[Mesa-dev] [PATCH] anv: fix alphaToCoverage when there is no color attachment

[Samuel Iglesias Gonsálvez]

CL#3532 added a test for alpha to coverage without a color attachment.
First the test draws a primitive with alpha 0 and a subpass with only
a depth buffer. No writes to a depth buffer are expected. Then a
second draw with a color buffer and the same depth buffer is done to
verify the depth buffer still has the original clear values.

This behavior is not explicitly forbidden by the Vulkan spec, so
it seems it is allowed.

When there is no color attachment for a given output, we discard it
so at the end we have an FS assembly like:

Native code for unnamed fragment shader (null)
SIMD16 shader: 1 instructions. 0 loops. 4 cycles. 0:0 spills:fills.
Promoted 0 constants. Compacted 16 to 16 bytes (0%)
   START B0 (4 cycles)
sendc(16)       null<1>UW       g120<0,1,0>F    0x90031000

render MsgDesc: RT write SIMD16 LastRT Surface = 0 mlen 8 rlen 0 { align1 1H EOT };

As g120 is not initialized, we see random writes to the depth buffer
due to the alphaToCoverage enablement. This commit fixes that by
keeping the output and creating a null render target for it.

Fixes tests:

dEQP-VK.pipeline.multisample.alpha_to_coverage_no_color_attachment.*

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias at igalia.com>
---
 src/intel/vulkan/anv_pipeline.c | 35 +++++++++++++++++++++++++--------
 1 file changed, 27 insertions(+), 8 deletions(-)

diff --git a/src/intel/vulkan/anv_pipeline.c b/src/intel/vulkan/anv_pipeline.c
index e2024212bd9..70a958bf3a8 100644
--- a/src/intel/vulkan/anv_pipeline.c
+++ b/src/intel/vulkan/anv_pipeline.c
@@ -792,7 +792,9 @@ anv_pipeline_compile_gs(const struct brw_compiler *compiler,
 
 static void
 anv_pipeline_link_fs(const struct brw_compiler *compiler,
-                     struct anv_pipeline_stage *stage)
+                     struct anv_pipeline_stage *stage,
+                     const struct anv_subpass *subpass,
+                     const VkPipelineMultisampleStateCreateInfo *ms_info)
 {
    unsigned num_rts = 0;
    const int max_rt = FRAG_RESULT_DATA7 - FRAG_RESULT_DATA0 + 1;
@@ -843,12 +845,28 @@ anv_pipeline_link_fs(const struct brw_compiler *compiler,
       const unsigned rt = var->data.location - FRAG_RESULT_DATA0;
       if (rt >= MAX_RTS ||
           !(stage->key.wm.color_outputs_valid & (1 << rt))) {
-         /* Unused or out-of-bounds, throw it away */
-         deleted_output = true;
-         var->data.mode = nir_var_function_temp;
-         exec_node_remove(&var->node);
-         exec_list_push_tail(&impl->locals, &var->node);
-         continue;
+         if (rt == 0 && ms_info && ms_info->alphaToCoverageEnable &&
+             subpass->depth_stencil_attachment && rt_to_bindings[rt] == -1) {
+            /* Don't throw away the unused output, because we needed it for
+             * calculate correctly the alpha to coverage samples when there
+             * is no color buffer attached at location 0.
+             */
+            rt_to_bindings[rt] = num_rts;
+            /* We need a null render target */
+            rt_bindings[rt_to_bindings[rt]] = (struct anv_pipeline_binding) {
+               .set = ANV_DESCRIPTOR_SET_COLOR_ATTACHMENTS,
+               .binding = 0,
+               .index = UINT32_MAX,
+            };
+            num_rts++;
+         } else {
+            /* Unused or out-of-bounds, throw it away */
+            deleted_output = true;
+            var->data.mode = nir_var_function_temp;
+            exec_node_remove(&var->node);
+            exec_list_push_tail(&impl->locals, &var->node);
+            continue;
+         }
       }
 
       /* Give it the new location */
@@ -1075,7 +1093,8 @@ anv_pipeline_compile_graphics(struct anv_pipeline *pipeline,
          anv_pipeline_link_gs(compiler, &stages[s], next_stage);
          break;
       case MESA_SHADER_FRAGMENT:
-         anv_pipeline_link_fs(compiler, &stages[s]);
+         anv_pipeline_link_fs(compiler, &stages[s],
+                              pipeline->subpass, info->pMultisampleState);
          break;
       default:
          unreachable("Invalid graphics shader stage");
-- 
2.19.1