnu.org

--- Comment #3 from Manuel López-Ibáñez <manu at gcc dot gnu.org> 2012-06-07 14:02:12 UTC ---
Could you send a doc patch to gcc-patches@ ? Thanks!

[Bug c/50347] unexpected -Wconversion error from gcc builtin

[mpolacek at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50347

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |documentation
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2014-03-25
                 CC|                            |mpolacek at gcc dot gnu.org
           Assignee|unassigned at gcc dot gnu.org      |mpolacek at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #4 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
__builtin_bswap* were fixed in r191774.  I have a patch for ffs* builtins.

[Bug c/50347] unexpected -Wconversion error from gcc builtin

[mpolacek at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50347

--- Comment #5 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Author: mpolacek
Date: Thu Mar 27 15:59:07 2014
New Revision: 208870

URL: http://gcc.gnu.org/viewcvs?rev=208870&root=gcc&view=rev
Log:
    PR c/50347
    * doc/extend.texi (ffs Builtins): Change unsigned types to signed
    types.


Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/doc/extend.texi

[Bug c/50347] unexpected -Wconversion error from gcc builtin

[mpolacek at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50347

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED

--- Comment #6 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Fixed.