From 3ec446b0a2dd3c7d5162304081e32fe0638b2280 Mon Sep 17 00:00:00 2001
From: wheremyfoodat <44909372+wheremyfoodat@users.noreply.github.com>
Date: Thu, 29 Jun 2023 00:56:01 +0300
Subject: [PATCH] [GPU] Implement vertex cache

---
 src/core/PICA/gpu.cpp | 31 ++++++++++++++++++++++++++++++-
 1 file changed, 30 insertions(+), 1 deletion(-)

diff --git a/src/core/PICA/gpu.cpp b/src/core/PICA/gpu.cpp
index 4b896807..3b00b438 100644
--- a/src/core/PICA/gpu.cpp
+++ b/src/core/PICA/gpu.cpp
@@ -1,6 +1,7 @@
 #include "PICA/gpu.hpp"
 
 #include <array>
+#include <bitset>
 #include <cstdio>
 #include <cstddef>
 
@@ -121,9 +122,19 @@ void GPU::drawArrays() {
 	// Total number of input attributes to shader. Differs between GS and VS. Currently stubbed to the VS one, as we don't have geometry shaders.
 	const u32 inputAttrCount = (regs[PICA::InternalRegs::VertexShaderInputBufferCfg] & 0xf) + 1;
 	const u64 inputAttrCfg = getVertexShaderInputConfig();
+
+	// When doing indexed rendering, we have a cache of vertices to avoid processing attributes and shaders for a single vertex many times
+	constexpr bool vertexCacheEnabled = true;
+	constexpr size_t vertexCacheSize = 64;
+
+	struct {
+		std::bitset<vertexCacheSize> validBits{0};           // Shows which tags are valid. If the corresponding bit is 1, then there's an entry
+		std::array<u32, vertexCacheSize> ids;                // IDs (ie indices of the cached vertices in the 3DS vertex buffer)
+		std::array<u32, vertexCacheSize> bufferPositions;    // Positions of the cached vertices in our own vertex buffer
+	} vertexCache;
 		
 	for (u32 i = 0; i < vertexCount; i++) {
-		u32 vertexIndex; // Index of the vertex in the VBO
+		u32 vertexIndex; // Index of the vertex in the VBO for indexed rendering
 
 		if constexpr (!indexed) {
 			vertexIndex = i + regs[PICA::InternalRegs::VertexOffsetReg];
@@ -139,6 +150,24 @@ void GPU::drawArrays() {
 			}
 		}
 
+		// Check if the vertex corresponding to the index is in cache
+		if constexpr (indexed && vertexCacheEnabled) {
+			auto& cache = vertexCache;
+			size_t tag = vertexIndex % vertexCacheSize;
+			// Cache hit
+			if (cache.validBits[tag] && cache.ids[tag] == vertexIndex) {
+				vertices[i] = vertices[cache.bufferPositions[tag]];
+				continue;
+			}
+
+			// Cache miss. Set cache entry, fetch attributes and run shaders as normal
+			else {
+				cache.validBits[tag] = true;
+				cache.ids[tag] = vertexIndex;
+				cache.bufferPositions[tag] = i;
+			}
+		}
+
 		int attrCount = 0;
 		int buffer = 0; // Vertex buffer index for non-fixed attributes