renderer/metal: prefer low‐power GPUs

Some Intel MacBook Pro laptops have both an integrated and discrete
GPU and support automatically switching between them. The system
uses the integrated GPU by default, but the default Metal device on
those systems is the discrete GPU. This means that Metal‐using
applications activate it by default, presumably as the intended
audience is high‐performance graphics applications.

This is unfortunate for productivity applications like terminals,
however, as the discrete GPU decreases battery life and worsens the
thermal throttling problems these machines have always had. Prefer
to use an integrated GPU when present and not using an external GPU.

The behaviour should be unchanged on Apple Silicon, as the platform
only supports one GPU. I have confirmed that the resulting app runs,
works, and doesn’t activate the AMD GPU on my MacBook Pro, but have
not done any measurements of the resulting performance impact. If
it is considered sufficiently noticeable, a GPU preference setting
could be added.

See <https://github.com/zed-industries/zed/issues/5124>,
<https://github.com/zed-industries/zed/pull/13685>,
<https://github.com/zed-industries/zed/pull/14738>, and
<https://github.com/zed-industries/zed/pull/14744> for discussion,
measurements, and changes relating to this issue in the Zed
project. The logic implemented here reflects what Zed ended up
settling on.

The [Metal documentation] recommends using
`MTLCopyAllDevicesWithObserver` to receive notifications of when
the list of available GPUs changes, such as when [external GPUs
are connected or disconnected]. I didn’t bother implementing that
because it seemed like a lot of fussy work to deal with migrating
everything to a new GPU on the fly just for a niche use case on a
legacy platform. Zed doesn’t implement it and I haven’t heard
about anyone complaining that their computer caught fire when they
unplugged an external GPU, so hopefully it’s fine.

[Metal documentation]: https://developer.apple.com/documentation/metal/gpu_devices_and_work_submission/multi-gpu_systems/finding_multiple_gpus_on_an_intel-based_mac
[external GPUs are connected or disconnected]: https://developer.apple.com/documentation/metal/gpu_devices_and_work_submission/multi-gpu_systems/handling_external_gpu_additions_and_removals

Closes: #2572

src/renderer/Metal.zig

@@ -175,7 +175,7 @@ pub const GPUState = struct {
     instance: InstanceBuffer, // MTLBuffer
 
     pub fn init() !GPUState {
-        const device = objc.Object.fromId(mtl.MTLCreateSystemDefaultDevice());
+        const device = try chooseDevice();
         const queue = device.msgSend(objc.Object, objc.sel("newCommandQueue"), .{});
         errdefer queue.release();
 
@@ -200,6 +200,25 @@ pub const GPUState = struct {
         return result;
     }
 
+    fn chooseDevice() error{NoMetalDevice}!objc.Object {
+        const devices = objc.Object.fromId(mtl.MTLCopyAllDevices());
+        defer devices.release();
+        var chosen_device: ?objc.Object = null;
+        var iter = devices.iterate();
+        while (iter.next()) |device| {
+            // We want a GPU that’s connected to a display.
+            if (device.getProperty(bool, "isHeadless")) continue;
+            chosen_device = device;
+            // If the user has an eGPU plugged in, they probably want
+            // to use it. Otherwise, integrated GPUs are better for
+            // battery life and thermals.
+            if (device.getProperty(bool, "isRemovable") or
+                device.getProperty(bool, "isLowPower")) break;
+        }
+        const device = chosen_device orelse return error.NoMetalDevice;
+        return device.retain();
+    }
+
     pub fn deinit(self: *GPUState) void {
         // Wait for all of our inflight draws to complete so that
         // we can cleanly deinit our GPU state.

src/renderer/metal/api.zig

@@ -175,4 +175,4 @@ pub const MTLSize = extern struct {
     depth: c_ulong,
 };
 
-pub extern "c" fn MTLCreateSystemDefaultDevice() ?*anyopaque;
+pub extern "c" fn MTLCopyAllDevices() ?*anyopaque;