Skip zero-size buffer uploads and readbacks on SDL-GPU (#2249)

sdl-gpu/src/main/java/com/gtnewhorizons/angelica/sdlgpu/SDLGPURenderBackend.java

@@ -3425,6 +3425,7 @@ public class SDLGPURenderBackend extends RenderBackend {
     }
 
     private void downloadBufferBlocking(long handle, int offset, int size, ByteBuffer out) {
+        if (size <= 0) return;
         flushAndSubmitMidFrame();
         final long cb = SDL_AcquireGPUCommandBuffer(device.getDevice());
         if (cb == 0) return;

sdl-gpu/src/main/java/com/gtnewhorizons/angelica/sdlgpu/resource/ResourceManager.java

@@ -1585,6 +1585,7 @@ public final class ResourceManager {
     public boolean arenaUpload(ByteBuffer data, long dstHandle, long dstOffset, boolean cycle) {
         final FrameManager.FrameState f = frameManager.frame();
         final int size = data.remaining();
+        if (size == 0) return true;
         if (!f.arena.fits(size)) {
             return false;
         }
@@ -1682,6 +1683,7 @@ public final class ResourceManager {
         if (srcOff < 0 || size < 0 || srcOff + size > src.capacity()) {
             throw new IllegalStateException("uploadRangeToBuffer: bad source range srcOff=" + srcOff + " size=" + size + " src.capacity=" + src.capacity());
         }
+        if (size == 0) return;
         final long xfer = acquireTransferBuffer(size);
         if (xfer == 0) return;
 

sdl-gpu/src/test/java/com/gtnewhorizons/angelica/sdlgpu/glsm/GlsmSdlEmptyBufferTransferTest.java

@@ -0,0 +1,66 @@
+package com.gtnewhorizons.angelica.sdlgpu.glsm;
+
+import com.gtnewhorizons.angelica.glsm.GLStateManager;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Tag;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.lwjgl.opengl.GL15;
+import org.lwjgl.opengl.GL40;
+import org.lwjgl.opengl.GL43;
+import org.lwjgl.system.MemoryUtil;
+
+import java.nio.ByteBuffer;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+@Tag("glsm-sdl")
+@ExtendWith(GlsmSdlFrameExtension.class)
+class GlsmSdlEmptyBufferTransferTest {
+
+    private int id;
+    private ByteBuffer empty;
+
+    @BeforeEach
+    void setUp() {
+        id = GLStateManager.glGenBuffers();
+        empty = MemoryUtil.memAlloc(16);
+        empty.limit(0);
+    }
+
+    @AfterEach
+    void tearDown() {
+        MemoryUtil.memFree(empty);
+        GLStateManager.glDeleteBuffers(id);
+    }
+
+    @Test
+    void emptyBufferDataReplacesTheStoreWithoutACopy() {
+        final int target = GL40.GL_DRAW_INDIRECT_BUFFER;
+        GLStateManager.glBindBuffer(target, id);
+        GLStateManager.glBufferData(target, 256L, GL15.GL_STATIC_DRAW);
+        GlsmSdlHeadlessRig.resourceManager().flushUploadArena();
+        GLStateManager.glBufferData(target, empty, GL15.GL_STATIC_DRAW);
+        assertEquals(0L, GlsmSdlHeadlessRig.frameManager().frame().arenaCopyPass);
+        assertEquals(0, GLStateManager.glGetBufferParameteri(target, GL15.GL_BUFFER_SIZE));
+    }
+
+    @Test
+    void emptyStorageBufferSubDataIssuesNoCopy() {
+        final int target = GL43.GL_SHADER_STORAGE_BUFFER;
+        GLStateManager.glBindBuffer(target, id);
+        GLStateManager.glBufferData(target, 256L, GL15.GL_DYNAMIC_DRAW);
+        final int before = GlsmSdlHeadlessRig.frameManager().frame().pendingUploadCommands;
+        GLStateManager.glBufferSubData(target, 0, empty);
+        assertEquals(before, GlsmSdlHeadlessRig.frameManager().frame().pendingUploadCommands);
+    }
+
+    @Test
+    void emptyGetBufferSubDataDoesNotReachTheDevice() {
+        final int target = GL43.GL_SHADER_STORAGE_BUFFER;
+        GLStateManager.glBindBuffer(target, id);
+        GLStateManager.glBufferData(target, 256L, GL15.GL_DYNAMIC_DRAW);
+        GLStateManager.glGetBufferSubData(target, 0, empty);
+    }
+}