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[2/3] CTF for extern variable fix [PR105089]

Message ID 20220330233135.1762317-3-indu.bhagat@oracle.com
State New
Headers show
Series Fix PR debug/105089 | expand

Commit Message

Indu Bhagat March 30, 2022, 11:31 p.m. UTC 
The CTF format cannot differentiate between a non-defining extern
variable declaration vs. a defining variable declaration (unlike DWARF).
So, the correct behaviour wrt the compiler generating CTF for such
extern variables (i.e., when both the defining and non-defining decl
are present in the same CU) is to simply emit the CTF variable
correspoding to the defining declaration.

To carry out the above, following changes are introduced via the patch:

1. The CTF container (ctfc.h) now keeps track of the non-defining declarations
(by noting the DWARF attribute DW_AT_specification) in a new ctfc_ignore_vars
hashtable.  Such book-keeping is necessary because the CTF container should
not rely on the order of DWARF DIEs presented to it at generation time.

2. At the time of ctf_add_variable (), the DW_AT_specification DIE if present
is added in the ctfc_ignore_vars hashtable.  The CTF variable generation for
the defining declaration continues as normal.

3. If the ctf_add_variable () is asked to generate CTF variable for a DIE
present in the ctfc_ignore_vars, it skips generating CTF for it.

4. Recall that CTF variables are pre-processed before emission.  Till now, the
only pre-processing that was being done was to sort them in order of their
names.  Now an additional step is added:  If the CTF variable which
corresponds to the non-defining declaration is indeed present in the ctfc_vars
hashtable (because the corresponding DWARF DIE was encountered first by the
CTF generation engine), skip that CTF variable from output.

An important side effect of such a workflow above is that CTF for the C type
of the non-defining decl will remain in the CTF dictionary (and will be
emitted in the output section as well).  This type can, however, be pruned by
the link-time de-duplicator as usual, if deemed unused.

2022-03-30  Indu Bhagat  <indu.bhagat@oracle.com>

gcc/ChangeLog:

	PR debug/105089
	* ctfc.cc (ctf_dvd_ignore_insert): New function.
	(ctf_dvd_ignore_lookup): Likewise.
	(ctf_add_variable): Keep track of non-defining decl DIEs.
	(new_ctf_container): Initialize the new hash-table.
	(ctfc_delete_container): Empty hash-table.
	* ctfc.h (struct ctf_container): Add new hash-table.
	(ctf_dvd_ignore_lookup): New declaration.
	(ctf_add_variable): Add additional argument.
	* ctfout.cc (ctf_dvd_preprocess_cb): Skip adding CTF variable
	record for non-defining decl for which a defining decl exists
	in the same TU.
	(ctf_preprocess): Defer updating the number of global objts
	until here.
	(output_ctf_header): Use ctfc_vars_list_count as some CTF
	variables may not make it to the final output.
	(output_ctf_vars): Likewise.
	* dwarf2ctf.cc (gen_ctf_variable): Skip generating CTF variable
	if this is known to be a non-defining decl DIE.
---
 gcc/ctfc.cc      | 62 ++++++++++++++++++++++++++++++++++++++++++++++--
 gcc/ctfc.h       |  6 ++++-
 gcc/ctfout.cc    | 24 +++++++++++++------
 gcc/dwarf2ctf.cc | 18 ++++++++++++--
 4 files changed, 98 insertions(+), 12 deletions(-)
diff mbox series

Patch

diff --git a/gcc/ctfc.cc b/gcc/ctfc.cc
index 6fe44d2e8d49..f24e7bff9487 100644
--- a/gcc/ctfc.cc
+++ b/gcc/ctfc.cc
@@ -179,6 +179,40 @@  ctf_dvd_lookup (const ctf_container_ref ctfc, dw_die_ref die)
   return NULL;
 }
 
+/* Insert a dummy CTF variable into the list of variables to be ignored.  */
+
+static void
+ctf_dvd_ignore_insert (ctf_container_ref ctfc, ctf_dvdef_ref dvd)
+{
+  bool existed = false;
+  ctf_dvdef_ref entry = dvd;
+
+  ctf_dvdef_ref * item = ctfc->ctfc_ignore_vars->find_slot (entry, INSERT);
+  if (*item == NULL)
+     *item = dvd;
+  else
+    existed = true;
+  /* Duplicate variable records not expected to be inserted.  */
+  gcc_assert (!existed);
+}
+
+/* Lookup the dummy CTF variable given the DWARF die for the non-defining
+   decl to be ignored.  */
+
+bool
+ctf_dvd_ignore_lookup (const ctf_container_ref ctfc, dw_die_ref die)
+{
+  ctf_dvdef_t entry;
+  entry.dvd_key = die;
+
+  ctf_dvdef_ref * slot = ctfc->ctfc_ignore_vars->find_slot (&entry, NO_INSERT);
+
+  if (slot)
+    return true;
+
+  return false;
+}
+
 /* Append member definition to the list.  Member list is a singly-linked list
    with list start pointing to the head.  */
 
@@ -666,9 +700,10 @@  ctf_add_member_offset (ctf_container_ref ctfc, dw_die_ref sou,
 
 int
 ctf_add_variable (ctf_container_ref ctfc, const char * name, ctf_id_t ref,
-		  dw_die_ref die, unsigned int external_vis)
+		  dw_die_ref die, unsigned int external_vis,
+		  dw_die_ref die_var_decl)
 {
-  ctf_dvdef_ref dvd;
+  ctf_dvdef_ref dvd, dvd_ignore;
 
   gcc_assert (name);
 
@@ -680,6 +715,24 @@  ctf_add_variable (ctf_container_ref ctfc, const char * name, ctf_id_t ref,
       dvd->dvd_name = ctf_add_string (ctfc, name, &(dvd->dvd_name_offset));
       dvd->dvd_visibility = external_vis;
       dvd->dvd_type = ref;
+
+      /* If DW_AT_specification attribute exists, keep track of it as this is
+	 the non-defining declaration corresponding to the variable.  We will
+	 skip emitting CTF variable for such incomplete, non-defining
+	 declarations.
+	 There could be some non-defining declarations, however, for which a
+	 defining declaration does not show up in the same CU.  For such
+	 cases, the compiler continues to emit CTF variable record as
+	 usual.  */
+      if (die_var_decl)
+	{
+	  dvd_ignore = ggc_cleared_alloc<ctf_dvdef_t> ();
+	  dvd_ignore->dvd_key = die_var_decl;
+	  /* It's alright to leave other fields as zero.  No valid CTF
+	     variable will be added for these DW_TAG_variable DIEs.  */
+	  ctf_dvd_ignore_insert (ctfc, dvd_ignore);
+	}
+
       ctf_dvd_insert (ctfc, dvd);
 
       if (strcmp (name, ""))
@@ -900,6 +953,8 @@  new_ctf_container (void)
     = hash_table<ctfc_dtd_hasher>::create_ggc (100);
   tu_ctfc->ctfc_vars
     = hash_table<ctfc_dvd_hasher>::create_ggc (100);
+  tu_ctfc->ctfc_ignore_vars
+    = hash_table<ctfc_dvd_hasher>::create_ggc (10);
 
   return tu_ctfc;
 }
@@ -952,6 +1007,9 @@  ctfc_delete_container (ctf_container_ref ctfc)
       ctfc->ctfc_vars->empty ();
       ctfc->ctfc_types = NULL;
 
+      ctfc->ctfc_ignore_vars->empty ();
+      ctfc->ctfc_ignore_vars = NULL;
+
       ctfc_delete_strtab (&ctfc->ctfc_strtable);
       ctfc_delete_strtab (&ctfc->ctfc_aux_strtable);
       if (ctfc->ctfc_vars_list)
diff --git a/gcc/ctfc.h b/gcc/ctfc.h
index 4ce756c728a7..001e544ef083 100644
--- a/gcc/ctfc.h
+++ b/gcc/ctfc.h
@@ -274,6 +274,8 @@  typedef struct GTY (()) ctf_container
   hash_table <ctfc_dtd_hasher> * GTY (()) ctfc_types;
   /* CTF variables.  */
   hash_table <ctfc_dvd_hasher> * GTY (()) ctfc_vars;
+  /* CTF variables to be ignored.  */
+  hash_table <ctfc_dvd_hasher> * GTY (()) ctfc_ignore_vars;
 
   /* CTF string table.  */
   ctf_strtable_t ctfc_strtable;
@@ -394,6 +396,8 @@  extern ctf_dtdef_ref ctf_dtd_lookup (const ctf_container_ref ctfc,
 				     dw_die_ref die);
 extern ctf_dvdef_ref ctf_dvd_lookup (const ctf_container_ref ctfc,
 				     dw_die_ref die);
+extern bool ctf_dvd_ignore_lookup (const ctf_container_ref ctfc,
+				   dw_die_ref die);
 
 extern const char * ctf_add_string (ctf_container_ref, const char *,
 				    uint32_t *, int);
@@ -430,7 +434,7 @@  extern int ctf_add_member_offset (ctf_container_ref, dw_die_ref, const char *,
 extern int ctf_add_function_arg (ctf_container_ref, dw_die_ref,
 				 const char *, ctf_id_t);
 extern int ctf_add_variable (ctf_container_ref, const char *, ctf_id_t,
-			     dw_die_ref, unsigned int);
+			     dw_die_ref, unsigned int, dw_die_ref);
 
 extern ctf_id_t ctf_lookup_tree_type (ctf_container_ref, const tree);
 extern ctf_id_t get_btf_id (ctf_id_t);
diff --git a/gcc/ctfout.cc b/gcc/ctfout.cc
index 28a873b2027d..3cf89b94f993 100644
--- a/gcc/ctfout.cc
+++ b/gcc/ctfout.cc
@@ -212,6 +212,13 @@  ctf_dvd_preprocess_cb (ctf_dvdef_ref * slot, void * arg)
   ctf_dvdef_ref var = (ctf_dvdef_ref) *slot;
   ctf_container_ref arg_ctfc = dvd_arg->dvd_arg_ctfc;
 
+  /* If the CTF variable corresponds to an extern variable declaration with
+     a defining declaration later on, skip it.  Only CTF variable
+     corresponding to the defining declaration for the extern variable is
+     desirable.  */
+  if (ctf_dvd_ignore_lookup (arg_ctfc, var->dvd_key))
+    return 1;
+
   ctf_preprocess_var (arg_ctfc, var);
 
   /* Keep track of global objts.  */
@@ -276,16 +283,16 @@  static void
 ctf_preprocess (ctf_container_ref ctfc)
 {
   size_t num_ctf_types = ctfc->ctfc_types->elements ();
+  size_t num_ctf_vars = ctfc_get_num_ctf_vars (ctfc);
 
   /* Initialize an array to keep track of the CTF variables at global
-     scope.  */
-  size_t num_global_objts = ctfc->ctfc_num_global_objts;
+     scope.  At this time, size it conservatively.  */
+  size_t num_global_objts = num_ctf_vars;
   if (num_global_objts)
     {
       ctfc->ctfc_gobjts_list = ggc_vec_alloc<ctf_dvdef_t*>(num_global_objts);
     }
 
-  size_t num_ctf_vars = ctfc_get_num_ctf_vars (ctfc);
   if (num_ctf_vars)
     {
       ctf_dvd_preprocess_arg_t dvd_arg;
@@ -299,8 +306,11 @@  ctf_preprocess (ctf_container_ref ctfc)
 	 list for sorting.  */
       ctfc->ctfc_vars->traverse<void *, ctf_dvd_preprocess_cb> (&dvd_arg);
       /* Sort the list.  */
-      qsort (ctfc->ctfc_vars_list, num_ctf_vars, sizeof (ctf_dvdef_ref),
-	     ctf_varent_compare);
+      qsort (ctfc->ctfc_vars_list, ctfc->ctfc_vars_list_count,
+	     sizeof (ctf_dvdef_ref), ctf_varent_compare);
+      /* Update the actual number of the generated CTF variables at global
+	 scope.  */
+      ctfc->ctfc_num_global_objts = dvd_arg.dvd_global_obj_idx;
     }
 
   /* Initialize an array to keep track of the CTF functions types for global
@@ -476,7 +486,7 @@  output_ctf_header (ctf_container_ref ctfc)
       /* Vars appear after function index.  */
       varoff = funcidxoff + ctfc->ctfc_num_global_funcs * sizeof (uint32_t);
       /* CTF types appear after vars.  */
-      typeoff = varoff + ctfc_get_num_ctf_vars (ctfc) * sizeof (ctf_varent_t);
+      typeoff = varoff + (ctfc->ctfc_vars_list_count) * sizeof (ctf_varent_t);
       /* The total number of bytes for CTF types is the sum of the number of
 	 times struct ctf_type_t, struct ctf_stype_t are written, plus the
 	 amount of variable length data after each one of these.  */
@@ -595,7 +605,7 @@  static void
 output_ctf_vars (ctf_container_ref ctfc)
 {
   size_t i;
-  size_t num_ctf_vars = ctfc_get_num_ctf_vars (ctfc);
+  unsigned int num_ctf_vars = ctfc->ctfc_vars_list_count;
   if (num_ctf_vars)
     {
       /* Iterate over the list of sorted vars and output the asm.  */
diff --git a/gcc/dwarf2ctf.cc b/gcc/dwarf2ctf.cc
index 747b2f661072..a6329ab6ee48 100644
--- a/gcc/dwarf2ctf.cc
+++ b/gcc/dwarf2ctf.cc
@@ -808,12 +808,26 @@  gen_ctf_variable (ctf_container_ref ctfc, dw_die_ref die)
   if (ctf_dvd_lookup (ctfc, die))
     return;
 
+  /* Do not generate CTF variable records for non-defining incomplete
+     declarations.  Such declarations can be known via the DWARF
+     DW_AT_specification attribute.  */
+  if (ctf_dvd_ignore_lookup (ctfc, die))
+    return;
+
+  /* The value of the DW_AT_specification attribute, if present, is a
+     reference to the debugging information entry representing the
+     non-defining declaration.  */
+  dw_die_ref decl = get_AT_ref (die, DW_AT_specification);
+
   /* Add the type of the variable.  */
   var_type_id = gen_ctf_type (ctfc, var_type);
 
   /* Generate the new CTF variable and update global counter.  */
-  (void) ctf_add_variable (ctfc, var_name, var_type_id, die, external_vis);
-  ctfc->ctfc_num_global_objts += 1;
+  (void) ctf_add_variable (ctfc, var_name, var_type_id, die, external_vis,
+			   decl);
+  /* Skip updating the number of global objects at this time.  This is updated
+     later after pre-processing as some CTF variable records although
+     generated now, will not be emitted later.  [PR105089].  */
 }
 
 /* Add a CTF function record for the given input DWARF DIE.  */