[wasm][gc] Exclude asm.js and small modules from sampling

The histograms currently mostly contain very small modules (having
0 MB generated and 0 MB freed). Many of those are asm.js modules.
Just recording the modules that are actually interesting for wasm
code GC will give us more meaningful data.

R=mstarzinger@chromium.org

Bug: v8:8217
Change-Id: I1d9ba8134c2f3617f896afc42dc9e87c7852c319
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1611679Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61505}

src/wasm/wasm-code-manager.cc

@@ -1330,14 +1330,19 @@ void NativeModule::SampleCodeSize(
       break;
     case kSampling: {
       histogram = counters->wasm_module_code_size_mb();
-      // Also, add a sample of freed code size, absolute and relative.
-      size_t freed_size = code_allocator_.freed_code_size();
+      // If this is a wasm module of >= 2MB, also sample the freed code size,
+      // absolute and relative. Code GC does not happen on asm.js modules, and
+      // small modules will never trigger GC anyway.
       size_t generated_size = code_allocator_.generated_code_size();
+      if (generated_size >= 2 * MB && module()->origin == kWasmOrigin) {
+        size_t freed_size = code_allocator_.freed_code_size();
         DCHECK_LE(freed_size, generated_size);
         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_size / generated_size);
-      counters->wasm_module_freed_code_size_percent()->AddSample(freed_percent);
+        counters->wasm_module_freed_code_size_percent()->AddSample(
+            freed_percent);
+      }
       break;
     }
   }