video_core/buffer_cache: reuse unchanged small uniform buffers within command buffer

(cherry picked from commit 4aa5cd89fd1862751b3297036f2d21a0dc924b23)
(cherry picked from commit f1bdd5b13a1bdca81243b4aa1d8529f0485d3671)

2 files changed, 139 insertions, 2 deletions

diff --git a/src/video_core/buffer_cache/buffer_cache.h b/src/video_core/buffer_cache/buffer_cache.h
index cd176dec27..481b2f0f32 100644
--- a/src/video_core/buffer_cache/buffer_cache.h
+++ b/src/video_core/buffer_cache/buffer_cache.h
@@ -15,6 +15,7 @@
 
 #include "common/range_sets.inc"
 #include "video_core/buffer_cache/buffer_cache_base.h"
+#include "video_core/buffer_cache/uniform_stream_reuse.h"
 #include "video_core/guest_memory.h"
 #include "video_core/host1x/gpu_device_memory_manager.h"
 #include "video_core/nx2.h"
@@ -28,6 +29,9 @@ using Core::DEVICE_PAGESIZE;
 template <class P>
 BufferCache<P>::BufferCache(Tegra::MaxwellDeviceMemoryManager& device_memory_, Runtime& runtime_)
     : runtime{runtime_}, device_memory{device_memory_}, memory_tracker{device_memory} {
+    // A new runtime has a new scheduler (its ticks restart) and a new stream buffer: whatever the
+    // stream uniform reuse remembers refers to the old ones. Before the early return below.
+    VideoCommon::ResetUniformStreamReuse<Runtime>();
     // Ensure the first slot is used for the null buffer
     void(slot_buffers.insert(runtime, NullBufferParams{}));
     gpu_modified_ranges.Clear();
@@ -1078,8 +1082,20 @@ void BufferCache<P>::BindHostGraphicsUniformBuffer(size_t stage, u32 index, u32
         channel_state->fast_bound_uniform_buffers[stage] |= 1u << binding_index;
         channel_state->uniform_buffer_binding_sizes[stage][binding_index] = size;
         // Stream buffer path to avoid stalling on non-Nvidia drivers or Vulkan
-        const std::span<u8> span = runtime.BindMappedUniformBuffer(stage, binding_index, size);
-        device_memory.ReadBlockUnsafe(device_addr, span.data(), size);
+        bool streamed = false;
+        if constexpr (!IS_OPENGL && VideoCommon::StreamUniformReusable<Runtime>) {
+            // The same bytes as this slot's previous bind, in the same command buffer: bind that
+            // stream allocation again instead of allocating and copying (uniform_stream_reuse.h).
+            // EDEN_NX2_UBOREUSE=0 turns it off.
+            if (VideoCommon::UniformStreamReuseEnabled()) {
+                streamed = VideoCommon::BindSmallUniformReusing(
+                    runtime, device_memory, stage, index, binding_index, device_addr, size);
+            }
+        }
+        if (!streamed) {
+            const std::span<u8> span = runtime.BindMappedUniformBuffer(stage, binding_index, size);
+            device_memory.ReadBlockUnsafe(device_addr, span.data(), size);
+        }
         return;
     }
     // Classic cached path
diff --git a/src/video_core/buffer_cache/uniform_stream_reuse.h b/src/video_core/buffer_cache/uniform_stream_reuse.h
new file mode 100644
index 0000000000..7ba31c4717
--- /dev/null
+++ b/src/video_core/buffer_cache/uniform_stream_reuse.h
@@ -0,0 +1,121 @@
+// SPDX-FileCopyrightText: Copyright 2026 Eden Emulator Project
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+#pragma once
+
+#include <array>
+#include <concepts>
+#include <cstddef>
+#include <cstdlib>
+#include <cstring>
+#include <span>
+#include <type_traits>
+
+#include "common/common_types.h"
+
+namespace VideoCommon {
+
+/// What a buffer cache runtime offers for stream uniform reuse.
+///  - StreamUniformRef: a small trivially copyable description of one stream allocation (the
+///    host buffer, its offset and whatever else the runtime needs to add it to a descriptor
+///    payload again, e.g. the device address of the descriptor buffer path).
+///  - LastStreamUniformRef(): the allocation made by the latest BindMappedUniformBuffer call.
+///  - RebindStreamUniform(ref, size): adds `ref` to the descriptor payload exactly as
+///    BindMappedUniformBuffer added the allocation the first time.
+template <class R>
+concept StreamUniformReusable = requires(R& runtime, u32 size) {
+    typename R::StreamUniformRef;
+    requires std::is_trivially_copyable_v<typename R::StreamUniformRef>;
+    { runtime.LastStreamUniformRef() } -> std::same_as<typename R::StreamUniformRef>;
+    runtime.RebindStreamUniform(runtime.LastStreamUniformRef(), size);
+    { runtime.CurrentTick() } -> std::convertible_to<u64>;
+};
+
+/// EDEN_NX2_UBOREUSE=0 turns the reuse off (every small uniform buffer is copied, as before).
+[[nodiscard]] inline bool UniformStreamReuseEnabled() noexcept {
+    static const bool enabled = [] {
+        const char* const value = std::getenv("EDEN_NX2_UBOREUSE");
+        return value == nullptr || value[0] != '0';
+    }();
+    return enabled;
+}
+
+template <class Ref>
+class UniformStreamReuse {
+public:
+    static constexpr u32 MaxBytes = 4096; // BufferCache's DEFAULT_SKIP_CACHE_SIZE
+    static constexpr size_t NumStages = 5;
+    static constexpr size_t NumSlots = 18; // Maxwell constant buffers per stage
+
+    struct Slot {
+        Ref ref{};
+        u64 tick{};
+        u32 size{}; // 0: nothing streamed yet
+        std::array<u8, MaxBytes> bytes{};
+    };
+
+    void Reset() noexcept {
+        for (auto& stage : slots) {
+            for (Slot& slot : stage) {
+                slot.size = 0;
+                slot.tick = 0;
+            }
+        }
+    }
+
+    std::array<std::array<Slot, NumSlots>, NumStages> slots{};
+    std::array<u8, MaxBytes> scratch{};
+};
+
+template <class Ref>
+[[nodiscard]] inline UniformStreamReuse<Ref>& UniformStreamReuseState() noexcept {
+    static UniformStreamReuse<Ref> state;
+    return state;
+}
+
+/// Forgets every slot. Call it when a buffer cache (and so its runtime, scheduler and stream
+/// buffer) is created: ticks restart with a new scheduler and the old references are dead.
+template <class Runtime>
+inline void ResetUniformStreamReuse() noexcept {
+    if constexpr (StreamUniformReusable<Runtime>) {
+        UniformStreamReuseState<typename Runtime::StreamUniformRef>().Reset();
+    }
+}
+
+/// Binds the small uniform buffer at `device_addr` for (stage, cbuf `index`) through the stream
+/// buffer, reusing the slot's previous allocation when the bytes and the command buffer are the
+/// same. Returns false (nothing bound) when the buffer does not qualify, so the caller binds it
+/// the classic way.
+template <class Runtime, class DeviceMemory>
+[[nodiscard]] bool BindSmallUniformReusing(Runtime& runtime, DeviceMemory& device_memory,
+                                           size_t stage, u32 index, u32 binding_index,
+                                           u64 device_addr, u32 size) {
+    using Reuse = UniformStreamReuse<typename Runtime::StreamUniformRef>;
+    if (size == 0 || size > Reuse::MaxBytes || stage >= Reuse::NumStages ||
+        index >= Reuse::NumSlots) {
+        return false;
+    }
+    Reuse& reuse = UniformStreamReuseState<typename Runtime::StreamUniformRef>();
+    typename Reuse::Slot& slot = reuse.slots[stage][index];
+    u8* const scratch = reuse.scratch.data();
+    device_memory.ReadBlockUnsafe(device_addr, scratch, size);
+
+    const bool same_bytes =
+        slot.size == size && std::memcmp(slot.bytes.data(), scratch, size) == 0;
+    if (same_bytes && slot.tick == runtime.CurrentTick()) {
+        runtime.RebindStreamUniform(slot.ref, size);
+        return true;
+    }
+    const std::span<u8> span = runtime.BindMappedUniformBuffer(stage, binding_index, size);
+    std::memcpy(span.data(), scratch, size);
+    slot.ref = runtime.LastStreamUniformRef();
+    // After the allocation: the tick the stream region is tagged with is the one current now.
+    slot.tick = runtime.CurrentTick();
+    if (!same_bytes) {
+        slot.size = size;
+        std::memcpy(slot.bytes.data(), scratch, size);
+    }
+    return true;
+}
+
+} // namespace VideoCommon