[binutils-gdb] treat Ada DW_TAG_unspecified_type DIEs as stub types

[Joel Brobecker <brobecke at sourceware dot org>]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=74a2f8ffb83172de1af0da6751a65c08a722986f

commit 74a2f8ffb83172de1af0da6751a65c08a722986f
Author: Joel Brobecker <brobecker@adacore.com>
Date:   Mon Jan 1 22:41:36 2018 -0500

    treat Ada DW_TAG_unspecified_type DIEs as stub types
    
    Consider the gdb.ada/taft_type.exp testcase, which exercises
    the situation where a variable is defined using a type which
    is a pointer to an incomplete type, with the actual type
    definition being provided by another unit. Up to now, the
    strategy used by GNAT when generating the DWARF debugging info
    was to produce a incomplete DW_TAG_enumeration_type DIE with
    a DW_AT_declaration flag attached to it:
    
            .uleb128 0x4    # (DIE (0x3e) DW_TAG_enumeration_type)
            .long   .LASF4  # DW_AT_name: "pck__empty"
                            # DW_AT_declaration
    
    However, a more standard way for the compiler to describe
    this kind of type is to use the DW_TAG_unspecified_type tag.
    When the compiler is enhanced to do so, we'll need to treat
    such types as stubs -- we only do so with types from Ada units,
    however, as the meaning of this TAG is intentionally left
    permissive and language-specific by the DWARF standard.
    
    Without this patch, running the testcase above with an enhanced
    compiler now yields:
    
        (gdb) print w.e.all
        Attempt to dereference a generic pointer.
        FAIL: gdb.ada/taft_type.exp: print w.e.all
    
    gdb/ChangeLog:
    
    	* dwarf2read.c (read_unspecified_type): Treat
            DW_TAG_enumeration_type DIEs from Ada units as stubs.
    
    Tested on x86_64-linux, fixes the FAIL in gdb.ada/taft_type.exp above.

Diff:
---
 gdb/ChangeLog    | 5 +++++
 gdb/dwarf2read.c | 9 +++++++--
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 9eaa15f..3199eaa 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,8 @@
+2018-01-02  Joel Brobecker  <brobecker@adacore.com>
+
+	* dwarf2read.c (read_unspecified_type): Treat
+	DW_TAG_enumeration_type DIEs from Ada units as stubs.
+
 2018-01-01  Joel Brobecker  <brobecker@adacore.com>
 
 	Update copyright year range in all GDB files.
diff --git a/gdb/dwarf2read.c b/gdb/dwarf2read.c
index b4f5cd1..4dbd5c3 100644
--- a/gdb/dwarf2read.c
+++ b/gdb/dwarf2read.c
@@ -17693,11 +17693,16 @@ read_unspecified_type (struct die_info *die, struct dwarf2_cu *cu)
 {
   struct type *type;
 
-  /* For now, we only support the C meaning of an unspecified type: void.  */
-
   type = init_type (cu->objfile, TYPE_CODE_VOID, 0, NULL);
   TYPE_NAME (type) = dwarf2_name (die, cu);
 
+  /* In Ada, an unspecified type is typically used when the description
+     of the type is defered to a different unit.  When encountering
+     such a type, we treat it as a stub, and try to resolve it later on,
+     when needed.  */
+  if (cu->language == language_ada)
+    TYPE_STUB (type) = 1;
+
   return set_die_type (die, type, cu);
 }