improve FrameSkipper when disregarding skipping

It is legal for the app to draw a frame even when FrameSkipper 
detects that a frame should be skipped. In that case we used to
overwrite the most recent fence, but this wasn't ideal. We now
proceed as if the fence had signaled, i.e. we destroy it and
move all the fences up, creating a new one at the end of the 
delayed array.

filament/src/FrameSkipper.cpp

@@ -16,16 +16,22 @@
 
 #include "FrameSkipper.h"
 
-#include <utils/Log.h>
+#include <backend/DriverEnums.h>
+
+#include <utils/compiler.h>
 #include <utils/debug.h>
 
+#include <algorithm>
+
+#include <stddef.h>
+
 namespace filament {
 
 using namespace utils;
 using namespace backend;
 
 FrameSkipper::FrameSkipper(size_t latency) noexcept
-        : mLast(latency - 1) {
+        : mLast(std::max(latency, MAX_FRAME_LATENCY) - 1) {
     assert_invariant(latency <= MAX_FRAME_LATENCY);
 }
 
@@ -41,31 +47,32 @@ void FrameSkipper::terminate(DriverApi& driver) noexcept {
 
 bool FrameSkipper::beginFrame(DriverApi& driver) noexcept {
     auto& fences = mDelayedFences;
-    auto fence = fences.front();
-    if (fence) {
-        auto status = driver.getFenceStatus(fence);
-        if (status == FenceStatus::TIMEOUT_EXPIRED) {
-            // Sync not ready, skip frame
+    if (fences.front()) {
+        // Do we have a latency old fence?
+        auto status = driver.getFenceStatus(fences.front());
+        if (UTILS_UNLIKELY(status == FenceStatus::TIMEOUT_EXPIRED)) {
+            // The fence hasn't signaled yet, skip this frame
             return false;
         }
         assert_invariant(status == FenceStatus::CONDITION_SATISFIED);
-        driver.destroyFence(fence);
     }
-    // shift all fences down by 1
-    std::move(fences.begin() + 1, fences.end(), fences.begin());
-    fences.back() = {};
     return true;
 }
 
 void FrameSkipper::endFrame(DriverApi& driver) noexcept {
-    // If the user produced a new frame despite the fact that the previous one wasn't finished
-    // (i.e. FrameSkipper::beginFrame() returned false), we need to make sure to replace
-    // a fence that might be here already)
-    auto& fence = mDelayedFences[mLast];
-    if (fence) {
-        driver.destroyFence(fence);
+    auto& fences = mDelayedFences;
+    size_t const last = mLast;
+
+    // pop the oldest fence and advance the other ones
+    if (fences.front()) {
+        driver.destroyFence(fences.front());
     }
-    fence = driver.createFence();
+    std::move(fences.begin() + 1, fences.end(), fences.begin());
+
+    // add a new fence to the end
+    assert_invariant(!fences[last]);
+
+    fences[last] = driver.createFence();
 }
 
 } // namespace filament

filament/src/FrameSkipper.h

@@ -22,6 +22,9 @@
 
 #include <array>
 
+#include <stddef.h>
+#include <stdint.h>
+
 namespace filament {
 
 /*
@@ -60,7 +63,7 @@ class FrameSkipper {
 private:
     using Container = std::array<backend::Handle<backend::HwFence>, MAX_FRAME_LATENCY>;
     mutable Container mDelayedFences{};
-    size_t mLast;
+    uint8_t const mLast;
 };
 
 } // namespace filament