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Re: [PR25529] Convert (unsigned t * 2)/2 into unsigned (t & 0x7FFFFFFF)

From: Marc Glisse <marc dot glisse at inria dot fr>
To: Richard Biener <richard dot guenther at gmail dot com>
Cc: "gcc-patches at gcc dot gnu dot org" <gcc-patches at gcc dot gnu dot org>, "Hurugalawadi, Naveen" <Naveen dot Hurugalawadi at caviumnetworks dot com>
Date: Tue, 7 Jul 2015 11:24:35 +0200 (CEST)
Subject: Re: [PR25529] Convert (unsigned t * 2)/2 into unsigned (t & 0x7FFFFFFF)
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Reply-to: gcc-patches at gcc dot gnu dot org

On Tue, 7 Jul 2015, Richard Biener wrote:

On Tue, Jul 7, 2015 at 8:06 AM, Marc Glisse <marc.glisse@inria.fr> wrote:

On Tue, 7 Jul 2015, Hurugalawadi, Naveen wrote:

Please find attached the patch "PR25529.patch" that converts the pattern:-
(unsigned * 2)/2 is into unsigned &0x7FFFFFFF



+/* Simplify (unsigned t * 2)/2 -> unsigned t & 0x7FFFFFFF.  */
+(for div (trunc_div ceil_div floor_div round_div exact_div)
+ (simplify
+  (div (mult @0 INTEGER_CST@1) INTEGER_CST@1)

You don't need to repeat INTEGER_CST, the second time @1 is enough.

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

What happens if you write t*3/3?


Huh, and you posted this patch twice?  See my reply to the other copy
for the correctness issues and better handling of exact_div

They are not the same, one is for left shifts and the other one for rightshifts. And that makes a big difference: in t*c/c, the division is alwaysexact, so all divisions are equivalent. This is not the case for t/c*c.


--
Marc Glisse

Follow-Ups:
- Re: [PR25529] Convert (unsigned t * 2)/2 into unsigned (t & 0x7FFFFFFF)
  - From: Richard Biener

References:
- [PR25529] Convert (unsigned t * 2)/2 into unsigned (t & 0x7FFFFFFF)
  - From: Hurugalawadi, Naveen
- Re: [PR25529] Convert (unsigned t * 2)/2 into unsigned (t & 0x7FFFFFFF)
  - From: Marc Glisse
- Re: [PR25529] Convert (unsigned t * 2)/2 into unsigned (t & 0x7FFFFFFF)
  - From: Richard Biener

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