[PATCH v4 1/2] [gdb] Untied inheritance virtuality and loc kind

[Bruno Larsen <blarsen@redhat.com>]

DWARF specifications, when talking about C++ virtual classes, don't tie
their existance or non-existance to how the location is described, but
the original offset decoding did, assuming that we could use BITPOS for
non-virtual inheritance.

The new code assumes that if there isn't virtuality at play, we probably
have BITPOS, but we may need to calculate an expression, so allows for
fallthrough.

A new testcase was added, where we use a trivial locexpr instead of a
bitpos, to showcase one of this instances
---
 gdb/gnu-v3-abi.c                              |  15 +-
 .../gdb.dwarf2/dw2-inheritance-locexpr-1.exp  | 233 ++++++++++++++++++
 .../gdb.dwarf2/dw2-inheritance-locexpr.c      |  69 ++++++
 gdb/value.c                                   |   8 +-
 4 files changed, 319 insertions(+), 6 deletions(-)
 create mode 100644 gdb/testsuite/gdb.dwarf2/dw2-inheritance-locexpr-1.exp
 create mode 100644 gdb/testsuite/gdb.dwarf2/dw2-inheritance-locexpr.c

diff --git a/gdb/gnu-v3-abi.c b/gdb/gnu-v3-abi.c
index 7aac0ca4f9c..ae165afd8f2 100644
--- a/gdb/gnu-v3-abi.c
+++ b/gdb/gnu-v3-abi.c
@@ -458,10 +458,17 @@ gnuv3_baseclass_offset (struct type *type, int index,
   gdbarch = type->arch ();
   ptr_type = builtin_type (gdbarch)->builtin_data_ptr;
 
-  /* If it isn't a virtual base, this is easy.  The offset is in the
-     type definition.  */
-  if (!BASETYPE_VIA_VIRTUAL (type, index))
-    return TYPE_BASECLASS_BITPOS (type, index) / 8;
+  if (!BASETYPE_VIA_VIRTUAL (type, index)){
+      /* If it isn't a virtual base, this is easy.
+         The offset is probably in the type definition.
+	 However, the DWARF specification doesn't tie virtuality with location
+	 so we may need to resolve location expressions.
+	 To reuse code, we just fall-through in that case */
+      if (type->field (index).loc_kind () == FIELD_LOC_KIND_BITPOS){
+	  cur_base_offset = TYPE_BASECLASS_BITPOS (type, index) / 8;
+	return TYPE_BASECLASS_BITPOS (type, index) / 8;
+      }
+  }
 
   /* If we have a DWARF expression for the offset, evaluate it.  */
   if (type->field (index).loc_kind () == FIELD_LOC_KIND_DWARF_BLOCK)
diff --git a/gdb/testsuite/gdb.dwarf2/dw2-inheritance-locexpr-1.exp b/gdb/testsuite/gdb.dwarf2/dw2-inheritance-locexpr-1.exp
new file mode 100644
index 00000000000..aa511a31a88
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/dw2-inheritance-locexpr-1.exp
@@ -0,0 +1,233 @@
+# Copyright 2018-2021 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
+
+# This test can only be run on targets which support DWARF-2 and use gas.
+if ![dwarf2_support] {
+    return 0
+}
+
+# We'll place the output of Dwarf::assemble in pr28030.S.
+standard_testfile dw2-inheritance-locexpr.c .S
+
+# ${testfile} is now "pr28030".  srcfile2 is "pr28030.S".
+set executable ${testfile}
+set asm_file [standard_output_file ${srcfile2}]
+
+# We need to know the size of integer and address types in order
+# to write some of the debugging info we'd like to generate.
+#
+# For that, we ask GDB by debugging our pr28030 program.
+# Any program would do, but since we already have pr28030
+# specifically for this testcase, might as well use that.
+if [prepare_for_testing "failed to prepare" ${testfile} ${srcfile}] {
+    return -1
+}
+set long_size [get_sizeof "long" -1]
+# gdb always assumes references are implemented as pointers.
+set addr_size [get_sizeof "void *" -1]
+set struct_A_size [get_sizeof "g_A" 4]
+set struct_B_size [get_sizeof "g_B" 4]
+set struct_C_size [get_sizeof "g_C" 4]
+
+# Create the DWARF.
+Dwarf::assemble $asm_file {
+    global srcdir subdir srcfile srcfile2
+    global long_size addr_size struct_A_size struct_B_size struct_C_size
+    declare_labels L
+
+    cu {} {
+	DW_TAG_compile_unit {
+	    {DW_AT_language @DW_LANG_C_plus_plus}
+	    {name pr28030.c}
+	    {stmt_list $L DW_FORM_sec_offset}
+        } {
+	    declare_labels int_label class_A_label class_B_label class_C_label
+
+	    int_label: DW_TAG_base_type {
+		{DW_AT_byte_size ${long_size} DW_FORM_udata}
+		{DW_AT_encoding @DW_ATE_signed}
+		{DW_AT_name "int"}
+	    }
+
+	    class_A_label: DW_TAG_class_type {
+		{DW_AT_name "A"}
+		{DW_AT_byte_size $struct_A_size DW_FORM_sdata}
+	    } {
+		DW_TAG_member {
+		    {DW_AT_name "a"}
+		    {DW_AT_type :$int_label}
+		    {DW_AT_data_member_location 0*$long_size DW_FORM_udata}
+		}
+		DW_TAG_member {
+		    {DW_AT_name "x"}
+		    {DW_AT_type :$int_label}
+		    {DW_AT_data_member_location 1*$long_size DW_FORM_udata}
+		}
+	    }
+
+	    class_B_label: DW_TAG_class_type {
+		{DW_AT_name "B"}
+		{DW_AT_byte_size $struct_B_size DW_FORM_sdata}
+	    } {
+		DW_TAG_inheritance {
+		    {DW_AT_type :$class_A_label}
+		    {DW_AT_data_member_location {
+			DW_OP_constu 0
+			DW_OP_plus
+			DW_OP_stack_value } SPECIAL_expr}
+		    {DW_AT_accessibility 1 DW_FORM_data1}
+		}
+		DW_TAG_member {
+		    {DW_AT_name "b"}
+		    {DW_AT_type :$int_label}
+		    {DW_AT_data_member_location 2*$long_size DW_FORM_udata}
+		}
+	    }
+
+	    class_C_label: DW_TAG_class_type {
+		{DW_AT_name "C"}
+		{DW_AT_byte_size $struct_C_size DW_FORM_sdata}
+	    } {
+		DW_TAG_inheritance {
+		    {DW_AT_type :$class_A_label}
+		    {DW_AT_data_member_location { DW_OP_constu 9000 } SPECIAL_expr}
+		    {DW_AT_accessibility 1 DW_FORM_data1}
+		}
+		DW_TAG_member {
+		    {DW_AT_name "b"}
+		    {DW_AT_type :$int_label}
+		    {DW_AT_data_member_location 2*$long_size DW_FORM_udata}
+		}
+	    }
+
+	    DW_TAG_variable {
+		{DW_AT_name "g_A"}
+		{DW_AT_type :$class_A_label}
+		{DW_AT_external 1 flag}
+		{DW_AT_location {DW_OP_addr [gdb_target_symbol "g_A"]} SPECIAL_expr}
+	    }
+
+	    DW_TAG_variable {
+		{DW_AT_name "g_B"}
+		{DW_AT_type :$class_B_label}
+		{DW_AT_external 1 flag}
+		{DW_AT_location {DW_OP_addr [gdb_target_symbol "g_B"]} SPECIAL_expr}
+	    }
+
+	    DW_TAG_variable {
+		{DW_AT_name "g_C"}
+		{DW_AT_type :$class_C_label}
+		{DW_AT_external 1 flag}
+		{DW_AT_location {DW_OP_addr [gdb_target_symbol "g_C"]} SPECIAL_expr}
+	    }
+
+	    DW_TAG_subprogram {
+		{MACRO_AT_func { "foo" }}
+		{DW_AT_type :${class_B_label}}
+		{DW_AT_external 1 flag}
+	    }
+	    DW_TAG_subprogram {
+		{MACRO_AT_func { "bar" }}
+		{DW_AT_type :${class_C_label}}
+		{DW_AT_external 1 flag}
+	    }
+	    DW_TAG_subprogram {
+		{MACRO_AT_func { "main" }}
+		{DW_AT_type :${int_label}}
+		{DW_AT_external 1 DW_FORM_flag}
+	    }
+	}
+    }
+
+    lines {version 2} L {
+	include_dir "${srcdir}/${subdir}"
+	file_name "$srcfile" 1
+
+	lassign [function_range main [list ${srcdir}/${subdir}/$srcfile]] \
+	    main_start main_len
+	set main_end "$main_start + $main_len"
+	lassign [function_range foo [list ${srcdir}/${subdir}/$srcfile]] \
+	    foo_start foo_len
+	set foo_end "$foo_start + $foo_len"
+	lassign [function_range bar [list ${srcdir}/${subdir}/$srcfile]] \
+	    bar_start bar_len
+	set bar_end "$bar_start + $bar_len"
+
+	# Generate a line table program.  An attempt was made to make it
+	# reasonably accurate as it made debugging the test case easier.
+	program {
+	    DW_LNE_set_address $main_start
+	    line [gdb_get_line_number "main prologue"]
+	    DW_LNS_copy
+	    DW_LNE_set_address main_label
+	    line [gdb_get_line_number "main foo call"]
+	    DW_LNS_copy
+	    DW_LNE_set_address main_bar_label
+	    line [gdb_get_line_number "main bar call"]
+	    DW_LNS_copy
+	    DW_LNE_set_address main_label2
+	    line [gdb_get_line_number "main return"]
+	    DW_LNS_copy
+	    DW_LNE_set_address $main_end
+	    line [expr [gdb_get_line_number "main end"] + 1]
+	    DW_LNS_copy
+	    DW_LNE_end_sequence
+
+	    DW_LNE_set_address $foo_start
+	    line [gdb_get_line_number "foo prologue"]
+	    DW_LNS_copy
+	    DW_LNE_set_address foo_label
+	    line [gdb_get_line_number "foo return"]
+	    DW_LNS_copy
+	    line [gdb_get_line_number "foo end"]
+	    DW_LNS_copy
+	    DW_LNE_set_address $foo_end
+	    DW_LNS_advance_line 1
+	    DW_LNS_copy
+	    DW_LNE_end_sequence
+
+	    DW_LNE_set_address $bar_start
+	    line [gdb_get_line_number "bar prologue"]
+	    DW_LNS_copy
+	    DW_LNE_set_address bar_label
+	    line [gdb_get_line_number "bar return"]
+	    DW_LNS_copy
+	    line [gdb_get_line_number "bar end"]
+	    DW_LNS_copy
+	    DW_LNE_set_address $bar_end
+	    DW_LNS_advance_line 1
+	    DW_LNS_copy
+	    DW_LNE_end_sequence
+
+	}
+    }
+}
+
+if [prepare_for_testing "failed to prepare" ${executable} [list ${asm_file} ${srcfile}] {}] {
+    return -1
+}
+
+if ![runto_main] {
+    return -1
+}
+
+gdb_test "step" "foo .. at .* foo end.*" "step into foo"
+gdb_test "finish" "= {<A> = {a = 5, x = 6}, b = 7}" "finish out of foo"
+
+if ![runto_main] {
+    return -1
+}
diff --git a/gdb/testsuite/gdb.dwarf2/dw2-inheritance-locexpr.c b/gdb/testsuite/gdb.dwarf2/dw2-inheritance-locexpr.c
new file mode 100644
index 00000000000..0de0cb04b34
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/dw2-inheritance-locexpr.c
@@ -0,0 +1,69 @@
+/* Copyright (C) 2018-2021 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   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/>.  */
+
+typedef struct A {
+    long a;
+    long x;
+} A;
+
+struct A g_A = {3, 4};
+
+typedef struct B {
+    long a;
+    long x;
+    long b;
+} B;
+
+struct B g_B = { 5, 6, 7 };
+
+typedef struct C {
+    long a;
+    long x;
+    long b;
+} C;
+
+struct C g_C = { 8, 9, 10 };
+
+B
+foo ()
+{						/* foo prologue */
+  asm ("foo_label: .globl foo_label");
+  return g_B;					/* foo return */
+}						/* foo end */
+
+C
+bar()
+{						/* bar prologue */
+  asm ("bar_label: .globl bar_label");
+  return g_C;					/* bar return */
+}						/* bar end */
+
+int
+main (void)
+{						/* main prologue */
+  B v;
+  C ret;
+  asm ("main_label: .globl main_label");
+
+  v = foo ();					/* main foo call */
+
+  asm ("main_bar_label: .globl main_bar_label");
+  ret = bar();					/* main bar call */
+
+  asm ("main_label2: .globl main_label2");
+  return 0;					/* main return */
+}						/* main end */
diff --git a/gdb/value.c b/gdb/value.c
index 08cccf711b2..14b4ff9ef91 100644
--- a/gdb/value.c
+++ b/gdb/value.c
@@ -3085,8 +3085,12 @@ value_primitive_field (struct value *arg1, LONGEST offset,
 
       /* We special case virtual inheritance here because this
 	 requires access to the contents, which we would rather avoid
-	 for references to ordinary fields of unavailable values.  */
-      if (BASETYPE_VIA_VIRTUAL (arg_type, fieldno))
+	 for references to ordinary fields of unavailable values.  
+	 TODO: we should probably try to change this special case, we are
+	 starting to add validation to baseclass_offset.
+	 */
+      if (BASETYPE_VIA_VIRTUAL (arg_type, fieldno) || 
+	  arg_type->field (fieldno).loc_kind () != FIELD_LOC_KIND_BITPOS)
 	boffset = baseclass_offset (arg_type, fieldno,
 				    value_contents (arg1).data (),
 				    value_embedded_offset (arg1),
-- 
2.31.1