ParallelCompile: Adjust the max compiler threads for WebGL

If the extension hasn't been requested, the limit is 2, otherwise
at least 4 and no more than half of hardware_concurrency().

Bug: 953687
Change-Id: I3051a4ee18b10c6c21487f7e06a6170aed1b5e87
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1575970
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653669}

third_party/blink/renderer/modules/webgl/khr_parallel_shader_compile.cc

@@ -25,6 +25,8 @@
 
 #include "third_party/blink/renderer/modules/webgl/khr_parallel_shader_compile.h"
 
+#include <thread>
+
 #include "gpu/command_buffer/client/gles2_interface.h"
 
 namespace blink {
@@ -34,8 +36,9 @@ KHRParallelShaderCompile::KHRParallelShaderCompile(
     : WebGLExtension(context) {
   context->ExtensionsUtil()->EnsureExtensionEnabled(
       "GL_KHR_parallel_shader_compile");
-  // Use 2 background threads per WebGL context by default.
-  context->ContextGL()->MaxShaderCompilerThreadsKHR(2);
+
+  GLuint max_threads = std::max(4u, std::thread::hardware_concurrency() / 2);
+  context->ContextGL()->MaxShaderCompilerThreadsKHR(max_threads);
 }
 
 WebGLExtensionName KHRParallelShaderCompile::GetName() const {

third_party/blink/renderer/modules/webgl/webgl_rendering_context_base.cc

@@ -1226,6 +1226,11 @@ void WebGLRenderingContextBase::InitializeNewContext() {
   for (int i = 0; i < kWebGLExtensionNameCount; ++i)
     extension_enabled_[i] = false;
 
+  // This limits the count of threads if the extension is yet to be requested.
+  if (String(ContextGL()->GetString(GL_EXTENSIONS))
+          .Contains("GL_KHR_parallel_shader_compile")) {
+    ContextGL()->MaxShaderCompilerThreadsKHR(2);
+  }
   is_web_gl2_formats_types_added_ = false;
   is_web_gl2_tex_image_source_formats_types_added_ = false;
   is_web_gl2_internal_formats_copy_tex_image_added_ = false;