radeonsi/gfx10: fix the vertex order for triangle strips emitted by a GS

Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>

diff --git a/src/gallium/drivers/radeonsi/gfx10_shader_ngg.c b/src/gallium/drivers/radeonsi/gfx10_shader_ngg.c
index 80739ff1d02890b97ff96ad1936303a0d8f4a958..23cb1ee34cad816b4be3515bae754d842533d30d 100644 (file)
--- a/src/gallium/drivers/radeonsi/gfx10_shader_ngg.c
+++ b/src/gallium/drivers/radeonsi/gfx10_shader_ngg.c
@@ -958,10 +958,28 @@ void gfx10_ngg_gs_emit_vertex(struct si_shader_context *ctx,
        const LLVMValueRef iscompleteprim =
                LLVMBuildICmp(builder, LLVMIntUGE, curverts, tmp, "");
 
+       /* Since the geometry shader emits triangle strips, we need to
+        * track which primitive is odd and swap vertex indices to get
+        * the correct vertex order.
+        */
+       LLVMValueRef is_odd = ctx->i1false;
+       if (stream == 0 && u_vertices_per_prim(sel->gs_output_prim) == 3) {
+               tmp = LLVMBuildAnd(builder, curverts, ctx->i32_1, "");
+               is_odd = LLVMBuildICmp(builder, LLVMIntEQ, tmp, ctx->i32_1, "");
+       }
+
        tmp = LLVMBuildAdd(builder, curverts, ctx->ac.i32_1, "");
        LLVMBuildStore(builder, tmp, ctx->gs_curprim_verts[stream]);
 
+       /* The per-vertex primitive flag encoding:
+        *   bit 0: whether this vertex finishes a primitive
+        *   bit 1: whether the primitive is odd (if we are emitting triangle strips)
+        */
        tmp = LLVMBuildZExt(builder, iscompleteprim, ctx->ac.i8, "");
+       tmp = LLVMBuildOr(builder, tmp,
+                         LLVMBuildShl(builder,
+                                      LLVMBuildZExt(builder, is_odd, ctx->ac.i8, ""),
+                                      ctx->ac.i8_1, ""), "");
        LLVMBuildStore(builder, tmp, ngg_gs_get_emit_primflag_ptr(ctx, vertexptr, stream));
 
        tmp = LLVMBuildLoad(builder, ctx->gs_generated_prims[stream], "");
@@ -1206,13 +1224,13 @@ void gfx10_ngg_gs_emit_epilogue(struct si_shader_context *ctx)
        tmp = LLVMBuildICmp(builder, LLVMIntULT, tid, num_emit_threads, "");
        ac_build_ifcc(&ctx->ac, tmp, 5140);
        {
+               LLVMValueRef flags;
                struct ngg_prim prim = {};
                prim.num_vertices = verts_per_prim;
 
                tmp = ngg_gs_vertex_ptr(ctx, tid);
-               tmp = LLVMBuildLoad(builder, ngg_gs_get_emit_primflag_ptr(ctx, tmp, 0), "");
-               prim.isnull = LLVMBuildICmp(builder, LLVMIntEQ, tmp,
-                                           LLVMConstInt(ctx->ac.i8, 0, false), "");
+               flags = LLVMBuildLoad(builder, ngg_gs_get_emit_primflag_ptr(ctx, tmp, 0), "");
+               prim.isnull = LLVMBuildNot(builder, LLVMBuildTrunc(builder, flags, ctx->i1, ""), "");
 
                for (unsigned i = 0; i < verts_per_prim; ++i) {
                        prim.index[i] = LLVMBuildSub(builder, vertlive_scan.result_exclusive,
@@ -1220,6 +1238,38 @@ void gfx10_ngg_gs_emit_epilogue(struct si_shader_context *ctx)
                        prim.edgeflag[i] = ctx->ac.i1false;
                }
 
+               /* Geometry shaders output triangle strips, but NGG expects triangles.
+                * We need to change the vertex order for odd triangles to get correct
+                * front/back facing by swapping 2 vertex indices, but we also have to
+                * keep the provoking vertex in the same place.
+                *
+                * If the first vertex is provoking, swap index 1 and 2.
+                * If the last vertex is provoking, swap index 0 and 1.
+                */
+               if (verts_per_prim == 3) {
+                       LLVMValueRef is_odd = LLVMBuildLShr(builder, flags, ctx->ac.i8_1, "");
+                       is_odd = LLVMBuildTrunc(builder, is_odd, ctx->i1, "");
+                       LLVMValueRef flatshade_first =
+                               LLVMBuildICmp(builder, LLVMIntEQ,
+                                             si_unpack_param(ctx, ctx->vs_state_bits, 4, 2),
+                                             ctx->i32_0, "");
+
+                       struct ngg_prim in = prim;
+                       prim.index[0] = LLVMBuildSelect(builder, flatshade_first,
+                                                       in.index[0],
+                                                       LLVMBuildSelect(builder, is_odd,
+                                                                       in.index[1], in.index[0], ""), "");
+                       prim.index[1] = LLVMBuildSelect(builder, flatshade_first,
+                                                       LLVMBuildSelect(builder, is_odd,
+                                                                       in.index[2], in.index[1], ""),
+                                                       LLVMBuildSelect(builder, is_odd,
+                                                                       in.index[0], in.index[1], ""), "");
+                       prim.index[2] = LLVMBuildSelect(builder, flatshade_first,
+                                                       LLVMBuildSelect(builder, is_odd,
+                                                                       in.index[1], in.index[2], ""),
+                                                       in.index[2], "");
+               }
+
                build_export_prim(ctx, &prim);
        }
        ac_build_endif(&ctx->ac, 5140);