[COMMITTED] irange: keep better track of powers of 2.

[Aldy Hernandez <aldyh@redhat.com>]

When setting the nonzero bits to a mask containing only one bit, set
the range immediately, as it can be devined from the mask.  This helps
us keep better track of powers of two.

For example, with this patch a nonzero mask of 0x8000 is set to a
range of [0,0][0x8000,0x8000] with a nonzero mask of 0x8000.

Tested on x86-64 Linux.

p.s. Thanks for the bug report Uli.

gcc/ChangeLog:

	* value-range.cc (irange::set_nonzero_bits): Set range when known.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/popcount6.c: New test.
---
 gcc/testsuite/gcc.dg/tree-ssa/popcount6.c | 12 ++++++++++++
 gcc/value-range.cc                        | 13 +++++++++++++
 2 files changed, 25 insertions(+)
 create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/popcount6.c

diff --git a/gcc/testsuite/gcc.dg/tree-ssa/popcount6.c b/gcc/testsuite/gcc.dg/tree-ssa/popcount6.c
new file mode 100644
index 00000000000..1406ad9d33b
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/popcount6.c
@@ -0,0 +1,12 @@
+// { dg-do compile }
+// { dg-options "-O2 -fdump-tree-evrp" }
+
+int g(int n)
+{
+  n &= 0x8000;
+  if (n == 0)
+    return 1;
+  return __builtin_popcount(n);
+}
+
+// { dg-final { scan-tree-dump "return 1;" "evrp" } }
diff --git a/gcc/value-range.cc b/gcc/value-range.cc
index 754379add19..6154d73ccf5 100644
--- a/gcc/value-range.cc
+++ b/gcc/value-range.cc
@@ -2930,6 +2930,19 @@ irange::set_nonzero_bits (const wide_int_ref &bits)
       set_nonzero_bits (NULL);
       return;
     }
+  // If we have only one bit set in the mask, we can figure out the
+  // range immediately.
+  if (wi::popcount (bits) == 1)
+    {
+      bool has_zero = contains_p (build_zero_cst (type ()));
+      set (type (), bits, bits);
+      if (has_zero)
+	{
+	  int_range<2> zero;
+	  zero.set_zero (type ());
+	  union_ (zero);
+	}
+    }
   set_nonzero_bits (wide_int_to_tree (type (), bits));
 }
 
-- 
2.37.1