Re: c/7696: Spurious shift warning

Re: c/7696: Spurious shift warning

[Christian Ehrhardt]

The following reply was made to PR c/7696; it has been noted by GNATS.

From: "Christian Ehrhardt" <ehrhardt@mathematik.uni-ulm.de>
To: gcc-gnats@gcc.gnu.org, schwab@suse.de, gcc-bugs@gcc.gnu.org,
  nobody@gcc.gnu.org, gcc-prs@gcc.gnu.org
Cc:  
Subject: Re: c/7696: Spurious shift warning
Date: Sun, 6 Apr 2003 14:17:56 +0200

 http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=7696
 
 PR 7696 started out as a spurious warning on ia64 but it turns out that
 this is probably a wrong code generation bug on any 64bit arch. The
 source code in question is this:
 $ cat 7696.c
                struct ia64_psr {
                  unsigned long cpl : 2;
                } x = {1};
 		
                main ()
                {
                  unsigned long y =  ((unsigned long) x.cpl) << 40;
 		 printf ("%lx %d\n", y, sizeof (y));
 		 return 0;
                }
 $ gcc -m64 7696.c 
 7696.c: In function `main':
 7696.c:7: warning: left shift count >= width of type
 This program will print 
 $ a.out
 100 8
 
 According to gdb the cast is thrown away early (build_binary_op doesn't see
 it at all), probably because the bitfield is of type unsigned long. But
 later on default_conversion sees that the precision is only two and converts
 x.cpl to an integer (instead of a long). This oviouly hurts on arches where
 int has 32 bit but long has 64.
 
 Two different work arounds can be used to cure the problem: either
 declare the bitfield as int or cast x.cpl to long long instead of long.
 Obviously none of these is a fix. I'll see if I can come up with a
 patch.
 
      regards   Christian
 
 -- 
 THAT'S ALL FOLKS!

Re: c/7696: Spurious shift warning

[Falk Hueffner]

The following reply was made to PR c/7696; it has been noted by GNATS.

From: Falk Hueffner <falk.hueffner@student.uni-tuebingen.de>
To: "Christian Ehrhardt" <ehrhardt@mathematik.uni-ulm.de>
Cc: gcc-gnats@gcc.gnu.org, schwab@suse.de, gcc-bugs@gcc.gnu.org,
   nobody@gcc.gnu.org, gcc-prs@gcc.gnu.org
Subject: Re: c/7696: Spurious shift warning
Date: 06 Apr 2003 16:50:37 +0200

 "Christian Ehrhardt" <ehrhardt@mathematik.uni-ulm.de> writes:
 
 >                struct ia64_psr {
 >                  unsigned long cpl : 2;
 >                } x = {1};
 > 		
 >                  unsigned long y =  ((unsigned long) x.cpl) << 40;
 > 
 > According to gdb the cast is thrown away early (build_binary_op
 > doesn't see it at all), probably because the bitfield is of type
 > unsigned long.
 
 All casts of bit-fields to the same type get discarded. This is a very
 long standing bug and has already been reported as c/3325, c/3326,
 c/6346, c/6515 and target/9620. We also have xfailing
 gcc.c-torture/execute/bitfld-1.c for it.
 
 -- 
 	Falk

Re: c/7696: Spurious shift warning

[bangerth]

Synopsis: Spurious shift warning

State-Changed-From-To: open->analyzed
State-Changed-By: bangerth
State-Changed-When: Tue Jan  7 18:15:39 2003
State-Changed-Why:
    Confirmed. Still exists in present 3.3 and 3.4

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=7696

c/7696: Spurious shift warning

[Andreas Schwab]

>Number:         7696
>Category:       c
>Synopsis:       Spurious shift warning
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri Aug 23 04:56:01 PDT 2002
>Closed-Date:
>Last-Modified:
>Originator:     
>Release:        3.2
>Organization:
>Environment:
System: Linux sykes 2.4.18-SMP #1 SMP Thu May 16 09:03:14 UTC 2002 ia64 unknown unknown GNU/Linux
Architecture: ia64

	
host: ia64-suse-linux-gnu
build: ia64-suse-linux-gnu
target: ia64-suse-linux-gnu
configured with: ../configure --enable-threads=posix --prefix=/usr --with-local-prefix=/usr/local --infodir=/usr/share/info --mandir=/usr/share/man --libdir=/usr/lib --enable-languages=c,c++,f77,objc,java,ada --enable-libgcj --with-gxx-include-dir=/usr/include/g++ --with-slibdir=/lib --with-system-zlib --enable-shared --enable-__cxa_atexit ia64-suse-linux
>Description:
There is a spurious warning about shift count too big, although the
operand ist cast to the proper type.
>How-To-Repeat:
$ cat shift.c
struct ia64_psr {
  unsigned long cpl : 2;
};

unsigned long foo (struct ia64_psr *c)
{
  return (unsigned long) c->cpl << 62;
}
$ gcc -O2 -g -Wall    -c -o shift.o shift.c
shift.c: In function `foo':
shift.c:7: warning: left shift count >= width of type
>Fix:
	
>Release-Note:
>Audit-Trail:
>Unformatted: