Generalizing the x+(x&1) pattern, one can round up x to a multiple of
a 2^k by adding the negative of x modulo 2^k. But it is fewer
instructions, and presumably requires fewer registers, to do the more
common (x+m)&~m where m=2^k-1.
Signed-off-by: Rasmus Villemoes <r...@rasmusvillemoes.dk>
---
gcc/match.pd | 9 ++++++
gcc/testsuite/gcc.dg/20150120-4.c | 59 +++++++++++++++++++++++++++++++++++++++
2 files changed, 68 insertions(+)
create mode 100644 gcc/testsuite/gcc.dg/20150120-4.c
diff --git gcc/match.pd gcc/match.pd
index 47865f1..93c2298 100644
--- gcc/match.pd
+++ gcc/match.pd
@@ -273,6 +273,15 @@ along with GCC; see the file COPYING3. If not see
(if (TREE_CODE (@2) != SSA_NAME || has_single_use (@2))
(bit_ior @0 (bit_not @1))))
+/* x + ((-x) & m) -> (x + m) & ~m when m == 2^k-1. */
+(simplify
+ (plus:c @0 (bit_and@2 (negate @0) CONSTANT_CLASS_P@1))
+ (with { tree cst = fold_binary (PLUS_EXPR, TREE_TYPE (@1),
+ @1, build_one_cst (TREE_TYPE (@1))); }
+ (if ((TREE_CODE (@2) != SSA_NAME || has_single_use (@2))
+ && cst && integer_pow2p (cst))
+ (bit_and (plus @0 @1) (bit_not @1)))))
+
(simplify
(abs (negate @0))
(abs @0))
diff --git gcc/testsuite/gcc.dg/20150120-4.c gcc/testsuite/gcc.dg/20150120-4.c
new file mode 100644
index 0000000..c3552bf
--- /dev/null
+++ gcc/testsuite/gcc.dg/20150120-4.c
@@ -0,0 +1,59 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-original" } */
+
+/* x + ((-x) & m) -> (x + m) & ~m for m one less than a pow2. */
+int
+fn1 (int x)
+{
+ return x + ((-x) & 7);
+}
+int
+fn2 (int x)
+{
+ return ((-x) & 7) + x;
+}
+unsigned int
+fn3 (unsigned int x)
+{
+ return x + ((-x) & 7);
+}
+unsigned int
+fn4 (unsigned int x)
+{
+ return ((-x) & 7) + x;
+}
+unsigned int
+fn5 (unsigned int x)
+{
+ return x + ((-x) % 8);
+}
+unsigned int
+fn6 (unsigned int x)
+{
+ return ((-x) % 8) + x;
+}
+int
+fn7 (int x)
+{
+ return x + ((-x) & 9);
+}
+int
+fn8 (int x)
+{
+ return ((-x) & 9) + x;
+}
+unsigned int
+fn9 (unsigned int x)
+{
+ return x + ((-x) & ~0U);
+}
+unsigned int
+fn10 (unsigned int x)
+{
+ return ((-x) & ~0U) + x;
+}
+
+
+/* { dg-final { scan-tree-dump-times "x \\+ 7" 6 "original" } } */
+/* { dg-final { scan-tree-dump-times "-x & 9" 2 "original" } } */
+/* { dg-final { scan-tree-dump-times "return 0" 2 "original" } } */
--
2.1.3
