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Re: [PATCH] Reassociate X == CST1 || X == CST2 if popcount (CST2 - CST1) == 1 into ((X - CST1) & ~(CST2 - CST1)) == 0


On 10/15/13 02:12, Jakub Jelinek wrote:
On Tue, Oct 15, 2013 at 03:57:23PM +0800, Zhenqiang Chen wrote:
Is it OK?

Ok, except two comments apparently still need updating.

+/* Optimize X == CST1 || X == CST2
+   if popcount (CST1 ^ CST2) == 1 into
+   (X & ~(CST1 ^ CST2)) == (CST1 & ~(CST1 ^ CST2)).
+   Similarly for ranges.  E.g.
+   X != 2 && X != 3 && X != 10 && X != 11
+   will be transformed by the previous optimization into
+   (X - 2U) <= 1U && (X - 10U) <= 1U
+   and this loop can transform that into
+   ((X & ~8) - 2U) <= 1U.  */

Here the example is using != and &&, so it is transformed into:
    !((X - 2U) <= 1U || (X - 10U) <= 1U)
(everything is negated at the end, and note || instead of &&,
with && it could fold into !(0))
and finally into:
    !(((X & ~8) - 2U) <= 1U)
Or alternatively change the first expression into:
   X == 2 || X == 3 || X == 10 || X == 11
and the second to:
   (X - 2U) <= 1U || (X - 10U) <= 1U
and the third keep as is.

+/* Optimize X == CST1 || X == CST2
+   if popcount (CST2 - CST1) == 1 into
+   ((X - CST1) & ~(CST2 - CST1)) == 0.
+   Similarly for ranges.  E.g.
+   X == 43 || X == 76 || X == 44 || X == 78 || X == 77 || X == 46
+   || X == 75 || X == 45
+   will be transformed by the previous optimization into
+   (X - 43U) <= 3U && (X - 75U) <= 3U
+   and this loop can transform that into
+   ((X - 43U) & ~(75U - 43U)) <= 3U.  */

Here the example is using == and ||, so the only wrong thing in there
is that the second expression should be
    (X - 43U) <= 3U || (X - 75U) <= 3U

Ok with those changes.
I fixed up the comments & ChangeLog entry and committed on Zhenqiang's behalf.

jeff


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