[Bug tree-optimization/45397] New: [4.5/4.6 Regression] Issues with integer narrowing conversions

[Bug tree-optimization/45397] New: [4.5/4.6 Regression] Issues with integer narrowing conversions

[jakub at gcc dot gnu dot org]

In http://gcc.gnu.org/ml/gcc/2010-08/msg00326.html
Revital complained about MAX_EXPR no longer being recognized in:
int foo (const unsigned char *tmp, int i, int val)
{
  return (unsigned char)(((tmp[i] + val)>0xFF)?0xFF:(((tmp[i] +
val)<0)?0:(tmp[i] + val)));
}
It is still recognized when using:
int bar (const unsigned char *tmp, int i, int val)
{
  int x = (((tmp[i] + val)>0xFF)?0xFF:(((tmp[i] + val)<0)?0:(tmp[i] + val)));
  return (unsigned char)x;
}
The regression is caused by folding being delayed in the C FE, while before
the inner COND_EXPR has been optimized by fold_ternary* into MAX_EXPR, now it
isn't immediately, thus convert_to_integer on the narrowing conversion
propagates the (unsigned char) narrowing casts down into the operands and when
fold_ternary* is actually called, it is too late, as the operands aren't
considered equal.

Perhaps it would be nice for -O2 to consider for e.g. int a and b
that (unsigned char) (a + b) is the same as (unsigned char) a + (unsigned char)
b during VN.


-- 
           Summary: [4.5/4.6 Regression] Issues with integer narrowing
                    conversions
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: jakub at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45397

[Bug tree-optimization/45397] [4.5/4.6 Regression] Issues with integer narrowing conversions

[jakub at gcc dot gnu dot org]

------- Comment #1 from jakub at gcc dot gnu dot org  2010-08-24 13:35 -------
On the C FE side, perhaps these optimizations should be moved over from
convert_to_integer to e.g. c_fully_fold_internal, where the operand would be
folded first and only afterwards the narrowing conversion optimization would be
applied.  That would solve this testcase too, but wouldn't help the case where
the narrowing conversion is done by the user already.

int foo (const unsigned char *a, int b, int c)
{
  int x = (unsigned char) (a[b] + c);
  int y = a[b] + c;
  int z = (unsigned char) y;
  return x == z;
}

is another testcase which isn't currently optimized at the tree level (the RTL
level can figure this out though).


-- 

jakub at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jsm28 at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45397

[Bug tree-optimization/45397] [4.5/4.6 Regression] Issues with integer narrowing conversions

[jakub at gcc dot gnu dot org]

------- Comment #2 from jakub at gcc dot gnu dot org  2010-08-24 13:41 -------
Created an attachment (id=21556)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=21556&action=view)
minmax.patch

I've also noticed that the folder only recognizes the inner MAX_EXPR, but
doesn't recognize the outer MIN_EXPR.  Apparently (when convert_to_integer
doesn't harm it, i.e. the bar testcase instead of foo) phiopt1 later on figures
this out.  The attached (untested) patch handles that case early, but not sure
it is worth it.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45397

[Bug tree-optimization/45397] [4.5/4.6 Regression] Issues with integer narrowing conversions

[rguenth at gcc dot gnu dot org]

------- Comment #3 from rguenth at gcc dot gnu dot org  2010-08-24 14:13 -------
Confirmed.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2010-08-24 14:13:05
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45397

[Bug tree-optimization/45397] [4.5/4.6 Regression] Issues with integer narrowing conversions

[rguenth at gcc dot gnu dot org]

-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |4.5.2


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45397

[Bug tree-optimization/45397] [4.5/4.6 Regression] Issues with integer narrowing conversions

[rguenth at gcc dot gnu dot org]

-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P2


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45397

[Bug tree-optimization/45397] [4.5/4.6 Regression] Issues with integer narrowing conversions

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45397

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.5.2                       |4.5.3

--- Comment #4 from Richard Guenther <rguenth at gcc dot gnu.org> 2010-12-16 13:03:18 UTC ---
GCC 4.5.2 is being released, adjusting target milestone.