Implement device-ram client hints header

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

Index: third_party/WebKit/Source/core/loader/FrameFetchContext.cpp
diff --git a/third_party/WebKit/Source/core/loader/FrameFetchContext.cpp b/third_party/WebKit/Source/core/loader/FrameFetchContext.cpp
index 39dedd72a6c594cf47d4bfc1f7607cefb57bfdbd..a7e87ab9b844f57c2ba0a4ecaec812b9de376925 100644
--- a/third_party/WebKit/Source/core/loader/FrameFetchContext.cpp
+++ b/third_party/WebKit/Source/core/loader/FrameFetchContext.cpp
@@ -606,6 +606,20 @@ void FrameFetchContext::ModifyRequestForCSP(ResourceRequest& resource_request) {
   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,
@@ -613,6 +627,9 @@ void FrameFetchContext::AddClientHintsIfNecessary(
   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();
@@ -623,6 +640,13 @@ void FrameFetchContext::AddClientHintsIfNecessary(
       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())));