gdb: change regcache list to be a map

One regcache object is created for each stopped thread and is stored in
the regcache::regcaches linked list.  Looking up a regcache for a given
thread is therefore in O(number of threads).  Stopping all threads then
becomes O((number of threads) ^ 2).  It becomes noticeable when
debugging thousands of threads, which is typical with GPU targets.  This
patch replaces the linked list with an std::unordered_multimap, indexed
by (target, ptid).

I originally designed it using an std::unordered_map with (target, ptid,
arch) as the key, because that's how lookups are done (in
get_thread_arch_aspace_regcache).  However, the registers_changed_ptid
function, also somewhat on the hot path (it is used when resuming
threads), needs to delete all regcaches associated to a given (target,
ptid) tuple.  Using (target, ptid) as a key allows to do this more
efficiently (see exception below).  If the key of the map was (target,
ptid, arch), we'd have to walk all items of the map.

The lookup (in get_thread_arch_aspace_regcache), walks over all existing
regcaches belonging to this (target, ptid), looking to find the one with
the right arch.  This is ok, as there will be very few regcaches for a
given key (typically one).  Lookups become faster when the number of
threads grows, compared to the linked list.  With a small number of
threads, it will probably be a bit slower to do a map lookup than to
walk a few linked list nodes, but I don't think it will be noticeable in
practice.

The function registers_changed_ptid deletes all regcaches related to a
given (target, ptid).  We must now handle the different cases
separately:

- NULL target and minus_one_ptid: we delete all the entries
- NULL target and non-minus_one_ptid: invalid (checked by assert)
- non-NULL target and non-minus_one_ptid: we delete all the entries
  associated to that tuple, this is done efficiently
- a non-NULL target and minus_one_ptid: we delete all the entries
  associated to that target, whatever the ptid.  This is the slightly
  annoying case, as we can't easily look up all items having this target
  in their key.  I implemented it by walking the list, which is not
  ideal.

The function regcache_thread_ptid_changed is called when a thread
changes ptid.  It is implemented efficiently using the map, although
that's not very important: it is not called often, mostly when creating
an inferior, on some specific platforms.

Note: In hash_target_ptid, I am combining hash values from std::hash by
summing them.  I don't think it's ideal, since std::hash is just the
identity function for base types.  But I don't know what would be better
to reduce the change of collisions.  If anybody has a better idea, I'd
be interested.

This patch is a tiny bit from ROCm GDB [1] we would like to merge
upstream.  Laurent Morichetti gave be these performance numbers:

The benchmark used is:

  time ./gdb --data-directory=data-directory /extra/lmoriche/hip/samples/0_Intro/bit_extract/bit_extract -ex "set pagination off" -ex "set breakpoint pending on" -ex "b bit_extract_kernel if \$_thread == 5" -ex run -ex c -batch

It measures the time it takes to continue from a conditional breakpoint with
2048 threads at that breakpoint, one of them reporting the breakpoint.

baseline:
real    0m10.227s
real    0m10.177s
real    0m10.362s

with patch:
real    0m8.356s
real    0m8.424s
real    0m8.494s

[1] https://github.com/ROCm-Developer-Tools/ROCgdb

gdb/ChangeLog:

	* regcache.c (struct target_ptid): New struct.
	(hash_target_ptid): New struct.
	(target_ptid_regcache_map): New type.
	(regcaches): Change type to target_ptid_regcache_map.
	(get_thread_arch_aspace_regcache): Update to regcaches' new
	type.
	(regcache_thread_ptid_changed): Likewise.
	(registers_changed_ptid): Likewise.
	(regcaches_size): Likewise.
	(regcaches_test): Update.
	(regcache_thread_ptid_changed): Update.
	* gdbsupport/ptid.h (hash_ptid): New struct.

Change-Id: Iabb0a1111707936ca111ddb13f3b09efa83d3402

2 files changed, 119 insertions, 60 deletions

diff --git a/gdb/regcache.c b/gdb/regcache.c
index 46fbb07..8ca59fd 100644
--- a/gdb/regcache.c
+++ b/gdb/regcache.c
@@ -29,7 +29,7 @@
 #include "reggroups.h"
 #include "observable.h"
 #include "regset.h"
-#include <forward_list>
+#include <unordered_map>
 
 /*
  * DATA STRUCTURE
@@ -313,32 +313,73 @@ reg_buffer::assert_regnum (int regnum) const
     gdb_assert (regnum < gdbarch_num_regs (arch ()));
 }
 
-/* Global structure containing the current regcache.  */
+/* Key type for target_ptid_regcache_map.  */
+
+struct target_ptid
+{
+  target_ptid (process_stratum_target *target, ptid_t ptid)
+    : target (target), ptid (ptid)
+  {}
+
+  process_stratum_target *target;
+  ptid_t ptid;
+
+  bool operator== (const target_ptid &other) const
+  {
+    return (this->target == other.target
+	    && this->ptid == other.ptid);
+  }
+};
+
+/* Hash function for target_ptid.  */
+
+struct hash_target_ptid
+{
+  size_t operator() (const target_ptid &val) const
+  {
+    std::hash<process_stratum_target *> h_ptr;
+    hash_ptid h_ptid;
+
+    return h_ptr (val.target) + h_ptid (val.ptid);
+  }
+};
+
+/* Type to map a (target, ptid) tuple to a regcache.  */
+
+using target_ptid_regcache_map
+  = std::unordered_multimap<target_ptid, regcache_up, hash_target_ptid>;
+
+/* Global structure containing the existing regcaches.  */
 
 /* NOTE: this is a write-through cache.  There is no "dirty" bit for
    recording if the register values have been changed (eg. by the
    user).  Therefore all registers must be written back to the
    target when appropriate.  */
-static std::forward_list<regcache *> regcaches;
+static target_ptid_regcache_map regcaches;
 
 struct regcache *
 get_thread_arch_aspace_regcache (process_stratum_target *target,
-				 ptid_t ptid, struct gdbarch *gdbarch,
+				 ptid_t ptid, gdbarch *arch,
 				 struct address_space *aspace)
 {
   gdb_assert (target != nullptr);
 
-  for (const auto &regcache : regcaches)
-    if (regcache->target () == target
-	&& regcache->ptid () == ptid
-	&& regcache->arch () == gdbarch)
-      return regcache;
-
-  regcache *new_regcache = new regcache (target, gdbarch, aspace);
+  /* Look up the regcache for this (target, ptid, arch).  */
+  target_ptid key (target, ptid);
+  auto range = regcaches.equal_range (key);
+  for (auto it = range.first; it != range.second; it++)
+    {
+      if (it->second->arch () == arch)
+	return it->second.get ();
+    }
 
-  regcaches.push_front (new_regcache);
+  /* It does not exist, create it.  */
+  regcache *new_regcache = new regcache (target, arch, aspace);
   new_regcache->set_ptid (ptid);
 
+  /* Insert it in the map.  */
+  regcaches.insert (std::make_pair (key, new_regcache));
+
   return new_regcache;
 }
 
@@ -417,10 +458,19 @@ static void
 regcache_thread_ptid_changed (process_stratum_target *target,
 			      ptid_t old_ptid, ptid_t new_ptid)
 {
-  for (auto &regcache : regcaches)
+  target_ptid old_key (target, old_ptid);
+  target_ptid new_key (target, new_ptid);
+
+  auto range = regcaches.equal_range (old_key);
+  for (auto it = range.first; it != range.second;)
     {
-      if (regcache->ptid () == old_ptid && regcache->target () == target)
-	regcache->set_ptid (new_ptid);
+      regcache_up rc = std::move (it->second);
+      rc->set_ptid (new_ptid);
+
+      /* Remove old before inserting new, to avoid rehashing, which would
+	 invalidate iterators.  */
+      it = regcaches.erase (it);
+      regcaches.insert (std::make_pair (new_key, std::move (rc)));
     }
 }
 
@@ -438,19 +488,36 @@ regcache_thread_ptid_changed (process_stratum_target *target,
 void
 registers_changed_ptid (process_stratum_target *target, ptid_t ptid)
 {
-  for (auto oit = regcaches.before_begin (), it = std::next (oit);
-       it != regcaches.end (); )
+  if (target == nullptr)
     {
-      struct regcache *regcache = *it;
-      if ((target == nullptr || regcache->target () == target)
-	  && regcache->ptid ().matches (ptid))
-	{
-	  delete regcache;
-	  it = regcaches.erase_after (oit);
-	}
-      else
-	oit = it++;
+      /* Since there can be ptid clashes between targets, it's not valid to
+	 pass a ptid without saying to which target it belongs.  */
+      gdb_assert (ptid == minus_one_ptid);
+
+      /* Delete all the regcaches.  */
+      regcaches.clear ();
     }
+  else if (ptid != minus_one_ptid)
+    {
+      /* Non-NULL target and non-minus_one_ptid, delete all regcaches belonging
+	 to this (TARGET, PTID).  */
+      regcaches.erase (target_ptid (target, ptid));
+    }
+  else
+    {
+       /* Non-NULL target and minus_one_ptid, delete all regcaches associated
+	  to this target.
+
+	  We unfortunately don't have an efficient way to do this.  Fall back
+	  to iterating all items to find all those belonging to TARGET.  */
+       for (auto it = regcaches.begin (); it != regcaches.end ();)
+	 {
+	   if (it->second->target () == target)
+	     it = regcaches.erase (it);
+	   else
+	     it++;
+	 }
+     }
 
   if ((target == nullptr || current_thread_target == target)
       && current_thread_ptid.matches (ptid))
@@ -1431,13 +1498,6 @@ register_dump::dump (ui_file *file)
 
 namespace selftests {
 
-static size_t
-regcaches_size ()
-{
-  return std::distance (regcaches.begin (),
-			  regcaches.end ());
-}
-
 /* Wrapper around get_thread_arch_aspace_regcache that does some self checks.  */
 
 static void
@@ -1457,7 +1517,7 @@ static void
 regcaches_test (void)
 {
   /* It is empty at the start.  */
-  SELF_CHECK (regcaches_size () == 0);
+  SELF_CHECK (regcaches.size () == 0);
 
   ptid_t ptid1 (1), ptid2 (2), ptid3 (3);
 
@@ -1469,52 +1529,52 @@ regcaches_test (void)
   test_get_thread_arch_aspace_regcache (&test_target1, ptid1,
 					target_gdbarch (),
 					NULL);
-  SELF_CHECK (regcaches_size () == 1);
+  SELF_CHECK (regcaches.size () == 1);
 
   /* Get regcache from (target1,ptid2), a new regcache is added to
      REGCACHES.  */
   test_get_thread_arch_aspace_regcache (&test_target1, ptid2,
 					target_gdbarch (),
 					NULL);
-  SELF_CHECK (regcaches_size () == 2);
+  SELF_CHECK (regcaches.size () == 2);
 
   /* Get regcache from (target1,ptid3), a new regcache is added to
      REGCACHES.  */
   test_get_thread_arch_aspace_regcache (&test_target1, ptid3,
 					target_gdbarch (),
 					NULL);
-  SELF_CHECK (regcaches_size () == 3);
+  SELF_CHECK (regcaches.size () == 3);
 
   /* Get regcache from (target1,ptid2) again, nothing is added to
      REGCACHES.  */
   test_get_thread_arch_aspace_regcache (&test_target1, ptid2,
 					target_gdbarch (),
 					NULL);
-  SELF_CHECK (regcaches_size () == 3);
+  SELF_CHECK (regcaches.size () == 3);
 
   /* Get regcache from (target2,ptid2), a new regcache is added to
      REGCACHES, since this time we're using a different target.  */
   test_get_thread_arch_aspace_regcache (&test_target2, ptid2,
 					target_gdbarch (),
 					NULL);
-  SELF_CHECK (regcaches_size () == 4);
+  SELF_CHECK (regcaches.size () == 4);
 
   /* Mark that (target1,ptid2) changed.  The regcache of (target1,
      ptid2) should be removed from REGCACHES.  */
   registers_changed_ptid (&test_target1, ptid2);
-  SELF_CHECK (regcaches_size () == 3);
+  SELF_CHECK (regcaches.size () == 3);
 
   /* Get the regcache from (target2,ptid2) again, confirming the
      registers_changed_ptid call above did not delete it.  */
   test_get_thread_arch_aspace_regcache (&test_target2, ptid2,
 					target_gdbarch (),
 					NULL);
-  SELF_CHECK (regcaches_size () == 3);
+  SELF_CHECK (regcaches.size () == 3);
 
   /* Confirm that marking all regcaches of all targets as changed
      clears REGCACHES.  */
   registers_changed_ptid (nullptr, minus_one_ptid);
-  SELF_CHECK (regcaches_size () == 0);
+  SELF_CHECK (regcaches.size () == 0);
 }
 
 class target_ops_no_register : public test_target_ops
@@ -1849,31 +1909,28 @@ regcache_thread_ptid_changed ()
   get_thread_arch_aspace_regcache (&target2.mock_target, old_ptid, arch,
 				   nullptr);
 
-  /* Return whether a regcache for (TARGET, PTID) exists in REGCACHES.  */
-  auto regcache_exists = [] (process_stratum_target *target, ptid_t ptid)
+  /* Return the count of regcaches for (TARGET, PTID) in REGCACHES.  */
+  auto regcache_count = [] (process_stratum_target *target, ptid_t ptid)
     {
-      for (regcache *rc : regcaches)
-	{
-	  if (rc->target () == target && rc->ptid () == ptid)
-	    return true;
-	}
+      target_ptid key (target, ptid);
 
-      return false;
+      auto range = regcaches.equal_range (key);
+      return std::distance (range.first, range.second);
     };
 
-  gdb_assert (regcaches_size () == 2);
-  gdb_assert (regcache_exists (&target1.mock_target, old_ptid));
-  gdb_assert (!regcache_exists (&target1.mock_target, new_ptid));
-  gdb_assert (regcache_exists (&target2.mock_target, old_ptid));
-  gdb_assert (!regcache_exists (&target2.mock_target, new_ptid));
+  gdb_assert (regcaches.size () == 2);
+  gdb_assert (regcache_count (&target1.mock_target, old_ptid) == 1);
+  gdb_assert (regcache_count (&target1.mock_target, new_ptid) == 0);
+  gdb_assert (regcache_count (&target2.mock_target, old_ptid) == 1);
+  gdb_assert (regcache_count (&target2.mock_target, new_ptid) == 0);
 
   thread_change_ptid (&target1.mock_target, old_ptid, new_ptid);
 
-  gdb_assert (regcaches_size () == 2);
-  gdb_assert (!regcache_exists (&target1.mock_target, old_ptid));
-  gdb_assert (regcache_exists (&target1.mock_target, new_ptid));
-  gdb_assert (regcache_exists (&target2.mock_target, old_ptid));
-  gdb_assert (!regcache_exists (&target2.mock_target, new_ptid));
+  gdb_assert (regcaches.size () == 2);
+  gdb_assert (regcache_count (&target1.mock_target, old_ptid) == 0);
+  gdb_assert (regcache_count (&target1.mock_target, new_ptid) == 1);
+  gdb_assert (regcache_count (&target2.mock_target, old_ptid) == 1);
+  gdb_assert (regcache_count (&target2.mock_target, new_ptid) == 0);
 }
 
 } // namespace selftests
diff --git a/gdb/regcache.h b/gdb/regcache.h
index dd0c2f2..9390f57 100644
--- a/gdb/regcache.h
+++ b/gdb/regcache.h
@@ -435,6 +435,8 @@ private:
 				   struct address_space *aspace);
 };
 
+using regcache_up = std::unique_ptr<regcache>;
+
 class readonly_detached_regcache : public readable_regcache
 {
 public: