[Bug c/35170] New: Suspected value range propagation [VRP] failure with ? :

[Bug c/35170] New: Suspected value range propagation [VRP] failure with ? :

[dps at simpson dot demon dot co dot uk]

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There is at least one simple case that the value range deduction apparently
fails to cover. The compiler generates a message about control reaching
possibly the end of a non-void function given the source file:

#include <string.h>

int ns_strcmp(const char *a, const char *b)
{
    int i;
    i= (a==NULL) ? 0 : 1;
    i |= (b==NULL) ? 0 : 2;
    switch(i)
    {
    case 0:                     /* a=NULL, b=NULL */
        return 0;               /* a=b */
        /* NOT REACHED */
    case 1:                     /* a=NULL, b!=NULL */
        return -1;              /* a<b */
        /* NOT REACHED */
    case 2:                     /* a!=NULL, b=NULL */
        return 1;               /* a>b */
        /* NOT REACHED */
    case 3:                     /* a!=NULL, b!=NULL */
        return strcmp(a, b);    /* Use strcmp */
        /* NOT REACHED */
    }
    /* NOT REACHED */
}

Reaching the end of the function is clearly impossible because i must be 0, 1
after  i=(a==NULL) ? 0 : 1 and 0, 1, 2, 3 or after the i |= (b==NULL) ? 0 : 2.
It would be nice if value range propagation could deduce that i>=0 and i<=3,
after which it would be sufficient to notice that all the possible values are
covered in the switch to deduce that the control can not reach beyond the
switch.

However gcc seems to miss this fact, even with -O3, and the generated assembly
seems to include coverage of both i<0 and i>3. Replacing |= with += does not
eliminate the warning about control possibly reaching the end of a non-void
function.

It would be even nicer if I could replace 2 with 4 and adjust the case values
accordingly and still not get a warning but that would require propagation of
more than a single value range. My processor is an AMD Athlon(tm) 64 X2 Dual
Core Processor 4400+, although I suspect the logic that misses the relevant
information is target independent.

I can avoid the warning by handling the last case after the switch.

host$ Linux dps 2.6.24 #9 SMP PREEMPT Sat Feb 2 18:17:42 GMT 2008 x86_64
GNU/Linux
host$ gcc --version
gcc (GCC) 4.3.0 20080209 (experimental)
Copyright (C) 2007 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

host$ gcc -c -O3 -Wall /tmp/message.c  
/tmp/message.c: In function ‘ns_strcmp’:
/tmp/message.c:24: warning: control reaches end of non-void function


-- 
           Summary: Suspected value range propagation [VRP] failure with ? :
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Severity: trivial
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: dps at simpson dot demon dot co dot uk
 GCC build triplet:  x86_64-unknown-linux-gnu
  GCC host triplet:  x86_64-unknown-linux-gnu
GCC target triplet:  x86_64-unknown-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35170

[Bug middle-end/35170] Suspected value range propagation [VRP] failure with ? :

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2008-02-12 01:54 -------
> /tmp/message.c:24: warning: control reaches end of non-void function

this warning happens before VRP anyways.

This is related to PR 14495 (or really the same).


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|c                           |middle-end
  GCC build triplet| x86_64-unknown-linux-gnu   |x86_64-unknown-linux-gnu
   GCC host triplet| x86_64-unknown-linux-gnu   |x86_64-unknown-linux-gnu
 GCC target triplet| x86_64-unknown-linux-gnu   |x86_64-unknown-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35170