[gcc r11-3746] Fix for PR97317.

[Aldy Hernandez]

https://gcc.gnu.org/g:1cde5d85bec20877bed063fc0046b9885231a0ae

commit r11-3746-g1cde5d85bec20877bed063fc0046b9885231a0ae
Author: Andrew MacLeod <amacleod@redhat.com>
Date:   Fri Oct 9 10:46:50 2020 +0200

    Fix for PR97317.
    
    gcc/ChangeLog:
    
            PR tree-optimization/97317
            * range-op.cc (operator_cast::op1_range): Handle casts where the precision
            of the RHS is only 1 greater than the precision of the LHS.
    
    gcc/testsuite/ChangeLog:
            * gcc.dg/pr97317.c: New test.

Diff:
---
 gcc/range-op.cc                | 25 ++++++++++++++++++-------
 gcc/testsuite/gcc.dg/pr97317.c | 11 +++++++++++
 2 files changed, 29 insertions(+), 7 deletions(-)

diff --git a/gcc/range-op.cc b/gcc/range-op.cc
index 22bc23c1bbf..d1a11b34894 100644
--- a/gcc/range-op.cc
+++ b/gcc/range-op.cc
@@ -1849,14 +1849,25 @@ operator_cast::op1_range (irange &r, tree type,
 							  type,
 							  converted_lhs,
 							  lim_range);
-	  // And union this with the entire outer types negative range.
-	  int_range_max neg (type,
-			     wi::min_value (TYPE_PRECISION (type),
-					    SIGNED),
-			     lim - 1);
-	  neg.union_ (lhs_neg);
+	  // lhs_neg now has all the negative versions of the LHS.
+	  // Now union in all the values from SIGNED MIN (0x80000) to
+	  // lim-1 in order to fill in all the ranges with the upper
+	  // bits set.
+
+	  // PR 97317.  If the lhs has only 1 bit less precision than the rhs,
+	  // we don't need to create a range from min to lim-1
+	  // calculate neg range traps trying to create [lim, lim - 1].
+	  wide_int min_val = wi::min_value (TYPE_PRECISION (type), SIGNED);
+	  if (lim != min_val)
+	    {
+	      int_range_max neg (type,
+				 wi::min_value (TYPE_PRECISION (type),
+						SIGNED),
+				 lim - 1);
+	      lhs_neg.union_ (neg);
+	    }
 	  // And finally, munge the signed and unsigned portions.
-	  r.union_ (neg);
+	  r.union_ (lhs_neg);
 	}
       // And intersect with any known value passed in the extra operand.
       r.intersect (op2);
diff --git a/gcc/testsuite/gcc.dg/pr97317.c b/gcc/testsuite/gcc.dg/pr97317.c
new file mode 100644
index 00000000000..f07327ac9a2
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr97317.c
@@ -0,0 +1,11 @@
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+
+struct a {
+  unsigned c : 17;
+};
+struct a b;
+int d(void) {
+  short e = b.c;
+  return e ? 0 : b.c;
+}