[wasm][gc] Add counters for amount of freed code

This adds two counters for collecting the absolute size of freed code,
and the percent of total generated code per module.

R=titzer@chromium.org

Bug: v8:8217
Change-Id: Ia065081104fbff6459791c919e0b18677ba45cc3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1573698
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60948}

src/counters-definitions.h

@@ -74,14 +74,19 @@ namespace internal {
   HR(wasm_memory_allocation_result, V8.WasmMemoryAllocationResult, 0, 3, 4)    \
   HR(wasm_address_space_usage_mb, V8.WasmAddressSpaceUsageMiB, 0, 1 << 20,     \
      128)                                                                      \
-  /* code size of live modules, collected on GC */                             \
+  /* committed code size per module, collected on GC */                        \
   HR(wasm_module_code_size_mb, V8.WasmModuleCodeSizeMiB, 0, 1024, 64)          \
-  /* code size of modules after baseline compilation */                        \
+  /* code size per module after baseline compilation */                        \
   HR(wasm_module_code_size_mb_after_baseline,                                  \
      V8.WasmModuleCodeSizeBaselineMiB, 0, 1024, 64)                            \
-  /* code size of modules after top-tier compilation */                        \
+  /* code size per module after top-tier compilation */                        \
   HR(wasm_module_code_size_mb_after_top_tier, V8.WasmModuleCodeSizeTopTierMiB, \
-     0, 1024, 64)
+     0, 1024, 64)                                                              \
+  /* freed code size per module, collected on GC */                            \
+  HR(wasm_module_freed_code_size_mb, V8.WasmModuleCodeSizeFreed, 0, 1024, 64)  \
+  /* percent of freed code size per module, collected on GC */                 \
+  HR(wasm_module_freed_code_size_percent, V8.WasmModuleCodeSizePercentFreed,   \
+     0, 100, 32)
 
 #define HISTOGRAM_TIMER_LIST(HT)                                               \
   /* Timer histograms, not thread safe: HT(name, caption, max, unit) */        \

src/wasm/wasm-code-manager.cc

@@ -1329,9 +1329,17 @@ void NativeModule::SampleCodeSize(
     case kAfterTopTier:
       histogram = counters->wasm_module_code_size_mb_after_top_tier();
       break;
-    case kSampling:
+    case kSampling: {
       histogram = counters->wasm_module_code_size_mb();
+      // Also, add a sample of freed code size, absolute and relative.
+      size_t freed_size = freed_code_size_.load(std::memory_order_relaxed);
+      int total_freed_mb = static_cast<int>(freed_size / MB);
+      counters->wasm_module_freed_code_size_mb()->AddSample(total_freed_mb);
+      int freed_percent =
+          static_cast<int>(100 * freed_code_size_ / generated_code_size_);
+      counters->wasm_module_freed_code_size_percent()->AddSample(freed_percent);
       break;
+    }
   }
   histogram->AddSample(code_size_mb);
 }
@@ -1386,11 +1394,14 @@ void NativeModule::FreeCode(Vector<WasmCode* const> codes) {
   // For now, we neither free the {WasmCode} objects, nor do we free any code.
   // We just zap the code to ensure it's not executed any more.
   // TODO(clemensh): Actually free the {WasmCode} objects and the code pages.
+  size_t code_size = 0;
   for (WasmCode* code : codes) {
     ZapCode(code->instruction_start(), code->instructions().size());
     FlushInstructionCache(code->instruction_start(),
                           code->instructions().size());
+    code_size += code->instructions().size();
   }
+  freed_code_size_.fetch_add(code_size);
 }
 
 void WasmCodeManager::FreeNativeModule(NativeModule* native_module) {

src/wasm/wasm-code-manager.h

@@ -523,6 +523,7 @@ class V8_EXPORT_PRIVATE NativeModule final {
   WasmEngine* const engine_;
   std::atomic<size_t> committed_code_space_{0};
   std::atomic<size_t> generated_code_size_{0};
+  std::atomic<size_t> freed_code_size_{0};
   int modification_scope_depth_ = 0;
   bool can_request_more_memory_;
   UseTrapHandler use_trap_handler_ = kNoTrapHandler;