[PATCH][RFC][match.pd] optimize (X & C) == N when C is power of 2

[Kyrill Tkachov <kyrylo dot tkachov at arm dot com>]

Hi all,

This transformation folds (X % C) == N into
X & ((1 << (size - 1)) | (C - 1))) == N
for constants C and N where N is positive and C is a power of 2.

The idea is similar to the existing X % C -> X & (C - 1) transformation
for unsigned values but this time when we have the comparison we use the
C - 1 mask (all 1s for power of 2 N) orred with the sign bit.

At runtime, if X is positive then X & ((1 << (size - 1)) | (C - 1)))
calculates X % C, which is compared against the positive N.

If X is negative then X & ((1 << (size - 1)) | (C - 1))) doesn't calculate
X % C but since we also catch the set sign bit, it will never compare equal
to N (which is positive), so we still get the right comparison result.

I don't have much experience with writing match.pd patterns, so I appreciate
any feedback on how to write this properly.

Bootstrapped and tested on arm, aarch64, x86_64.

Thanks,
Kyrill

2015-07-24  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

    * match.pd ((X % C) == N -> (X & ((1 << (size - 1)) | (C - 1))) == N):
    Transform when N is positive and C is a power of 2.

2015-07-24  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

    * gcc.dg/fold-mod-cmp-1.c: New test.

commit af785bad5cb32ef2bc35503ebbe65414b67ef8b1
Author: Kyrylo Tkachov <kyrylo.tkachov@arm.com>
Date:   Tue Jul 14 18:20:28 2015 +0100

    [match.pd] optimize (X & C) == N when C is power of 2

diff --git a/gcc/match.pd b/gcc/match.pd
index 9cf0278..02ad708 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -266,7 +266,9 @@ along with GCC; see the file COPYING3.  If not see
 /* Optimize TRUNC_MOD_EXPR by a power of two into a BIT_AND_EXPR,
    i.e. "X % C" into "X & (C - 1)", if X and C are positive.
    Also optimize A % (C << N)  where C is a power of 2,
-   to A & ((C << N) - 1).  */
+   to A & ((C << N) - 1).
+   Optimize (X % C) == N into (X & ((1 << (size - 1)) | (C - 1))) == N
+   when C is signed, a power of 2 and N is positive.  */
 (match (power_of_two_cand @1)
  INTEGER_CST@1)
 (match (power_of_two_cand @1)
@@ -278,7 +280,17 @@ along with GCC; see the file COPYING3.  If not see
 	|| tree_expr_nonnegative_p (@0))
 	&& tree_nop_conversion_p (type, TREE_TYPE (@3))
 	&& integer_pow2p (@2) && tree_int_cst_sgn (@2) > 0)
-   (bit_and @0 (convert (minus @1 { build_int_cst (TREE_TYPE (@1), 1); }))))))
+   (bit_and @0 (convert (minus @1 { build_int_cst (TREE_TYPE (@1), 1); })))))
+
+ (simplify
+  (eq (mod @0 (INTEGER_CST@1)) INTEGER_CST@2)
+  (if (integer_pow2p (@1) && tree_expr_nonnegative_p (@2)
+       && tree_to_uhwi (@2) < tree_to_uhwi (@1))
+   (eq (bit_and @0
+         (bit_ior (minus @1 { build_int_cst (TREE_TYPE (@1), 1); })
+         { build_int_cst (type, 1 << (tree_to_uhwi (TYPE_SIZE (type)) - 1)); })
+        )
+    @2))))
 
 /* X % Y is smaller than Y.  */
 (for cmp (lt ge)
diff --git a/gcc/testsuite/gcc.dg/fold-mod-cmp-1.c b/gcc/testsuite/gcc.dg/fold-mod-cmp-1.c
new file mode 100644
index 0000000..9dcf313
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/fold-mod-cmp-1.c
@@ -0,0 +1,12 @@
+/* { dg-do compile } */
+/* { dg-options "-fdump-tree-original" } */
+
+int
+modcmp (int a)
+{
+  return (a % 32) == 6;
+}
+
+/* The above should be simplified to (a & -2147483617) == 6.  */
+/* { dg-final { scan-tree-dump "& -2147483617" "original" } } */
+/* { dg-final { scan-tree-dump "== 6" "original" } } */