[irange] Return FALSE if updated bitmask is unchanged [PR110753]

The mask/value pair we track in the irange is a bit fickle in that it
can sometimes contradict the bitmask inherent in the range.  This can
happen when a series of calculations yield a combination such as:

	[3, 1000] MASK 0xfffffffe VALUE 0x0

The mask/value above implies that the lowest bit is a known 0, which
would exclude the 3 in the range.  At one time we tried keeping mask
and ranges 100% consistent, but the performance penalty was too high
(5% in VRP).  Also, it's unclear whether the intersection of two
incompatible known bits should make the whole range undefined, or
just the contradicting bits.  This is all documented in
irange::get_bitmask().  We could revisit both of these assumptions
in the future.

In this testcase IPA ends up with a range where the lower 2 bits are
expected to be 0, but the range is [1,1].

	[irange] long int [1, 1] MASK 0xfffffffffffffffc VALUE 0x0

This causes irange::union_bitmask() to think an update occurred, when
no semantic change happened, thus triggering an assert in IPA-cp.  We
could get rid of the assert, but it's cleaner to make
irange::{union,intersect}_bitmask always tell the truth.  Beside, the
ranger's cache also depends on union being truthful.

	PR ipa/110753

gcc/ChangeLog:

	* value-range.cc (irange::union_bitmask): Return FALSE if updated
	bitmask is semantically equivalent to the original mask.
	(irange::intersect_bitmask): Same.
	(irange::get_bitmask): Add comment.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/pr110753.c: New test.

2 files changed, 33 insertions, 0 deletions

diff --git a/gcc/testsuite/gcc.dg/tree-ssa/pr110753.c b/gcc/testsuite/gcc.dg/tree-ssa/pr110753.c
new file mode 100644
index 0000000..aa02487
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/pr110753.c
@@ -0,0 +1,15 @@
+// { dg-do compile }
+// { dg-options "-O2" }
+
+int a, b, c;
+static int d(long e, long f) { return f == 0 || e && f == 1 ?: f; }
+int g(void) {static int t; return t;}
+static void h(long e) {
+  b = e - 1;
+  a = d(b || d(e, 8), g());
+}
+int tt;
+void i(void) {
+  c = (__SIZE_TYPE__)&tt;
+  h(c);
+}
diff --git a/gcc/value-range.cc b/gcc/value-range.cc
index 2abf57b..76f88d9 100644
--- a/gcc/value-range.cc
+++ b/gcc/value-range.cc
@@ -1876,6 +1876,8 @@ irange::get_bitmask () const
   //
   // 3 is in the range endpoints, but is excluded per the known 0 bits
   // in the mask.
+  //
+  // See also the note in irange_bitmask::intersect.
   irange_bitmask bm = get_bitmask_from_range ();
   if (!m_bitmask.unknown_p ())
     bm.intersect (m_bitmask);
@@ -1915,10 +1917,18 @@ irange::intersect_bitmask (const irange &r)
     return false;
 
   irange_bitmask bm = get_bitmask ();
+  irange_bitmask save = bm;
   if (!bm.intersect (r.get_bitmask ()))
     return false;
 
   m_bitmask = bm;
+
+  // Updating m_bitmask may still yield a semantic bitmask (as
+  // returned by get_bitmask) which is functionally equivalent to what
+  // we originally had.  In which case, there's still no change.
+  if (save == get_bitmask ())
+    return false;
+
   if (!set_range_from_bitmask ())
     normalize_kind ();
   if (flag_checking)
@@ -1938,10 +1948,18 @@ irange::union_bitmask (const irange &r)
     return false;
 
   irange_bitmask bm = get_bitmask ();
+  irange_bitmask save = bm;
   if (!bm.union_ (r.get_bitmask ()))
     return false;
 
   m_bitmask = bm;
+
+  // Updating m_bitmask may still yield a semantic bitmask (as
+  // returned by get_bitmask) which is functionally equivalent to what
+  // we originally had.  In which case, there's still no change.
+  if (save == get_bitmask ())
+    return false;
+
   // No need to call set_range_from_mask, because we'll never
   // narrow the range.  Besides, it would cause endless recursion
   // because of the union_ in set_range_from_mask.