[wasm] Minor improvement to code GC

Decommitting memory ranges is expensive, so try to merge as many of them
as possible before decommitting.

Minor drive-by: Slightly extend a GC tracing message.

R=ahaas@chromium.org

Change-Id: I91e44db1212f0c9b70b8c8fccebe8fd33ff37d9c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2375825Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69564}

src/wasm/wasm-code-manager.cc

@@ -744,10 +744,14 @@ void WasmCodeAllocator::FreeCode(Vector<WasmCode* const> codes) {
   }
   freed_code_size_.fetch_add(code_size);
 
-  // Merge {freed_regions} into {freed_code_space_} and discard full pages.
-  base::MutexGuard guard(&mutex_);
+  // Merge {freed_regions} into {freed_code_space_} and put all ranges of full
+  // pages to decommit into {regions_to_decommit} (decommitting is expensive,
+  // so try to merge regions before decommitting).
+  DisjointAllocationPool regions_to_decommit;
   PageAllocator* allocator = GetPlatformPageAllocator();
   size_t commit_page_size = allocator->CommitPageSize();
+  {
+    base::MutexGuard guard(&mutex_);
     for (auto region : freed_regions.regions()) {
       auto merged_region = freed_code_space_.Merge(region);
       Address discard_start =
@@ -757,12 +761,16 @@ void WasmCodeAllocator::FreeCode(Vector<WasmCode* const> codes) {
           std::min(RoundDown(merged_region.end(), commit_page_size),
                    RoundUp(region.end(), commit_page_size));
       if (discard_start >= discard_end) continue;
-    size_t discard_size = discard_end - discard_start;
-    size_t old_committed = committed_code_space_.fetch_sub(discard_size);
-    DCHECK_GE(old_committed, discard_size);
+      regions_to_decommit.Merge({discard_start, discard_end - discard_start});
+    }
+  }
+
+  for (auto region : regions_to_decommit.regions()) {
+    size_t old_committed = committed_code_space_.fetch_sub(region.size());
+    DCHECK_GE(old_committed, region.size());
     USE(old_committed);
-    for (base::AddressRegion split_range : SplitRangeByReservationsIfNeeded(
-             {discard_start, discard_size}, owned_code_space_)) {
+    for (base::AddressRegion split_range :
+         SplitRangeByReservationsIfNeeded(region, owned_code_space_)) {
       code_manager_->Decommit(split_range);
     }
   }

src/wasm/wasm-engine.cc

@@ -1372,8 +1372,8 @@ void WasmEngine::TriggerGC(int8_t gc_sequence_index) {
     }
   }
   TRACE_CODE_GC(
-      "Starting GC. Total number of potentially dead code objects: %zu\n",
-      current_gc_info_->dead_code.size());
+      "Starting GC (nr %d). Number of potentially dead code objects: %zu\n",
+      current_gc_info_->gc_sequence_index, current_gc_info_->dead_code.size());
   // Ensure that there are outstanding isolates that will eventually finish this
   // GC. If there are no outstanding isolates, we finish the GC immediately.
   PotentiallyFinishCurrentGC();