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Date: Wed, 22 Jul 2015 12:10:00 -0000
Message-ID: <CAFiYyc2=EyQvrDK+4PfUMQwHLY1EK0ZDg5WoY+ZXQ8k5oP_7NQ@mail.gmail.com>
Subject: Re: [PR25529] Convert (unsigned t * 2)/2 into unsigned (t & 0x7FFFFFFF)
From: Richard Biener <richard.guenther@gmail.com>
To: "Hurugalawadi, Naveen" <Naveen.Hurugalawadi@caviumnetworks.com>
Cc: "marc.glisse@inria.fr" <marc.glisse@inria.fr>, GCC Patches <gcc-patches@gcc.gnu.org>
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On Tue, Jul 21, 2015 at 11:16 AM, Hurugalawadi, Naveen
<Naveen.Hurugalawadi@caviumnetworks.com> wrote:
> Hi,
>
>>> For signed types with TYPE_OVERFLOW_UNDEFINED
>>> you can simply cancel the operation (even for non-power-of-two multipliers).
>
> Thanks for the review and comments.
>
> Please find attached the modified patch as per your comments.
>
> Please review the same and let me know if any further modifications are required.
>
> Regression Tested on X86_64.

@@ -280,6 +280,20 @@ along with GCC; see the file COPYING3.  If not see
        && integer_pow2p (@2) && tree_int_cst_sgn (@2) > 0)
    (bit_and @0 (convert (minus @1 { build_int_cst (TREE_TYPE (@1), 1); }))))))

+/* Simplify (unsigned t * 2)/2 -> unsigned t & 0x7FFFFFFF.  */
+(simplify
+ (exact_div (mult @0 INTEGER_CST@1) @1)
+ (if (TYPE_OVERFLOW_UNDEFINED (TREE_TYPE (@0)))
+  @0))

The comment applies to the pattern below and the pattern above lacks a comment

+(simplify
+ (trunc_div (mult @0 integer_pow2p@1) @1)
+ (if (TYPE_UNSIGNED (TREE_TYPE (@0)))
+ (with { tree n2 = build_int_cst (TREE_TYPE (@0),
+                                 wi::exact_log2 (@1)); }
+  (bit_and @0 (rshift (lshift { build_minus_one_cst (TREE_TYPE (@0)); }
+                               { n2; }) { n2; })))))

please use

  (with
    {
      int n2 = wi::exact_log2 (@1);
      tree mask = wide_int_to_tree (type, wi::rshift (wi::lshift (-1, n2), n2));
    }
   (bit_and @0 { mask; }))))

in fact, the -1 << log2 >> log2 looks like it does wi::mask
(TYPE_PRECISION (type) - wi::exact_log2 (@1), false, TYPE_PRECISION
(type));
so using wi::mask is prefered here.

Thanks,
Richard.


> Thanks,
> Naveen
>
> gcc/testsuite/ChangeLog:
>
> 2015-07-21  Naveen H.S  <Naveen.Hurugalawadi@caviumnetworks.com>
>
>         PR middle-end/25529
>         * gcc.dg/pr25529.c: New test.
>
> gcc/ChangeLog:
>
> 2015-07-21  Naveen H.S  <Naveen.Hurugalawadi@caviumnetworks.com>
>
>         PR middle-end/25529
>         * match.pd (exact_div (mult @0 INTEGER_CST@1) @1) :     New simplifier.
>         (trunc_div (mult @0 integer_pow2p@1) @1) : New simplifier.