Re: [PR25530] Convert (unsigned t / 2) * 2 into (unsigned t & ~1)

[Richard Biener <richard dot guenther at gmail dot com>]

On Tue, Jul 7, 2015 at 6:52 AM, Hurugalawadi, Naveen
<Naveen.Hurugalawadi@caviumnetworks.com> wrote:
> Hi,
>
> Please find attached the patch PR25530.patch that converts the pattern:-
> (unsigned / 2) * 2 is into (unsigned & ~1).
>
> Please review and let me know if its okay.

For EXACT_DIV fold-const.c has

          /* ((T) (X /[ex] C)) * C cancels out if the conversion is
             sign-changing only.  */
          if (TREE_CODE (arg1) == INTEGER_CST
              && TREE_CODE (arg0) == EXACT_DIV_EXPR
              && operand_equal_p (arg1, TREE_OPERAND (arg0, 1), 0))
            return fold_convert_loc (loc, type, TREE_OPERAND (arg0, 0));

we know the remainder is zero for EXACT_DIV.  It also gives hints that
a sign-changing conversion is ok.

+/* Simplify (unsigned t / 2) * 2 -> unsigned t & ~1.  */
+/* PR25530.  */
+(for div (trunc_div ceil_div floor_div round_div exact_div)
+ (simplify
+  (mult (div @0 INTEGER_CST@1) INTEGER_CST@1)
+  (with { tree n2 = build_int_cst (TREE_TYPE (@0),
+                                  wi::exact_log2 (@1)); }
+  (if (TYPE_UNSIGNED (TREE_TYPE (@0)))
+   (bit_and @0 (lshift (rshift { build_minus_one_cst (TREE_TYPE (@0)); }
+                              { n2; }) { n2; }))))))

you should move the (with inside the (if to save work if the type is not
unsigned.  Also you are using wi::exact_log2 without checking whether
@1 was a power of two (I think exact_log2 returns -1 in this case).
Then expressing ~1 with the result expression is really excessive - you
should simply build this with @1 - 1 if @1 is a power of two.

So please handle exact_div differently, like fold-const.c does.

Also I am not sure ceil_div and floor_div can be handled this way.
(5 /[ceil] 2) * 2 == 6 but you compute it as 4.  So I am only convinced
trunc_div works this way.

Thanks,
Richard.

> Regression tested on AARH64 and x86_64.
>
> Thanks,
> Naveen
>
> gcc/testsuite/ChangeLog:
>
> 2015-07-07  Naveen H.S  <Naveen.Hurugalawadi@caviumnetworks.com>
>
>         PR middle-end/25530
>         * gcc.dg/pr25530.c: New test.
>
> gcc/ChangeLog:
>
> 2015-07-07  Naveen H.S  <Naveen.Hurugalawadi@caviumnetworks.com>
>
>         PR middle-end/25530
>         * match.pd (mult (div @0 INTEGER_CST@1) INTEGER_CST@1) :
>         New simplifier.