ac: add helper ac_build_triangle_strip_indices_to_triangle

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

src/amd/llvm/ac_llvm_build.c

@@ -4967,3 +4967,38 @@ LLVMValueRef ac_prefix_bitcount_2x64(struct ac_llvm_context *ctx,
 	return LLVMBuildAdd(builder, prefix_bcnt[0], prefix_bcnt[1], "");
 #endif
 }
+
+/**
+ * Convert triangle strip indices to triangle indices. This is used to decompose
+ * triangle strips into triangles.
+ */
+void ac_build_triangle_strip_indices_to_triangle(struct ac_llvm_context *ctx,
+						 LLVMValueRef is_odd,
+						 LLVMValueRef flatshade_first,
+						 LLVMValueRef index[3])
+{
+	LLVMBuilderRef builder = ctx->builder;
+	LLVMValueRef out[3];
+
+	/* 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.
+	 */
+	out[0] = LLVMBuildSelect(builder, flatshade_first,
+				 index[0],
+				 LLVMBuildSelect(builder, is_odd,
+						 index[1], index[0], ""), "");
+	out[1] = LLVMBuildSelect(builder, flatshade_first,
+				 LLVMBuildSelect(builder, is_odd,
+						 index[2], index[1], ""),
+				 LLVMBuildSelect(builder, is_odd,
+						 index[0], index[1], ""), "");
+	out[2] = LLVMBuildSelect(builder, flatshade_first,
+				 LLVMBuildSelect(builder, is_odd,
+						 index[1], index[2], ""),
+				 index[2], "");
+	memcpy(index, out, sizeof(out));
+}

src/amd/llvm/ac_llvm_build.h

@@ -795,6 +795,10 @@ LLVMValueRef ac_prefix_bitcount(struct ac_llvm_context *ctx,
 				LLVMValueRef mask, LLVMValueRef index);
 LLVMValueRef ac_prefix_bitcount_2x64(struct ac_llvm_context *ctx,
 				     LLVMValueRef mask[2], LLVMValueRef index);
+void ac_build_triangle_strip_indices_to_triangle(struct ac_llvm_context *ctx,
+						 LLVMValueRef is_odd,
+						 LLVMValueRef flatshade_first,
+						 LLVMValueRef index[3]);
 
 #ifdef __cplusplus
 }

src/gallium/drivers/radeonsi/gfx10_shader_ngg.c

@@ -1894,14 +1894,7 @@ 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.
+		/* 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, "");
@@ -1910,20 +1903,9 @@ void gfx10_ngg_gs_emit_epilogue(struct si_shader_context *ctx)
 					      si_unpack_param(ctx, ctx->vs_state_bits, 4, 2),
 					      ctx->i32_0, "");
 
-			struct ac_ngg_prim in = prim;
+			ac_build_triangle_strip_indices_to_triangle(&ctx->ac, is_odd,
-			prim.index[0] = LLVMBuildSelect(builder, flatshade_first,
+								    flatshade_first,
-							in.index[0],
+								    prim.index);
-							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], "");
 		}
 
 		ac_build_export_prim(&ctx->ac, &prim);

src/gallium/drivers/radeonsi/si_compute_prim_discard.c

@@ -656,21 +656,10 @@ void si_build_prim_discard_compute_shader(struct si_shader_context *ctx)
 			LLVMBuildXor(builder, first_is_odd,
 				     LLVMBuildTrunc(builder, thread_id, ctx->i1, ""), "");
 
-		/* Determine the primitive orientation.
+		/* Convert triangle strip indices to triangle indices. */
-		 * Only swap the vertices that are not the provoking vertex. We need to keep
+		ac_build_triangle_strip_indices_to_triangle(&ctx->ac, prim_is_odd,
-		 * the provoking vertex in place.
+							    LLVMConstInt(ctx->i1, key->opt.cs_provoking_vertex_first, 0),
-		 */
+							    index);
-		if (key->opt.cs_provoking_vertex_first) {
-			LLVMValueRef index1 = index[1];
-			LLVMValueRef index2 = index[2];
-			index[1] = LLVMBuildSelect(builder, prim_is_odd, index2, index1, "");
-			index[2] = LLVMBuildSelect(builder, prim_is_odd, index1, index2, "");
-		} else {
-			LLVMValueRef index0 = index[0];
-			LLVMValueRef index1 = index[1];
-			index[0] = LLVMBuildSelect(builder, prim_is_odd, index1, index0, "");
-			index[1] = LLVMBuildSelect(builder, prim_is_odd, index0, index1, "");
-		}
 	}
 
 	/* Execute the vertex shader for each vertex to get vertex positions. */