Implement device-ram client hints header

third_party/WebKit/Source/core/loader/FrameFetchContext.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 588 matching lines @@
     GetFrame()->Console().AddMessage(console_message);
 }
 
 void FrameFetchContext::ModifyRequestForCSP(ResourceRequest& resource_request) {
   // Record the latest requiredCSP value that will be used when sending this
   // request.
   GetFrame()->Loader().RecordLatestRequiredCSP();
   GetFrame()->Loader().ModifyRequestForCSP(resource_request, GetDocument());
 }
 
+float FrameFetchContext::ClientHintsDeviceRam(int physical_memory_mb) {

[panicker, 2017/05/24 17:27:20]

let's add a comment that this math with change and what we plan to do - to avoid
any confusion.

[fmeawad, 2017/05/24 18:24:57]

Done.

+  // The reported device-ram is rounded down to next power of 2 in GB. Ex.
+  // 3072MB will return 2, and 768MB will return 0.5.
+  DCHECK_GT(physical_memory_mb, 0);
+  int power = -1;

[Yoav Weiss, 2017/05/24 07:48:02]

I find the MSB counting here a bit confusing. Maybe start `power` at 0 and count
until `physical_memory_mb` is 1?

[fmeawad, 2017/05/24 18:24:57]

The loop does an extra iteration, to compensate, we init to -1. That saves a
subtraction operation. The logic might change to rounding instead of flooring.
Added a comment for now.

Also considering changing it to use:
https://graphics.stanford.edu/~seander/bithacks.html#RoundUpPowerOf2

+  // Extract the MSB location.
+  while (physical_memory_mb != 0) {
+    physical_memory_mb = physical_memory_mb >> 1;

[Yoav Weiss, 2017/05/24 07:48:02]

Nit: "physical_memory_mb >>= 1;" might be a bit clearer.

[fmeawad, 2017/05/24 18:24:56]

Done.

+    power++;
+  }
+  // Restore to the power of 2, and convert to GB.
+  return ((1 << power) / 1024.0);

[Yoav Weiss, 2017/05/24 07:48:02]

Implicit conversion from int to float... Can you make that conversion explicit?

[fmeawad, 2017/05/24 18:24:56]

Done.

+}
+
 void FrameFetchContext::AddClientHintsIfNecessary(
     const ClientHintsPreferences& hints_preferences,
     const FetchParameters::ResourceWidth& resource_width,
     ResourceRequest& request) {
   if (!RuntimeEnabledFeatures::clientHintsEnabled() || !GetDocument())
     return;
 
+  bool should_send_device_ram =
+      GetDocument()->GetClientHintsPreferences().ShouldSendDeviceRam() ||
+      hints_preferences.ShouldSendDeviceRam();
   bool should_send_dpr =
       GetDocument()->GetClientHintsPreferences().ShouldSendDPR() ||
       hints_preferences.ShouldSendDPR();
   bool should_send_resource_width =
       GetDocument()->GetClientHintsPreferences().ShouldSendResourceWidth() ||
       hints_preferences.ShouldSendResourceWidth();
   bool should_send_viewport_width =
       GetDocument()->GetClientHintsPreferences().ShouldSendViewportWidth() ||
       hints_preferences.ShouldSendViewportWidth();
 
+  if (should_send_device_ram) {
+    int physical_memory = Platform::Current()->AmountOfPhysicalMemoryMB();
+    request.AddHTTPHeaderField(
+        "device-ram",
+        AtomicString(String::Number(ClientHintsDeviceRam(physical_memory))));
+  }
+
   if (should_send_dpr) {
     request.AddHTTPHeaderField(
         "DPR", AtomicString(String::Number(GetDocument()->DevicePixelRatio())));
   }
 
   if (should_send_resource_width) {
     if (resource_width.is_set) {
       float physical_width =
           resource_width.width * GetDocument()->DevicePixelRatio();
       request.AddHTTPHeaderField(
@@ skipping 143 matching lines @@
 std::unique_ptr<WebURLLoader> FrameFetchContext::CreateURLLoader() {
   return GetFrame()->CreateURLLoader();
 }
 
 DEFINE_TRACE(FrameFetchContext) {
   visitor->Trace(document_loader_);
   BaseFetchContext::Trace(visitor);
 }
 
 }  // namespace blink