[PATCH] Do not print <synthetic pointer> when piece is optimized out

[Tom Tromey <tom@tromey.com>]

A user reported a bug where printing a certain array of integer types
would result in the nonsensical:

(gdb) p l_126
$1 = {6639779683436459270, <synthetic pointer>, <synthetic pointer>, <synthetic pointer>}

I tracked this down to some issues in the DWARF expression code.
First, check_pieced_synthetic_pointer did not account for the
situation where a location expression does not describe all the bits
of a value -- in this case it returned true, meaning there is a
synthetic pointer, but in fact these bits are optimized out.  (It
turns out this incorrect output had already been erroneously tested
for as well.)

Next, rw_pieced_value did not mark these bits as optimized-out,
either.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30296
---
 gdb/dwarf2/expr.c                             | 19 +++--
 .../gdb.dwarf2/opt-out-not-implptr.exp        | 76 +++++++++++++++++++
 gdb/testsuite/gdb.dwarf2/shortpiece.exp       |  4 +-
 3 files changed, 91 insertions(+), 8 deletions(-)
 create mode 100644 gdb/testsuite/gdb.dwarf2/opt-out-not-implptr.exp

diff --git a/gdb/dwarf2/expr.c b/gdb/dwarf2/expr.c
index 4aa4542d947..f12abd9599d 100644
--- a/gdb/dwarf2/expr.c
+++ b/gdb/dwarf2/expr.c
@@ -411,11 +411,11 @@ rw_pieced_value (value *v, value *from, bool check_optimized)
 	  break;
 
 	case DWARF_VALUE_IMPLICIT_POINTER:
-	    if (from != nullptr)
-	      {
-		v->mark_bits_optimized_out (offset, this_size_bits);
-		break;
-	      }
+	  if (from != nullptr)
+	    {
+	      v->mark_bits_optimized_out (offset, this_size_bits);
+	      break;
+	    }
 
 	  /* These bits show up as zeros -- but do not cause the value to
 	     be considered optimized-out.  */
@@ -435,6 +435,13 @@ rw_pieced_value (value *v, value *from, bool check_optimized)
       bits_to_skip = 0;
     }
 
+  if (offset < max_offset)
+    {
+      if (check_optimized)
+	return true;
+      v->mark_bits_optimized_out (offset, max_offset - offset);
+    }
+
   return false;
 }
 
@@ -493,7 +500,7 @@ check_pieced_synthetic_pointer (const value *value, LONGEST bit_offset,
 	return false;
     }
 
-  return true;
+  return bit_length == 0;
 }
 
 /* An implementation of an lval_funcs method to indirect through a
diff --git a/gdb/testsuite/gdb.dwarf2/opt-out-not-implptr.exp b/gdb/testsuite/gdb.dwarf2/opt-out-not-implptr.exp
new file mode 100644
index 00000000000..949bc23492a
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/opt-out-not-implptr.exp
@@ -0,0 +1,76 @@
+# Copyright 2023 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+load_lib dwarf.exp
+
+require dwarf2_support
+
+standard_testfile main.c -dw.S
+
+set asm_file [standard_output_file $srcfile2]
+
+Dwarf::assemble $asm_file {
+    cu {} {
+	compile_unit {} {
+	    declare_labels i64_type array
+
+	    i64_type: base_type {
+		{name "int64_t"}
+		{encoding @DW_ATE_signed}
+		{byte_size 8 DW_FORM_sdata}
+	    }
+
+	    array: DW_TAG_array_type {
+		{DW_AT_name array_type}
+		{DW_AT_type :$i64_type}
+	    } {
+		DW_TAG_subrange_type {
+		    {DW_AT_type        :$i64_type}
+		    {DW_AT_lower_bound 0 DW_FORM_data1}
+		    {DW_AT_upper_bound 3 DW_FORM_data1}
+		}
+	    }
+
+	    DW_TAG_variable {
+		{name noptr}
+		{type :$array}
+		{location {
+		    DW_OP_constu 6639779683436459270
+		    DW_OP_stack_value
+		    DW_OP_piece 8
+		} SPECIAL_expr}
+	    }
+	}
+    }
+}
+
+if {[build_executable ${testfile}.exp ${testfile} \
+	 [list $srcfile $asm_file] {nodebug}]} {
+    return -1
+}
+
+# We need --readnow because otherwise we never read in the CU we
+# created above.
+save_vars { GDBFLAGS } {
+    append set GDBFLAGS " -readnow"
+    clean_restart ${testfile}
+}
+
+if {![runto_main]} {
+    return -1
+}
+
+gdb_test "print noptr" \
+    " = \\{6639779683436459270, <optimized out>, <optimized out>, <optimized out>\\}"
diff --git a/gdb/testsuite/gdb.dwarf2/shortpiece.exp b/gdb/testsuite/gdb.dwarf2/shortpiece.exp
index dec219e95a8..82ea2c2eaa4 100644
--- a/gdb/testsuite/gdb.dwarf2/shortpiece.exp
+++ b/gdb/testsuite/gdb.dwarf2/shortpiece.exp
@@ -137,8 +137,8 @@ gdb_test "p s2" \
 # value's content.
 # This test verifies that GDB can print a pieced value casted to a
 # different type.
-gdb_test "p (int \[\]) s1" " = \\{1\\, 0\\}"
-gdb_test "p (short \[\]) s1" " = \\{1\\, 0\\, 0\\, <synthetic pointer>\\}"
+gdb_test "p (int \[\]) s1" " = \\{1, <optimized out>\\}"
+gdb_test "p (short \[\]) s1" " = \\{1, 0, 0, <optimized out>\\}"
 
 # Test for correct output if the size of the original object is not a
 # multiple of the array's element size.
-- 
2.39.2