Re: [PATCH v2 39/42] Pass existing_cu object to cutu_reader

[Simon Marchi <simark@simark.ca>]

On 2020-05-22 4:57 p.m., Tom Tromey wrote:
>>>>>> "Simon" == Simon Marchi via Gdb-patches <gdb-patches@sourceware.org> writes:
> 
> Simon> It is possible, seemingly for a special case described in
> Simon> find_partial_die, for cutu_reader to re-use an existing dwarf2_cu
> Simon> instead of creating a new one.  This happens
> Simon> ---
> 
> The description got cut off here, and there's no ChangeLog entry.
> 
> Tom
> 

Oops, I messed up, not sure what happened.

Here's the full version:

From 3d20153d73373a0c2281d2dd9a6ffe505f6346bc Mon Sep 17 00:00:00 2001
From: Simon Marchi <simon.marchi@efficios.com>
Date: Thu, 9 Apr 2020 13:46:41 -0400
Subject: [PATCH] Pass existing_cu object to cutu_reader

It is possible, seemingly for a special case described in
find_partial_die, for cutu_reader to re-use an existing dwarf2_cu
instead of creating a new one.  This happens when running this test, for
example:

    make check TESTS="gdb.dwarf2/fission-reread.exp"

Right now the, `use_existing_cu` flag tells cutu_reader to use the
dwarf2_cu object at dwarf2_per_cu_data::cu.  However, we'll remove that
field, so we need to find another solution.

This situation arises when some caller up the stack has already created
the dwarf2_cu to read a dwarf2_per_cu_data, but needs to re-read it with
some other parameters.  Therefore, it's possible to just have that
caller pass down the dwarf2_cu object to use as a `existing_cu`
parameter.  If `existing_cu` is NULL, it tells cutu_reader that it needs
to instantiate a new one.

gdb/ChangeLog:

	* dwarf2/read.c (class cutu_reader) <cutu_reader>: Replace
	`int use_existing_cu` parameter with `dwarf2_cu *existing_cu`.
	(init_tu_and_read_dwo_dies): Likewise.
	(cutu_reader::init_tu_and_read_dwo_dies): Likewise.
	(cutu_reader::cutu_reader): Likewise.
	(load_partial_comp_unit): Likewise.
	(process_psymtab_comp_unit): Update.
	(build_type_psymtabs_1): Update.
	(process_skeletonless_type_unit): Update.
	(load_full_comp_unit): Update.
	(find_partial_die): Update.
	(dwarf2_read_addr_index): Update.
	(read_signatured_type): Update.
---
 gdb/dwarf2/read.c | 57 ++++++++++++++++++++++++++---------------------
 1 file changed, 32 insertions(+), 25 deletions(-)

diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index a6b7f2c6d25..c2dc34477f7 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -912,7 +912,7 @@ class cutu_reader : public die_reader_specs
   cutu_reader (dwarf2_per_cu_data *this_cu,
 	       dwarf2_per_objfile *per_objfile,
 	       struct abbrev_table *abbrev_table,
-	       int use_existing_cu,
+	       dwarf2_cu *existing_cu,
 	       bool skip_partial);

   explicit cutu_reader (struct dwarf2_per_cu_data *this_cu,
@@ -933,7 +933,7 @@ class cutu_reader : public die_reader_specs
 private:
   void init_tu_and_read_dwo_dies (dwarf2_per_cu_data *this_cu,
 				  dwarf2_per_objfile *per_objfile,
-				  int use_existing_cu);
+				  dwarf2_cu *existing_cu);

   struct dwarf2_per_cu_data *m_this_cu;
   std::unique_ptr<dwarf2_cu> m_new_cu;
@@ -6902,7 +6902,7 @@ lookup_dwo_unit (dwarf2_cu *cu, die_info *comp_unit_die, const char *dwo_name)
 void
 cutu_reader::init_tu_and_read_dwo_dies (dwarf2_per_cu_data *this_cu,
 					dwarf2_per_objfile *per_objfile,
-					int use_existing_cu)
+					dwarf2_cu *existing_cu)
 {
   struct signatured_type *sig_type;

@@ -6912,24 +6912,28 @@ cutu_reader::init_tu_and_read_dwo_dies (dwarf2_per_cu_data *this_cu,
   sig_type = (struct signatured_type *) this_cu;
   gdb_assert (sig_type->dwo_unit != NULL);

-  if (use_existing_cu && this_cu->cu != NULL)
+  dwarf2_cu *cu;
+
+  if (existing_cu != nullptr)
     {
-      gdb_assert (this_cu->cu->dwo_unit == sig_type->dwo_unit);
+      cu = existing_cu;
+      gdb_assert (cu->dwo_unit == sig_type->dwo_unit);
       /* There's no need to do the rereading_dwo_cu handling that
 	 cutu_reader does since we don't read the stub.  */
     }
   else
     {
-      /* If !use_existing_cu, this_cu->cu must be NULL.  */
+      /* If an existing_cu is provided, this_cu->cu must be NULL.  */
       gdb_assert (this_cu->cu == NULL);
       m_new_cu.reset (new dwarf2_cu (this_cu, per_objfile));
+      cu = m_new_cu.get ();
     }

   /* A future optimization, if needed, would be to use an existing
      abbrev table.  When reading DWOs with skeletonless TUs, all the TUs
      could share abbrev tables.  */

-  if (read_cutu_die_from_dwo (this_cu->cu, sig_type->dwo_unit,
+  if (read_cutu_die_from_dwo (cu, sig_type->dwo_unit,
 			      NULL /* stub_comp_unit_die */,
 			      sig_type->dwo_unit->dwo_file->comp_dir,
 			      this, &info_ptr,
@@ -6948,13 +6952,13 @@ cutu_reader::init_tu_and_read_dwo_dies (dwarf2_per_cu_data *this_cu,
    Otherwise the table specified in the comp unit header is read in and used.
    This is an optimization for when we already have the abbrev table.

-   If USE_EXISTING_CU is non-zero, and THIS_CU->cu is non-NULL, then use it.
-   Otherwise, a new CU is allocated with xmalloc.  */
+   If EXISTING_CU is non-NULL, then use it.  Otherwise, a new CU is
+   allocated.  */

 cutu_reader::cutu_reader (dwarf2_per_cu_data *this_cu,
 			  dwarf2_per_objfile *dwarf2_per_objfile,
 			  struct abbrev_table *abbrev_table,
-			  int use_existing_cu,
+			  dwarf2_cu *existing_cu,
 			  bool skip_partial)
   : die_reader_specs {},
     m_this_cu (this_cu)
@@ -6962,7 +6966,6 @@ cutu_reader::cutu_reader (dwarf2_per_cu_data *this_cu,
   struct objfile *objfile = dwarf2_per_objfile->objfile;
   struct dwarf2_section_info *section = this_cu->section;
   bfd *abfd = section->get_bfd_owner ();
-  struct dwarf2_cu *cu;
   const gdb_byte *begin_info_ptr;
   struct signatured_type *sig_type = NULL;
   struct dwarf2_section_info *abbrev_section;
@@ -6983,7 +6986,7 @@ cutu_reader::cutu_reader (dwarf2_per_cu_data *this_cu,
       /* Narrow down the scope of possibilities to have to understand.  */
       gdb_assert (this_cu->is_debug_types);
       gdb_assert (abbrev_table == NULL);
-      init_tu_and_read_dwo_dies (this_cu, dwarf2_per_objfile, use_existing_cu);
+      init_tu_and_read_dwo_dies (this_cu, dwarf2_per_objfile, existing_cu);
       return;
     }

@@ -6994,9 +6997,11 @@ cutu_reader::cutu_reader (dwarf2_per_cu_data *this_cu,

   abbrev_section = get_abbrev_section_for_cu (this_cu);

-  if (use_existing_cu && this_cu->cu != NULL)
+  dwarf2_cu *cu;
+
+  if (existing_cu != nullptr)
     {
-      cu = this_cu->cu;
+      cu = existing_cu;
       /* If this CU is from a DWO file we need to start over, we need to
 	 refetch the attributes from the skeleton CU.
 	 This could be optimized by retrieving those attributes from when we
@@ -7008,7 +7013,7 @@ cutu_reader::cutu_reader (dwarf2_per_cu_data *this_cu,
     }
   else
     {
-      /* If !use_existing_cu, this_cu->cu must be NULL.  */
+      /* If an existing_cu is provided, this_cu->cu must be NULL.  */
       gdb_assert (this_cu->cu == NULL);
       m_new_cu.reset (new dwarf2_cu (this_cu, dwarf2_per_objfile));
       cu = m_new_cu.get ();
@@ -7561,7 +7566,7 @@ process_psymtab_comp_unit (dwarf2_per_cu_data *this_cu,
   if (this_cu->cu != NULL)
     free_one_cached_comp_unit (this_cu, per_objfile);

-  cutu_reader reader (this_cu, per_objfile, NULL, 0, false);
+  cutu_reader reader (this_cu, per_objfile, nullptr, nullptr, false);

   switch (reader.comp_unit_die->tag)
     {
@@ -7743,7 +7748,7 @@ build_type_psymtabs_1 (struct dwarf2_per_objfile *dwarf2_per_objfile)
 	}

       cutu_reader reader (&tu.sig_type->per_cu, dwarf2_per_objfile,
-			  abbrev_table.get (), 0, false);
+			  abbrev_table.get (), nullptr, false);
       if (!reader.dummy_p)
 	build_type_psymtabs_reader (&reader, reader.info_ptr,
 				    reader.comp_unit_die);
@@ -7848,7 +7853,8 @@ process_skeletonless_type_unit (void **slot, void *info)
   *slot = entry;

   /* This does the job that build_type_psymtabs_1 would have done.  */
-  cutu_reader reader (&entry->per_cu, dwarf2_per_objfile, NULL, 0, false);
+  cutu_reader reader (&entry->per_cu, dwarf2_per_objfile, nullptr, nullptr,
+		      false);
   if (!reader.dummy_p)
     build_type_psymtabs_reader (&reader, reader.info_ptr,
 				reader.comp_unit_die);
@@ -7984,9 +7990,10 @@ dwarf2_build_psymtabs_hard (struct dwarf2_per_objfile *dwarf2_per_objfile)

 static void
 load_partial_comp_unit (dwarf2_per_cu_data *this_cu,
-			dwarf2_per_objfile *per_objfile)
+			dwarf2_per_objfile *per_objfile,
+			dwarf2_cu *existing_cu)
 {
-  cutu_reader reader (this_cu, per_objfile, NULL, 1, false);
+  cutu_reader reader (this_cu, per_objfile, nullptr, existing_cu, false);

   if (!reader.dummy_p)
     {
@@ -9105,7 +9112,7 @@ load_full_comp_unit (dwarf2_per_cu_data *this_cu,
 {
   gdb_assert (! this_cu->is_debug_types);

-  cutu_reader reader (this_cu, per_objfile, NULL, 1, skip_partial);
+  cutu_reader reader (this_cu, per_objfile, NULL, this_cu->cu, skip_partial);
   if (reader.dummy_p)
     return;

@@ -18713,7 +18720,7 @@ find_partial_die (sect_offset sect_off, int offset_in_dwz, struct dwarf2_cu *cu)
 						 dwarf2_per_objfile);

       if (per_cu->cu == NULL || per_cu->cu->partial_dies == NULL)
-	load_partial_comp_unit (per_cu, cu->per_objfile);
+	load_partial_comp_unit (per_cu, cu->per_objfile, nullptr);

       per_cu->cu->last_used = 0;
       pd = per_cu->cu->find_partial_die (sect_off);
@@ -18732,7 +18739,7 @@ find_partial_die (sect_offset sect_off, int offset_in_dwz, struct dwarf2_cu *cu)
 	 DIEs alone (which can still be in use, e.g. in scan_partial_symbols),
 	 and clobber THIS_CU->cu->partial_dies with the hash table for the new
 	 set.  */
-      load_partial_comp_unit (per_cu, cu->per_objfile);
+      load_partial_comp_unit (per_cu, cu->per_objfile, cu);

       pd = per_cu->cu->find_partial_die (sect_off);
     }
@@ -19449,7 +19456,7 @@ dwarf2_read_addr_index (dwarf2_per_cu_data *per_cu,
     }
   else
     {
-      cutu_reader reader (per_cu, dwarf2_per_objfile, NULL, 0, false);
+      cutu_reader reader (per_cu, dwarf2_per_objfile, nullptr, nullptr, false);
       addr_base = reader.cu->addr_base;
       addr_size = reader.cu->header.addr_size;
     }
@@ -22831,7 +22838,7 @@ read_signatured_type (signatured_type *sig_type,
   gdb_assert (per_cu->is_debug_types);
   gdb_assert (per_cu->cu == NULL);

-  cutu_reader reader (per_cu, per_objfile, NULL, 0, false);
+  cutu_reader reader (per_cu, per_objfile, nullptr, nullptr, false);

   if (!reader.dummy_p)
     {
-- 
2.26.2