Re: fold x ^ y to 0 if x == y

[Richard Biener <rguenther at suse dot de>]

On Fri, 8 Jul 2016, Prathamesh Kulkarni wrote:

> Hi Richard,
> For the following test-case:
> 
> int f(int x, int y)
> {
>    int ret;
> 
>    if (x == y)
>      ret = x ^ y;
>    else
>      ret = 1;
> 
>    return ret;
> }
> 
> I was wondering if x ^ y should be folded to 0 since
> it's guarded by condition x == y ?
> 
> optimized dump shows:
> f (int x, int y)
> {
>   int iftmp.0_1;
>   int iftmp.0_4;
> 
>   <bb 2>:
>   if (x_2(D) == y_3(D))
>     goto <bb 3>;
>   else
>     goto <bb 4>;
> 
>   <bb 3>:
>   iftmp.0_4 = x_2(D) ^ y_3(D);
> 
>   <bb 4>:
>   # iftmp.0_1 = PHI <iftmp.0_4(3), 1(2)>
>   return iftmp.0_1;
> 
> }
> 
> The attached patch tries to fold for above case.
> I am checking if op0 and op1 are equal using:
> if (bitmap_intersect_p (vr1->equiv, vr2->equiv)
>    && operand_equal_p (vr1->min, vr1->max)
>    && operand_equal_p (vr2->min, vr2->max))
>   { /* equal /* }
> 
> I suppose intersection would check if op0 and op1 have equivalent ranges,
> and added operand_equal_p check to ensure that there is only one
> element within the range. Does that look correct ?
> Bootstrap+test in progress on x86_64-unknown-linux-gnu.

I think VRP is the wrong place to catch this and DOM should have but it
does

Optimizing block #3

1>>> STMT 1 = x_2(D) le_expr y_3(D)
1>>> STMT 1 = x_2(D) ge_expr y_3(D)
1>>> STMT 1 = x_2(D) eq_expr y_3(D)
1>>> STMT 0 = x_2(D) ne_expr y_3(D)
0>>> COPY x_2(D) = y_3(D)
0>>> COPY y_3(D) = x_2(D)
Optimizing statement ret_4 = x_2(D) ^ y_3(D);
  Replaced 'x_2(D)' with variable 'y_3(D)'
  Replaced 'y_3(D)' with variable 'x_2(D)'
  Folded to: ret_4 = x_2(D) ^ y_3(D);
LKUP STMT ret_4 = x_2(D) bit_xor_expr y_3(D)

heh, registering both reqivalencies is obviously not going to help...

The 2nd equivalence is from doing

      /* We already recorded that LHS = RHS, with canonicalization,
         value chain following, etc.

         We also want to record RHS = LHS, but without any 
canonicalization
         or value chain following.  */
      if (TREE_CODE (rhs) == SSA_NAME)
        const_and_copies->record_const_or_copy_raw (rhs, lhs,
                                                    SSA_NAME_VALUE (rhs));

generally recording both is not helpful.  Jeff?  This seems to be
r233207 (fix for PR65917) which must have regressed this testcase.

Richard.