RE: [PATCH] Debug support for global alias variable

["Natarajan, Kavitha" <Kavitha.Natarajan@amd.com>]

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[AMD Official Use Only - General]

Hi Pedro,

Thanks again for looking into the patch.

> >>  standard_testfile symbol-alias.c symbol-alias2.c
> >> +set using_clang [test_compiler_info clang-*] set old_clang [expr
> >> +[test_compiler_info {clang-1[0-3]-*-*}] \
> >> +                || [test_compiler_info {clang-[1-9]-*}]]
>
> These could use a short comment (see bottom of this email).
>
> Also, this assumes the fix will be in Clang 14, but Clang 14 has been released
> recently, and your fix will only be in Clang 15, right?  I caught this because in
> addition to testing against current main LLVM, I tested against a ROCm
> bundled Clang I had handy, which happens to be based on a LLVM that
> identifies as 14, and the tests FAILed, instead of XFAILing.  I think that above
> we want "clang-1[0-4]" instead of "clang-1[0-3]", right?

Yes, that's right. I had made these changes a while ago and missed to update it
with the version change.

> >> +
> >> +# Deferred Variable.
>
> I had never heard the term "deferred variable" before.  IIUC, it refers to the
> case of the alias aliasing a name that has not been seen in scope yet.  Is this
> term "deferred variable" a term you invented, or did you find it elsewhere?
> I'm just curious, and pondering adding a comment.

Well, if such a term doesn't exist, then I think I would have been influenced by the
clang sources where we refer emitting deferred variable declarations. I shall change
it if it doesn't suit here.

> >> +gdb_test "p g_def_var" " = 2"
> >> +gdb_test_multiple "p g_def_var_alias" "print alias of deferred variable" {
> >> +    -re -wrap " = 2" {
> >> +       pass $gdb_test_name
> >> +    }
> >> +    -re -wrap "has unknown type; cast it to its declared type" {
> >> +       if { $using_clang } {
>
> So all other tests pass with your fixed Clang, but this one will still xfail.
> Do you have more information about this?  Strange offhand that "alias of
> alias" works, but this one doesn't.  Is there some Clang bug open about this
> we could add a reference to here?  If not, could one be created?  If instead
> you think this is a GDB bug, then this should be a kfail ("known gdb fail")
> instead of an xfail, in which case we should have a bug open in gdb's bugzilla
> about it so we can reference it here.

This is a clang bug. For the alias of deferred declaration, clang does not generate
debug information. Clang has to emit the DIE with DW_TAG_imported_declaration tag.
So, this is an expected failure. I can add a comment for the xfail part.

> >> +extern int g_def_var_alias __attribute__ ((alias ("g_def_var")));
> >> +
> >> +int g_def_var = 2;
> >> +
> >> +extern int g_def_var2_alias __attribute__ ((alias ("g_def_var2")));
> >> +
> >> +int g_def_var2 = 3;
>
> Offhand, there doesn't seem to be any difference between g_def_var_alias
> and g_def_var2_alias?  If we really need both, then I'm thinking this could
> use some comment.

Yes, they are same. I just wanted to call out the xfail through g_def_var_alias that
no debug information is generated when the aliasee is declared later. In case of
g_def_var2_alias, though its aliasee is declared later, the debug info is generated
(as DW_TAG_variable) when g_def_var2_alias2 accesses it to generate its debug info.
When I am writing this, I am thinking of removing this xfail part itself. Anyway,
g_def_var2_alias would cover the same case when clang is fixed.

> >> +
> >> +extern int g_def_var2_alias2 __attribute__ ((alias
> >> +("g_def_var2_alias")));
> >> --
> >>
> >>
> >
>
> Here's a slightly tweaked version of your patch, that I am aiming at merging,
> once we resolve the nits pointed out above.  It just tweaks/adds comments
> in the code and the commit log, and tweaks formatting to be a bit more in
> line with GDB conventions.  Please double check.

I have made my changes (removing xfail and updating the comments in .exp)
on top of your patch and attached the same.

From 4ec4f0473f2df1468fe25e46c30e4b20b9c207c5 Mon Sep 17 00:00:00 2001
From: Kavitha Natarajan <kavitha.natarajan@amd.com>
Date: Mon, 13 Jun 2022 16:24:24 +0530
Subject: [PATCH] Debug support for global alias variable

Starting with (future) Clang 15 (since
https://reviews.llvm.org/D120989), Clang emits the DWARF information
of global alias variables as DW_TAG_imported_declaration.  However,
GDB does not handle it.  It incorrectly always reads this tag as
C++/Fortran imported declaration (type alias, namespace alias and
Fortran module).  This commit adds support to handle this tag as an
alias variable.

This change fixes the failures in the gdb.base/symbol-alias.exp
testcase with current git Clang.  This testcase is also updated to
test nested (recursive) aliases.
---
 gdb/dwarf2/read.c                       | 63 ++++++++++++++++--------
 gdb/testsuite/gdb.base/symbol-alias.exp | 64 +++++++++++++++++++++++--
 gdb/testsuite/gdb.base/symbol-alias2.c  | 12 +++++
 3 files changed, 115 insertions(+), 24 deletions(-)

diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index 64cb8d421b9..e22e09b4e2b 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -894,6 +894,10 @@ static void build_type_psymtabs_reader (cutu_reader *reader,

 static void dwarf2_build_psymtabs_hard (dwarf2_per_objfile *per_objfile);

+static void var_decode_location (struct attribute *attr,
+                                struct symbol *sym,
+                                struct dwarf2_cu *cu);
+
 static unsigned int peek_abbrev_code (bfd *, const gdb_byte *);

 static const gdb_byte *read_attribute (const struct die_reader_specs *,
@@ -1071,7 +1075,7 @@ static struct using_direct **using_directives (struct dwarf2_cu *cu);

 static void read_import_statement (struct die_info *die, struct dwarf2_cu *);

-static int read_namespace_alias (struct die_info *die, struct dwarf2_cu *cu);
+static bool read_alias (struct die_info *die, struct dwarf2_cu *cu);

 static struct type *read_module_type (struct die_info *die,
                                      struct dwarf2_cu *cu);
@@ -8664,9 +8668,10 @@ process_die (struct die_info *die, struct dwarf2_cu *cu)
       break;
     case DW_TAG_imported_declaration:
       cu->processing_has_namespace_info = true;
-      if (read_namespace_alias (die, cu))
+      if (read_alias (die, cu))
        break;
-      /* The declaration is not a global namespace alias.  */
+      /* The declaration is neither a global namespace nor a variable
+        alias.  */
       /* Fall through.  */
     case DW_TAG_imported_module:
       cu->processing_has_namespace_info = true;
@@ -9134,18 +9139,18 @@ dwarf2_physname (const char *name, struct die_info *die, struct dwarf2_cu *cu)
   return retval;
 }

-/* Inspect DIE in CU for a namespace alias.  If one exists, record
-   a new symbol for it.
+/* Inspect DIE in CU for a namespace alias or a variable with alias
+   attribute.  If one exists, record a new symbol for it.

-   Returns 1 if a namespace alias was recorded, 0 otherwise.  */
+   Returns true if an alias was recorded, false otherwise.  */

-static int
-read_namespace_alias (struct die_info *die, struct dwarf2_cu *cu)
+static bool
+read_alias (struct die_info *die, struct dwarf2_cu *cu)
 {
   struct attribute *attr;

-  /* If the die does not have a name, this is not a namespace
-     alias.  */
+  /* If the die does not have a name, this is neither a namespace
+     alias nor a variable alias.  */
   attr = dwarf2_attr (die, DW_AT_name, cu);
   if (attr != NULL)
     {
@@ -9171,26 +9176,42 @@ read_namespace_alias (struct die_info *die, struct dwarf2_cu *cu)
        {
          complaint (_("DIE at %s has too many recursively imported "
                       "declarations"), sect_offset_str (d->sect_off));
-         return 0;
+         return false;
        }

       if (attr != NULL)
        {
          struct type *type;
-         sect_offset sect_off = attr->get_ref_die_offset ();
-
-         type = get_die_type_at_offset (sect_off, cu->per_cu, cu->per_objfile);
-         if (type != NULL && type->code () == TYPE_CODE_NAMESPACE)
+         if (d->tag == DW_TAG_variable)
            {
-             /* This declaration is a global namespace alias.  Add
-                a symbol for it whose type is the aliased namespace.  */
-             new_symbol (die, type, cu);
-             return 1;
+             /* This declaration is a C/C++ global variable alias.
+                Add a symbol for it whose type is the same as the
+                aliased variable's.  */
+             type = die_type (d, imported_cu);
+             struct symbol *sym = new_symbol (die, type, cu);
+             attr = dwarf2_attr (d, DW_AT_location, imported_cu);
+             sym->set_aclass_index (LOC_UNRESOLVED);
+             if (attr != nullptr)
+               var_decode_location (attr, sym, cu);
+             return true;
+           }
+         else
+           {
+             sect_offset sect_off = attr->get_ref_die_offset ();
+             type = get_die_type_at_offset (sect_off, cu->per_cu,
+                                            cu->per_objfile);
+             if (type != nullptr && type->code () == TYPE_CODE_NAMESPACE)
+               {
+                 /* This declaration is a global namespace alias. Add
+                    a symbol for it whose type is the aliased
+                    namespace.  */
+                 new_symbol (die, type, cu);
+                 return true;
+               }
            }
        }
     }
-
-  return 0;
+  return false;
 }

 /* Return the using directives repository (global or local?) to use in the
diff --git a/gdb/testsuite/gdb.base/symbol-alias.exp b/gdb/testsuite/gdb.base/symbol-alias.exp
index 289f49bbc3f..5f6fa94dd76 100644
--- a/gdb/testsuite/gdb.base/symbol-alias.exp
+++ b/gdb/testsuite/gdb.base/symbol-alias.exp
@@ -15,6 +15,10 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.

 standard_testfile symbol-alias.c symbol-alias2.c
+set using_clang [test_compiler_info clang-*]
+# Clang versions prior to v15 do not emit debug info for aliases.
+set old_clang [expr [test_compiler_info {clang-1[0-4]-*-*}] \
+                || [test_compiler_info {clang-[1-9]-*}]]

 if { [prepare_for_testing "failed to prepare" ${testfile} [list $srcfile $srcfile2]] } {
     return -1
@@ -30,7 +34,61 @@ foreach f {"func" "func_alias"} {
     gdb_test "p *${f}()" "= {field1 = 1, field2 = 2}"
 }

-# Variables.
-foreach v {"g_var_s" "g_var_s_alias"} {
-    gdb_test "p $v" "= {field1 = 1, field2 = 2}"
+# Extern global variable.
+gdb_test "p g_var_s" "= {field1 = 1, field2 = 2}"
+foreach v {"g_var_s_alias" "g_var_s_alias2"} {
+    gdb_test_multiple "p $v" "print alias variable $v" {
+       -re -wrap " = {field1 = 1, field2 = 2}" {
+           pass $gdb_test_name
+       }
+       -re -wrap "has unknown type; cast it to its declared type" {
+           if { $old_clang } {
+               xfail $gdb_test_name
+           } else {
+               fail $gdb_test_name
+           }
+       }
+    }
+}
+
+# Static global variable.
+gdb_test "p g_var" " = 1"
+gdb_test_multiple "p g_var_alias" "print alias of static variable" {
+    -re -wrap " = 1" {
+       pass $gdb_test_name
+    }
+    -re -wrap "has unknown type; cast it to its declared type" {
+       if { $old_clang } {
+         xfail $gdb_test_name
+       } else {
+         fail $gdb_test_name
+       }
+    }
+}
+
+# Alias of a variable that is declared later in the scope.
+gdb_test "p g_def_var" " = 2"
+gdb_test_multiple "p g_def_var_alias" "print g_def_var_alias" {
+    -re -wrap " = 2" {
+       pass $gdb_test_name
+    }
+    -re -wrap "has unknown type; cast it to its declared type" {
+       if { $old_clang } {
+         xfail $gdb_test_name
+       } else {
+         fail $gdb_test_name
+       }
+    }
+}
+gdb_test_multiple "p g_def_var_alias2" "print g_def_var_alias2" {
+    -re -wrap " = 2" {
+       pass $gdb_test_name
+    }
+    -re -wrap "has unknown type; cast it to its declared type" {
+       if { $old_clang } {
+         xfail $gdb_test_name
+       } else {
+         fail $gdb_test_name
+       }
+    }
 }
diff --git a/gdb/testsuite/gdb.base/symbol-alias2.c b/gdb/testsuite/gdb.base/symbol-alias2.c
index 34f4e121e25..c1c0413ed60 100644
--- a/gdb/testsuite/gdb.base/symbol-alias2.c
+++ b/gdb/testsuite/gdb.base/symbol-alias2.c
@@ -23,6 +23,8 @@ struct S

 struct S g_var_s = { 1, 2 };

+static int g_var = 1;
+
 #ifdef __cplusplus
 /* So that the alias attribute below work without having to figure out
    this function's mangled name.  */
@@ -38,3 +40,13 @@ func (void)
 struct S *func_alias (void) __attribute__ ((alias ("func")));

 extern struct S g_var_s_alias __attribute__ ((alias ("g_var_s")));
+
+extern struct S g_var_s_alias2 __attribute__ ((alias ("g_var_s_alias")));
+
+extern int g_var_alias __attribute__ ((alias ("g_var")));
+
+extern int g_def_var_alias __attribute__ ((alias ("g_def_var")));
+
+int g_def_var = 2;
+
+extern int g_def_var_alias2 __attribute__ ((alias ("g_def_var_alias")));

base-commit: 8025a164287b588f450686a4b81f6001c33e4269
--
2.25.1


[-- Attachment #2: 0010-Debug-support-for-C-global-alias-variable-pedro.patch --]
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From 4ec4f0473f2df1468fe25e46c30e4b20b9c207c5 Mon Sep 17 00:00:00 2001
From: Kavitha Natarajan <kavitha.natarajan@amd.com>
Date: Mon, 13 Jun 2022 16:24:24 +0530
Subject: [PATCH] Debug support for global alias variable

Starting with (future) Clang 15 (since
https://reviews.llvm.org/D120989), Clang emits the DWARF information
of global alias variables as DW_TAG_imported_declaration.  However,
GDB does not handle it.  It incorrectly always reads this tag as
C++/Fortran imported declaration (type alias, namespace alias and
Fortran module).  This commit adds support to handle this tag as an
alias variable.

This change fixes the failures in the gdb.base/symbol-alias.exp
testcase with current git Clang.  This testcase is also updated to
test nested (recursive) aliases.
---
 gdb/dwarf2/read.c                       | 63 ++++++++++++++++--------
 gdb/testsuite/gdb.base/symbol-alias.exp | 64 +++++++++++++++++++++++--
 gdb/testsuite/gdb.base/symbol-alias2.c  | 12 +++++
 3 files changed, 115 insertions(+), 24 deletions(-)

diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index 64cb8d421b9..e22e09b4e2b 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -894,6 +894,10 @@ static void build_type_psymtabs_reader (cutu_reader *reader,
 
 static void dwarf2_build_psymtabs_hard (dwarf2_per_objfile *per_objfile);
 
+static void var_decode_location (struct attribute *attr,
+				 struct symbol *sym,
+				 struct dwarf2_cu *cu);
+
 static unsigned int peek_abbrev_code (bfd *, const gdb_byte *);
 
 static const gdb_byte *read_attribute (const struct die_reader_specs *,
@@ -1071,7 +1075,7 @@ static struct using_direct **using_directives (struct dwarf2_cu *cu);
 
 static void read_import_statement (struct die_info *die, struct dwarf2_cu *);
 
-static int read_namespace_alias (struct die_info *die, struct dwarf2_cu *cu);
+static bool read_alias (struct die_info *die, struct dwarf2_cu *cu);
 
 static struct type *read_module_type (struct die_info *die,
 				      struct dwarf2_cu *cu);
@@ -8664,9 +8668,10 @@ process_die (struct die_info *die, struct dwarf2_cu *cu)
       break;
     case DW_TAG_imported_declaration:
       cu->processing_has_namespace_info = true;
-      if (read_namespace_alias (die, cu))
+      if (read_alias (die, cu))
 	break;
-      /* The declaration is not a global namespace alias.  */
+      /* The declaration is neither a global namespace nor a variable
+	 alias.  */
       /* Fall through.  */
     case DW_TAG_imported_module:
       cu->processing_has_namespace_info = true;
@@ -9134,18 +9139,18 @@ dwarf2_physname (const char *name, struct die_info *die, struct dwarf2_cu *cu)
   return retval;
 }
 
-/* Inspect DIE in CU for a namespace alias.  If one exists, record
-   a new symbol for it.
+/* Inspect DIE in CU for a namespace alias or a variable with alias
+   attribute.  If one exists, record a new symbol for it.
 
-   Returns 1 if a namespace alias was recorded, 0 otherwise.  */
+   Returns true if an alias was recorded, false otherwise.  */
 
-static int
-read_namespace_alias (struct die_info *die, struct dwarf2_cu *cu)
+static bool
+read_alias (struct die_info *die, struct dwarf2_cu *cu)
 {
   struct attribute *attr;
 
-  /* If the die does not have a name, this is not a namespace
-     alias.  */
+  /* If the die does not have a name, this is neither a namespace
+     alias nor a variable alias.  */
   attr = dwarf2_attr (die, DW_AT_name, cu);
   if (attr != NULL)
     {
@@ -9171,26 +9176,42 @@ read_namespace_alias (struct die_info *die, struct dwarf2_cu *cu)
 	{
 	  complaint (_("DIE at %s has too many recursively imported "
 		       "declarations"), sect_offset_str (d->sect_off));
-	  return 0;
+	  return false;
 	}
 
       if (attr != NULL)
 	{
 	  struct type *type;
-	  sect_offset sect_off = attr->get_ref_die_offset ();
-
-	  type = get_die_type_at_offset (sect_off, cu->per_cu, cu->per_objfile);
-	  if (type != NULL && type->code () == TYPE_CODE_NAMESPACE)
+	  if (d->tag == DW_TAG_variable)
 	    {
-	      /* This declaration is a global namespace alias.  Add
-		 a symbol for it whose type is the aliased namespace.  */
-	      new_symbol (die, type, cu);
-	      return 1;
+	      /* This declaration is a C/C++ global variable alias.
+		 Add a symbol for it whose type is the same as the
+		 aliased variable's.  */
+	      type = die_type (d, imported_cu);
+	      struct symbol *sym = new_symbol (die, type, cu);
+	      attr = dwarf2_attr (d, DW_AT_location, imported_cu);
+	      sym->set_aclass_index (LOC_UNRESOLVED);
+	      if (attr != nullptr)
+		var_decode_location (attr, sym, cu);
+	      return true;
+	    }
+	  else
+	    {
+	      sect_offset sect_off = attr->get_ref_die_offset ();
+	      type = get_die_type_at_offset (sect_off, cu->per_cu,
+					     cu->per_objfile);
+	      if (type != nullptr && type->code () == TYPE_CODE_NAMESPACE)
+		{
+		  /* This declaration is a global namespace alias. Add
+		     a symbol for it whose type is the aliased
+		     namespace.  */
+		  new_symbol (die, type, cu);
+		  return true;
+		}
 	    }
 	}
     }
-
-  return 0;
+  return false;
 }
 
 /* Return the using directives repository (global or local?) to use in the
diff --git a/gdb/testsuite/gdb.base/symbol-alias.exp b/gdb/testsuite/gdb.base/symbol-alias.exp
index 289f49bbc3f..5f6fa94dd76 100644
--- a/gdb/testsuite/gdb.base/symbol-alias.exp
+++ b/gdb/testsuite/gdb.base/symbol-alias.exp
@@ -15,6 +15,10 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 standard_testfile symbol-alias.c symbol-alias2.c
+set using_clang [test_compiler_info clang-*]
+# Clang versions prior to v15 do not emit debug info for aliases.
+set old_clang [expr [test_compiler_info {clang-1[0-4]-*-*}] \
+		 || [test_compiler_info {clang-[1-9]-*}]]
 
 if { [prepare_for_testing "failed to prepare" ${testfile} [list $srcfile $srcfile2]] } {
     return -1
@@ -30,7 +34,61 @@ foreach f {"func" "func_alias"} {
     gdb_test "p *${f}()" "= {field1 = 1, field2 = 2}"
 }
 
-# Variables.
-foreach v {"g_var_s" "g_var_s_alias"} {
-    gdb_test "p $v" "= {field1 = 1, field2 = 2}"
+# Extern global variable.
+gdb_test "p g_var_s" "= {field1 = 1, field2 = 2}"
+foreach v {"g_var_s_alias" "g_var_s_alias2"} {
+    gdb_test_multiple "p $v" "print alias variable $v" {
+	-re -wrap " = {field1 = 1, field2 = 2}" {
+	    pass $gdb_test_name
+	}
+	-re -wrap "has unknown type; cast it to its declared type" {
+	    if { $old_clang } {
+		xfail $gdb_test_name
+	    } else {
+		fail $gdb_test_name
+	    }
+	}
+    }
+}
+
+# Static global variable.
+gdb_test "p g_var" " = 1"
+gdb_test_multiple "p g_var_alias" "print alias of static variable" {
+    -re -wrap " = 1" {
+	pass $gdb_test_name
+    }
+    -re -wrap "has unknown type; cast it to its declared type" {
+	if { $old_clang } {
+	  xfail $gdb_test_name
+	} else {
+	  fail $gdb_test_name
+	}
+    }
+}
+
+# Alias of a variable that is declared later in the scope.
+gdb_test "p g_def_var" " = 2"
+gdb_test_multiple "p g_def_var_alias" "print g_def_var_alias" {
+    -re -wrap " = 2" {
+	pass $gdb_test_name
+    }
+    -re -wrap "has unknown type; cast it to its declared type" {
+	if { $old_clang } {
+	  xfail $gdb_test_name
+	} else {
+	  fail $gdb_test_name
+	}
+    }
+}
+gdb_test_multiple "p g_def_var_alias2" "print g_def_var_alias2" {
+    -re -wrap " = 2" {
+	pass $gdb_test_name
+    }
+    -re -wrap "has unknown type; cast it to its declared type" {
+	if { $old_clang } {
+	  xfail $gdb_test_name
+	} else {
+	  fail $gdb_test_name
+	}
+    }
 }
diff --git a/gdb/testsuite/gdb.base/symbol-alias2.c b/gdb/testsuite/gdb.base/symbol-alias2.c
index 34f4e121e25..c1c0413ed60 100644
--- a/gdb/testsuite/gdb.base/symbol-alias2.c
+++ b/gdb/testsuite/gdb.base/symbol-alias2.c
@@ -23,6 +23,8 @@ struct S
 
 struct S g_var_s = { 1, 2 };
 
+static int g_var = 1;
+
 #ifdef __cplusplus
 /* So that the alias attribute below work without having to figure out
    this function's mangled name.  */
@@ -38,3 +40,13 @@ func (void)
 struct S *func_alias (void) __attribute__ ((alias ("func")));
 
 extern struct S g_var_s_alias __attribute__ ((alias ("g_var_s")));
+
+extern struct S g_var_s_alias2 __attribute__ ((alias ("g_var_s_alias")));
+
+extern int g_var_alias __attribute__ ((alias ("g_var")));
+
+extern int g_def_var_alias __attribute__ ((alias ("g_def_var")));
+
+int g_def_var = 2;
+
+extern int g_def_var_alias2 __attribute__ ((alias ("g_def_var_alias")));

base-commit: 8025a164287b588f450686a4b81f6001c33e4269
-- 
2.25.1