[PATCH] libdwP.h: Remove abbrev_lock

[Aaron Merey]

Improve __libdw_dieabbrev performance by removing abbrev_lock.  This
lock protects the Dwarf_Die abbrev member due to lazy loading.

Instead, eagerly load abbrev during Dwarf_Die initialization so that
the member is readonly and requires no locking.

Signed-off-by: Aaron Merey <amerey@redhat.com>
---
 libdw/dwarf_decl_file.c   |  3 +-
 libdw/dwarf_end.c         |  1 -
 libdw/dwarf_getscopevar.c |  3 +-
 libdw/libdwP.h            | 65 +++++++++++++++++----------------------
 libdw/libdw_findcu.c      |  1 -
 5 files changed, 32 insertions(+), 41 deletions(-)

diff --git a/libdw/dwarf_decl_file.c b/libdw/dwarf_decl_file.c
index 1b91a4e9..03040e12 100644
--- a/libdw/dwarf_decl_file.c
+++ b/libdw/dwarf_decl_file.c
@@ -50,7 +50,8 @@ dwarf_decl_file (Dwarf_Die *die)
 
   Dwarf_Files *files;
   size_t nfiles;
-  if (INTUSE(dwarf_getsrcfiles) (&CUDIE (attr_mem.cu), &files, &nfiles) != 0)
+  Dwarf_Die cudie = CUDIE (attr_mem.cu);
+  if (INTUSE(dwarf_getsrcfiles) (&cudie, &files, &nfiles) != 0)
     return NULL;
 
   if (idx >= nfiles)
diff --git a/libdw/dwarf_end.c b/libdw/dwarf_end.c
index 1628e448..51021d64 100644
--- a/libdw/dwarf_end.c
+++ b/libdw/dwarf_end.c
@@ -68,7 +68,6 @@ cu_free (void *arg)
      && p != p->dbg->fake_addr_cu)
     {
       Dwarf_Abbrev_Hash_free (&p->abbrev_hash);
-      rwlock_fini (p->abbrev_lock);
       rwlock_fini (p->split_lock);
       mutex_fini (p->src_lock);
       mutex_fini (p->str_off_base_lock);
diff --git a/libdw/dwarf_getscopevar.c b/libdw/dwarf_getscopevar.c
index 7b1416f3..306ccae0 100644
--- a/libdw/dwarf_getscopevar.c
+++ b/libdw/dwarf_getscopevar.c
@@ -40,7 +40,8 @@
 static int
 getfiles (Dwarf_Die *die, Dwarf_Files **files)
 {
-  return INTUSE(dwarf_getsrcfiles) (&CUDIE (die->cu), files, NULL);
+  Dwarf_Die cudie = CUDIE (die->cu);
+  return INTUSE(dwarf_getsrcfiles) (&cudie, files, NULL);
 }
 
 /* Fetch an attribute that should have a constant integer form.  */
diff --git a/libdw/libdwP.h b/libdw/libdwP.h
index de80cd4e..4a528c9c 100644
--- a/libdw/libdwP.h
+++ b/libdw/libdwP.h
@@ -455,10 +455,6 @@ struct Dwarf_CU
      Don't access directly, call __libdw_cu_locs_base.  */
   Dwarf_Off locs_base;
 
-  /* Synchronize access to the abbrev member of a Dwarf_Die that
-     refers to this Dwarf_CU.  Covers __libdw_die_abbrev. */
-  rwlock_define(, abbrev_lock);
-
   /* Synchronize access to the split member of this Dwarf_CU.
      Covers __libdw_find_split_unit.  */
   rwlock_define(, split_lock);
@@ -595,23 +591,6 @@ __libdw_first_die_off_from_cu (struct Dwarf_CU *cu)
 					  cu->unit_type);
 }
 
-#define CUDIE(fromcu)							      \
-  ((Dwarf_Die)								      \
-   {									      \
-     .cu = (fromcu),							      \
-     .addr = ((char *) (fromcu)->dbg->sectiondata[cu_sec_idx (fromcu)]->d_buf \
-	      + __libdw_first_die_off_from_cu (fromcu))			      \
-   })
-
-#define SUBDIE(fromcu)							      \
-  ((Dwarf_Die)								      \
-   {									      \
-     .cu = (fromcu),							      \
-     .addr = ((char *) (fromcu)->dbg->sectiondata[cu_sec_idx (fromcu)]->d_buf \
-	      + (fromcu)->start + (fromcu)->subdie_offset)		      \
-   })
-
-
 /* Prototype of a single .debug_macro operator.  */
 typedef struct
 {
@@ -820,12 +799,7 @@ __nonnull_attribute__ (1)
 __libdw_dieabbrev (Dwarf_Die *die, const unsigned char **readp)
 {
   if (unlikely (die->cu == NULL))
-    {
-      die->abbrev = DWARF_END_ABBREV;
-      return DWARF_END_ABBREV;
-    }
-
-  rwlock_wrlock (die->cu->abbrev_lock);
+    return DWARF_END_ABBREV;
 
   /* Do we need to get the abbreviation, or need to read after the code?  */
   if (die->abbrev == NULL || readp != NULL)
@@ -835,25 +809,20 @@ __libdw_dieabbrev (Dwarf_Die *die, const unsigned char **readp)
       const unsigned char *addr = die->addr;
 
       if (addr >= (const unsigned char *) die->cu->endp)
-	{
-	  die->abbrev = DWARF_END_ABBREV;
-	  rwlock_unlock (die->cu->abbrev_lock);
-	  return DWARF_END_ABBREV;
-	}
+	return DWARF_END_ABBREV;
 
       get_uleb128 (code, addr, die->cu->endp);
       if (readp != NULL)
 	*readp = addr;
 
-      /* Find the abbreviation.  */
+      /* Find the abbreviation.  To improve performance, this write is not
+	 protected by a lock.  It should only be reachable by a single thread
+	 when initializing the DIE.  */
       if (die->abbrev == NULL)
 	die->abbrev = __libdw_findabbrev (die->cu, code);
     }
 
-  Dwarf_Abbrev *result = die->abbrev;
-  rwlock_unlock (die->cu->abbrev_lock);
-
-  return result;
+  return die->abbrev;
 }
 
 /* Helper functions for form handling.  */
@@ -1112,6 +1081,28 @@ cu_sec_idx (struct Dwarf_CU *cu)
   return cu->sec_idx;
 }
 
+static inline Dwarf_Die
+CUDIE (Dwarf_CU *fromcu)
+{
+  Dwarf_Die res = {0};
+  res.cu = fromcu;
+  res.addr = ((char *) (fromcu)->dbg->sectiondata[cu_sec_idx (fromcu)]->d_buf
+              + __libdw_first_die_off_from_cu (fromcu));
+  (void) dwarf_tag (&res);  /* Set .abbrev  */
+  return res;
+}
+
+static inline Dwarf_Die
+SUBDIE (Dwarf_CU *fromcu)
+{
+  Dwarf_Die res = {0};
+  res.cu = fromcu;
+  res.addr = ((char *) (fromcu)->dbg->sectiondata[cu_sec_idx (fromcu)]->d_buf
+	      + (fromcu)->start + (fromcu)->subdie_offset);
+  (void) dwarf_tag (&res);  /* Set .abbrev  */
+  return res;
+}
+
 static inline bool
 is_cudie (Dwarf_Die *cudie)
 {
diff --git a/libdw/libdw_findcu.c b/libdw/libdw_findcu.c
index 0e4dcc37..857ebd38 100644
--- a/libdw/libdw_findcu.c
+++ b/libdw/libdw_findcu.c
@@ -177,7 +177,6 @@ __libdw_intern_next_unit (Dwarf *dbg, bool debug_types)
   newp->startp = data->d_buf + newp->start;
   newp->endp = data->d_buf + newp->end;
   eu_search_tree_init (&newp->locs_tree);
-  rwlock_init (newp->abbrev_lock);
   rwlock_init (newp->split_lock);
   mutex_init (newp->src_lock);
   mutex_init (newp->str_off_base_lock);
-- 
2.49.0