PR tree-optimization/71343: Optimize (X<<C)&(Y<<C) as (X&Y)<<C.

This patch is the first part of a solution to PR tree-optimization/71343,
a missed-optimization enhancement request where GCC fails to see that
(a<<2)+(b<<2) == a*4+b*4.

This piece is that (X<<C) op (Y<<C) can be simplified to (X op Y) << C,
for many binary operators, including AND, IOR, XOR, and (if overflow
isn't an issue) PLUS and MINUS.  Likewise, the right shifts (both logical
and arithmetic) and bit-wise logical operators can be simplified in a
similar fashion.  These all reduce the number of GIMPLE binary operations
from 3 to 2, by combining/eliminating a shift operation.

2022-08-15  Roger Sayle  <roger@nextmovesoftware.com>
	    Richard Biener  <rguenther@suse.de>

gcc/ChangeLog
	PR tree-optimization/71343
	* match.pd (op (lshift @0 @1) (lshift @2 @1)): Optimize the
	expression (X<<C) + (Y<<C) to (X+Y)<<C for multiple operators.
	(op (rshift @0 @1) (rshift @2 @1)): Likewise, simplify (X>>C)^(Y>>C)
	to (X^Y)>>C for binary logical operators, AND, IOR and XOR.

gcc/testsuite/ChangeLog
	PR tree-optimization/71343
	* gcc.dg/pr71343-1.c: New test case.

diff --git a/gcc/match.pd b/gcc/match.pd
index c22bc2ca1e811594bfe1ccd5369091b8eda56aaa..e7d10f427a94303378d4c28350059bd9fcb690d8 100644 (file)
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -982,6 +982,26 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
        && tree_nop_conversion_p (type, TREE_TYPE (@1)))
    (lshift @0 @2)))
 
+/* Shifts by constants distribute over several binary operations,
+   hence (X << C) + (Y << C) can be simplified to (X + Y) << C.  */
+(for op (plus minus)
+  (simplify
+    (op (lshift:s @0 @1) (lshift:s @2 @1))
+    (if (INTEGRAL_TYPE_P (type)
+        && TYPE_OVERFLOW_WRAPS (type)
+        && !TYPE_SATURATING (type))
+      (lshift (op @0 @2) @1))))
+
+(for op (bit_and bit_ior bit_xor)
+  (simplify
+    (op (lshift:s @0 @1) (lshift:s @2 @1))
+    (if (INTEGRAL_TYPE_P (type))
+      (lshift (op @0 @2) @1)))
+  (simplify
+    (op (rshift:s @0 @1) (rshift:s @2 @1))
+    (if (INTEGRAL_TYPE_P (type))
+      (rshift (op @0 @2) @1))))
+
 /* Fold (1 << (C - x)) where C = precision(type) - 1
    into ((1 << C) >> x). */
 (simplify

diff --git a/gcc/testsuite/gcc.dg/pr71343-1.c b/gcc/testsuite/gcc.dg/pr71343-1.c
new file mode 100644 (file)
index 0000000..146f5fc
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr71343-1.c
@@ -0,0 +1,56 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+
+unsigned int foo_plus(unsigned int a, unsigned int b)
+{
+  return (a << 2) + (b << 2);
+}
+
+unsigned int foo_and(unsigned int a, unsigned int b)
+{
+  return (a << 2) & (b << 2);
+}
+
+unsigned int foo_ior(unsigned int a, unsigned int b)
+{
+  return (a << 2) | (b << 2);
+}
+
+unsigned int foo_xor(unsigned int a, unsigned int b)
+{
+  return (a << 2) ^ (b << 2);
+}
+
+unsigned int bar_and(unsigned int a, unsigned int b)
+{
+  return (a >> 2) & (b >> 2);
+}
+
+unsigned int bar_ior(unsigned int a, unsigned int b)
+{
+  return (a >> 2) | (b >> 2);
+}
+
+unsigned int bar_xor(unsigned int a, unsigned int b)
+{
+  return (a >> 2) ^ (b >> 2);
+}
+
+int baz_and(int a, int b)
+{
+  return (a >> 2) & (b >> 2);
+}
+
+int baz_ior(int a, int b)
+{
+  return (a >> 2) | (b >> 2);
+}
+
+int baz_xor(int a, int b)
+{
+  return (a >> 2) ^ (b >> 2);
+}
+
+/* { dg-final { scan-tree-dump-times " << " 4 "optimized" } } */
+/* { dg-final { scan-tree-dump-times " >> " 6 "optimized" } } */
+