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[patch] fold-const.c: Fold (A & ~3) - (A & 3) into (A ^ 3) - 3.
- From: Kazu Hirata <kazu at cs dot umass dot edu>
- To: gcc-patches at gcc dot gnu dot org
- Date: Mon, 29 Sep 2003 12:38:00 -0400 (EDT)
- Subject: [patch] fold-const.c: Fold (A & ~3) - (A & 3) into (A ^ 3) - 3.
Hi,
Attached is a patch to fold (A & ~B) - (A & B) into (A ^ B) - B, where
B is any power of 2 minus 1.
Here is a quick proof:
(A & ~3) - (A & 3) = (A & ~3) + ~(A & 3) + 1
= (A & ~3) + (-4) + (~A & 3) + 1
= (A & ~3) + (~A & 3) - 3
= (A ^ 3) - 3
This occurs in the code that swaps byte order like so
array[(index & ~3) + (3 - (index & 3))]
With this patch, this is simplified down to
array[index ^ 3]
Tested on h8300 port. OK to apply?
Kazu Hirata
2003-09-29 Kazu Hirata <kazu@cs.umass.edu>
* fold-const.c (fold): Fold (A & ~B) - (A & B) into
(A ^ B) - B, where B is any power of 2 minus 1.
Index: fold-const.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/fold-const.c,v
retrieving revision 1.306
diff -u -r1.306 fold-const.c
--- fold-const.c 25 Sep 2003 02:12:13 -0000 1.306
+++ fold-const.c 29 Sep 2003 05:49:21 -0000
@@ -6017,6 +6017,29 @@
TREE_OPERAND (arg1, 1))),
arg0));
}
+
+ /* Fold (A & ~3) - (A & 3) into (A ^ 3) - 3. */
+ if (TREE_CODE (arg0) == BIT_AND_EXPR
+ && TREE_CODE (arg1) == BIT_AND_EXPR
+ && !TREE_SIDE_EFFECTS (TREE_OPERAND (arg0, 0))
+ && operand_equal_p (TREE_OPERAND (arg0, 0),
+ TREE_OPERAND (arg1, 0), 0)
+ && TREE_CODE (TREE_OPERAND (arg0, 1)) == INTEGER_CST
+ && TREE_CODE (TREE_OPERAND (arg1, 1)) == INTEGER_CST)
+ {
+ tree mask0 = TREE_OPERAND (arg0, 1);
+ tree mask1 = TREE_OPERAND (arg1, 1);
+ tree tem = fold (build1 (BIT_NOT_EXPR, type, mask0));
+
+ if (operand_equal_p (tem, mask1, 0)
+ && integer_pow2p (fold (build (PLUS_EXPR, type,
+ mask1, integer_one_node))))
+ {
+ tem = fold (build (BIT_XOR_EXPR, type,
+ TREE_OPERAND (arg0, 0), mask1));
+ return fold (build (MINUS_EXPR, type, tem, mask1));
+ }
+ }
}
/* See if ARG1 is zero and X - ARG1 reduces to X. */