Hi!
In x * x = ((x & -2) + (x & 1))^2 = ((x^2) & -4) + 2 * (x & -2) * (x & 1) + (x
& 1)^2
the first two terms are divisible by 4 and the last one is 0 or 1, so
(x * x) % 4U <= 1
and thus bit ccp can assume he second least significant bit of any power of two
is
0.
Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
2020-01-07 Jakub Jelinek <ja...@redhat.com>
PR tree-optimization/93156
* tree-ssa-ccp.c (bit_value_binop): For x * x note that the second
least significant bit is always clear.
* gcc.dg/tree-ssa/pr93156.c: New test.
--- gcc/tree-ssa-ccp.c.jj 2020-01-01 12:15:48.026609558 +0100
+++ gcc/tree-ssa-ccp.c 2020-01-06 15:39:16.807109578 +0100
@@ -1650,6 +1650,17 @@ bit_value_binop (enum tree_code code, tr
TYPE_SIGN (TREE_TYPE (rhs2)), TYPE_PRECISION (TREE_TYPE
(rhs2)),
value_to_wide_int (r2val), r2val.mask);
+ /* (x * x) & 2 == 0. */
+ if (code == MULT_EXPR && rhs1 == rhs2 && TYPE_PRECISION (type) > 1)
+ {
+ widest_int m = 2;
+ if (wi::sext (mask, TYPE_PRECISION (type)) != -1)
+ value = wi::bit_and_not (value, m);
+ else
+ value = 0;
+ mask = wi::bit_and_not (mask, m);
+ }
+
if (wi::sext (mask, TYPE_PRECISION (type)) != -1)
{
val.lattice_val = CONSTANT;
--- gcc/testsuite/gcc.dg/tree-ssa/pr93156.c.jj 2020-01-06 15:26:57.750286597
+0100
+++ gcc/testsuite/gcc.dg/tree-ssa/pr93156.c 2020-01-06 15:40:32.987957791
+0100
@@ -0,0 +1,23 @@
+/* PR tree-optimization/93156 */
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+/* { dg-final { scan-tree-dump-times "return 0;" 3 "optimized" } } */
+
+int
+foo (int x)
+{
+ return (x * x) & 2;
+}
+
+unsigned long long
+bar (unsigned long long x)
+{
+ return (x * x) & 2;
+}
+
+int
+baz (int x)
+{
+ x &= -2;
+ return (x * x) & 3;
+}
Jakub
