draw: cleanup and fix instance id computation

The instance id system value always starts at 0, even if the
specified start instance is larger than 0. Instead of implicitly
setting instance id to instance id plus start instance and then
having to subtract instance id when computing the buffer offsets
lets just set instance id to the proper instance id. This fixes
instance id computation and cleansup buffer offset computation.

Signed-off-by: Zack Rusin <[email protected]>
Reviewed-by: Roland Scheidegger <[email protected]>
Reviewed-by: Brian Paul <[email protected]>
Reviewed-by: Jose Fonseca <[email protected]>

2 files changed, 7 insertions, 7 deletions

diff --git a/src/gallium/auxiliary/draw/draw_llvm.c b/src/gallium/auxiliary/draw/draw_llvm.c
index 79e7a9b6d12..a3174b404c1 100644
--- a/src/gallium/auxiliary/draw/draw_llvm.c
+++ b/src/gallium/auxiliary/draw/draw_llvm.c
@@ -707,15 +707,14 @@ generate_fetch(struct gallivm_state *gallivm,
    if (velem->instance_divisor) {
       /* Index is equal to the start instance plus the number of current 
        * instance divided by the divisor. In this case we compute it as:
-       * index = start_instance + ((instance_id - start_instance) / divisor)
+       * index = start_instance + (instance_id  / divisor)
        */
       LLVMValueRef current_instance;
       index = lp_build_const_int32(gallivm, draw->start_instance);
-      current_instance = LLVMBuildSub(builder, instance_id, index, "");
-      current_instance = LLVMBuildUDiv(builder, current_instance,
+      current_instance = LLVMBuildUDiv(builder, instance_id,
                                        lp_build_const_int32(gallivm, velem->instance_divisor),
                                        "instance_divisor");
-      index = LLVMBuildAdd(builder, index, current_instance, "instance");
+      index = lp_build_uadd_overflow(gallivm, index, current_instance, &ofbit);
    }
 
    stride = lp_build_umul_overflow(gallivm, vb_stride, index, &ofbit);
diff --git a/src/gallium/auxiliary/draw/draw_pt.c b/src/gallium/auxiliary/draw/draw_pt.c
index ccde371ffcd..fcc24057db0 100644
--- a/src/gallium/auxiliary/draw/draw_pt.c
+++ b/src/gallium/auxiliary/draw/draw_pt.c
@@ -542,11 +542,12 @@ draw_vbo(struct draw_context *draw,
     */
 
    for (instance = 0; instance < info->instance_count; instance++) {
-      draw->instance_id = instance + info->start_instance;
+      unsigned instance_idx = instance + info->start_instance;
       draw->start_instance = info->start_instance;
+      draw->instance_id = instance;
       /* check for overflow */
-      if (draw->instance_id < instance ||
-          draw->instance_id < info->start_instance) {
+      if (instance_idx < instance ||
+          instance_idx < draw->start_instance) {
          /* if we overflown just set the instance id to the max */
          draw->instance_id = 0xffffffff;
       }