Stop draws from merging or reading past their own vertices

Incomplete primitives no longer emit indices: quads drop a one- or
two-vertex tail and draw a three-vertex tail as a triangle, triangle
lists ignore leftover vertices, and fans or strips under three vertices
emit nothing. Draws with no complete primitive are skipped. Previously an
incomplete quad indexed vertices that belonged to the next merged draw,
or past the end of the buffer.

Merging now stops before the 16-bit index offset would wrap, never folds
triangles into a single-instance line or point draw, and breaks after
GXInvalidateVtxCache or a vertex-format switch, so the next draw uploads
fresh arrays and uses its own format's shader.

From upstream patchzyy/Wiicompiled 6f14bde (#244, KartPad batch).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Wg7mB8ogCWmp9GH19Uc82B

aurora-main/tests/renderer_regression_test.cpp

@@ -0,0 +1,86 @@
+#include "gx_test_common.hpp"
+#include "gx/pipeline.hpp"
+
+using aurora::gx::g_gxState;
+
+namespace {
+std::vector<u8> draw(GXPrimitive primitive, u16 count, GXVtxFmt format = GX_VTXFMT0) {
+  std::vector<u8> bytes{static_cast<u8>(primitive | format), static_cast<u8>(count >> 8), static_cast<u8>(count)};
+  bytes.resize(3 + count);
+  return bytes;
+}
+} // namespace
+
+TEST_F(GXFifoTest, MaximumQuadCountTerminatesWithoutOutOfRangeIndices) {
+  g_gxState.lastVtxFmt = GX_VTXFMT0;
+  g_gxState.lastVtxSize = 1;
+  for (const u16 count : {65532, 65533, 65534, 65535}) {
+    g_gxState.stateDirty = true;
+    decode_fifo(draw(GX_QUADS, count));
+    const auto& indices = aurora::gfx::testing::last_pushed_indices();
+    ASSERT_EQ(indices.size(), (count / 4) * 6 + (count % 4 == 3 ? 3 : 0));
+    for (const auto index : indices)
+      ASSERT_LT(index, count);
+  }
+}
+
+TEST_F(GXFifoTest, IncompletePrimitivesNeverJoinAcrossDraws) {
+  g_gxState.lastVtxFmt = GX_VTXFMT0;
+  g_gxState.lastVtxSize = 1;
+  aurora::gfx::testing::use_draw_command_tracking(true);
+  decode_fifo(draw(GX_TRIANGLES, 4));
+  EXPECT_EQ(aurora::gfx::testing::last_pushed_indices(), (std::vector<u16>{0, 1, 2}));
+  decode_fifo(draw(GX_TRIANGLES, 5));
+  EXPECT_EQ(aurora::gfx::testing::last_pushed_indices(), (std::vector<u16>{4, 5, 6}));
+  const auto before = aurora::gfx::testing::last_pushed_indices();
+  decode_fifo(draw(GX_TRIANGLEFAN, 2));
+  EXPECT_EQ(aurora::gfx::testing::last_pushed_indices(), before);
+}
+
+TEST_F(GXFifoTest, MergeStopsBeforeSixteenBitIndexOverflow) {
+  g_gxState.lastVtxFmt = GX_VTXFMT0;
+  g_gxState.lastVtxSize = 1;
+  aurora::gfx::testing::use_draw_command_tracking(true);
+  decode_fifo(draw(GX_TRIANGLES, 65535));
+  decode_fifo(draw(GX_TRIANGLES, 3));
+  EXPECT_EQ(aurora::gfx::g_mergedDrawCallCount, 0u);
+  EXPECT_EQ(aurora::gfx::testing::last_pushed_indices(), (std::vector<u16>{0, 1, 2}));
+}
+
+TEST_F(GXFifoTest, VertexCacheInvalidationBreaksDrawMerging) {
+  g_gxState.lastVtxFmt = GX_VTXFMT0;
+  g_gxState.lastVtxSize = 1;
+  aurora::gfx::testing::use_draw_command_tracking(true);
+  decode_fifo(draw(GX_TRIANGLES, 3));
+  decode_fifo({GX_CMD_INVL_VC});
+  EXPECT_TRUE(g_gxState.stateDirty);
+  decode_fifo(draw(GX_TRIANGLES, 3));
+  EXPECT_EQ(aurora::gfx::g_mergedDrawCallCount, 0u);
+}
+
+TEST_F(GXFifoTest, EqualStrideVertexFormatChangeBreaksDrawMerging) {
+  aurora::gfx::testing::use_real_vertex_format_helpers(true);
+  g_gxState.vtxDesc[GX_VA_POS] = GX_DIRECT;
+  for (const auto format : {GX_VTXFMT0, GX_VTXFMT1}) {
+    g_gxState.vtxFmts[format].attrs[GX_VA_POS].cnt = GX_POS_XY;
+    g_gxState.vtxFmts[format].attrs[GX_VA_POS].type = GX_U8;
+  }
+  g_gxState.vtxFmts[GX_VTXFMT1].attrs[GX_VA_POS].frac = 1;
+  aurora::gfx::testing::use_draw_command_tracking(true);
+  for (const auto format : {GX_VTXFMT0, GX_VTXFMT1}) {
+    auto bytes = draw(GX_TRIANGLES, 3, format);
+    bytes.resize(9);
+    decode_fifo(bytes);
+  }
+  EXPECT_EQ(aurora::gfx::g_mergedDrawCallCount, 0u);
+}
+
+TEST_F(GXFifoTest, SingleExpandedPrimitiveCannotMergeWithTriangles) {
+  g_gxState.lastVtxFmt = GX_VTXFMT0;
+  g_gxState.lastVtxSize = 1;
+  aurora::gfx::testing::use_draw_command_tracking(true);
+  decode_fifo(draw(GX_POINTS, 1));
+  decode_fifo(draw(GX_TRIANGLES, 3));
+  EXPECT_EQ(aurora::gfx::g_mergedDrawCallCount, 0u);
+  EXPECT_EQ(aurora::gfx::testing::last_pushed_indices(), (std::vector<u16>{0, 1, 2}));
+}