nir: add FRAG_RESULT_DUAL_SRC_BLEND and an option to use it

This is potentially nicer for some drivers. AMD drivers will use it.

mesa_frag_result_get_color_index will be used often.

Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/38604>

src/compiler/nir/nir.h

@@ -1999,7 +1999,7 @@ typedef enum {
 typedef struct nir_io_semantics {
    unsigned location : 7;  /* gl_vert_attrib, gl_varying_slot, or gl_frag_result */
    unsigned num_slots : 6; /* max 32, may be pessimistic with const indexing */
-   unsigned dual_source_blend_index : 1;
+   unsigned dual_source_blend_index : 1; /* used without nir_io_use_frag_result_dual_src_blend */
    unsigned fb_fetch_output : 1;  /* for GL_KHR_blend_equation_advanced */
    unsigned fb_fetch_output_coherent : 1;
    unsigned gs_streams : 8;       /* xxyyzzww: 2-bit stream index for each component */

src/compiler/nir/nir_lower_blend.c

@@ -586,10 +586,12 @@ consume_dual_stores(nir_builder *b, nir_intrinsic_instr *store, void *data)
    store->instr.pass_flags = 0;
 
    nir_io_semantics sem = nir_intrinsic_io_semantics(store);
-   if (sem.dual_source_blend_index == 0)
+   int rt = 0;
+   if (sem.dual_source_blend_index)
+      rt = color_index_for_location(sem.location);
+   else if (sem.location != FRAG_RESULT_DUAL_SRC_BLEND)
       return false;
 
-   int rt = color_index_for_location(sem.location);
    assert(rt >= 0 && rt < 8 && "bounds for dual-source blending");
 
    outputs[rt] = store->src[0].ssa;

src/compiler/nir/nir_lower_io.c

@@ -600,6 +600,17 @@ emit_store(struct lower_io_state *state, nir_def *data,
    }
 
    int location = var->data.location;
+   bool dual_src_blend = var->data.index > 0;
+
+   /* Set FRAG_RESULT_DUAL_SRC_BLEND if the driver prefers that. */
+   if (dual_src_blend &&
+       b->shader->options->io_options & nir_io_use_frag_result_dual_src_blend) {
+      assert(b->shader->info.stage == MESA_SHADER_FRAGMENT);
+      assert(location == FRAG_RESULT_COLOR || location == FRAG_RESULT_DATA0);
+
+      location = FRAG_RESULT_DUAL_SRC_BLEND;
+      dual_src_blend = false;
+   }
 
    /* Maximum values in nir_io_semantics. */
    assert(num_slots <= 63);
@@ -608,7 +619,7 @@ emit_store(struct lower_io_state *state, nir_def *data,
    nir_io_semantics semantics = { 0 };
    semantics.location = location;
    semantics.num_slots = num_slots;
-   semantics.dual_source_blend_index = var->data.index;
+   semantics.dual_source_blend_index = dual_src_blend;
    semantics.gs_streams = gs_streams;
    semantics.medium_precision = is_medium_precision(b->shader, var);
    semantics.per_view = var->data.per_view;

src/compiler/nir/nir_shader_compiler_options.h

@@ -206,6 +206,16 @@ typedef enum {
     */
    nir_io_assign_color_input_bases_after_all_other_inputs = BITFIELD_BIT(11),
 
+   /**
+    * Whether nir_lower_io should use FRAG_RESULT_DUAL_SRC_BLEND instead of
+    * nir_io_semantics::dual_source_blend_index.
+    *
+    * This has the advantage that it behaves like a normal output and its
+    * presence is reflected in shader_info::outputs_written instead of
+    * shader_info::fs::color_is_dual_source.
+    */
+   nir_io_use_frag_result_dual_src_blend = BITFIELD_BIT(11),
+
    /* Options affecting the GLSL compiler or Gallium are below. */
 
    /**

src/compiler/shader_enums.c

@@ -468,6 +468,7 @@ gl_frag_result_name(gl_frag_result result)
       ENUM(FRAG_RESULT_DATA5),
       ENUM(FRAG_RESULT_DATA6),
       ENUM(FRAG_RESULT_DATA7),
+      ENUM(FRAG_RESULT_DUAL_SRC_BLEND),
    };
    STATIC_ASSERT(ARRAY_SIZE(names) == FRAG_RESULT_MAX);
    return NAME(result);
@@ -487,3 +488,19 @@ mesa_scope_name(mesa_scope scope)
    };
    return NAME(scope);
 }
+
+int
+mesa_frag_result_get_color_index(gl_frag_result result)
+{
+   switch (result) {
+   case FRAG_RESULT_COLOR:
+      return 0;
+   case FRAG_RESULT_DUAL_SRC_BLEND:
+      return 1;
+   default:
+      if (result >= FRAG_RESULT_DATA0 && result <= FRAG_RESULT_DATA7)
+         return result - FRAG_RESULT_DATA0;
+
+      return -1;
+   }
+}

src/compiler/shader_enums.h

@@ -1026,11 +1026,18 @@ typedef enum
    FRAG_RESULT_DATA5,
    FRAG_RESULT_DATA6,
    FRAG_RESULT_DATA7,
+
+   /* The color output that sets the values for the SRC1 blend factors, also
+    * known as dual source blending. This is typically the second color output,
+    * and DATA1-DATA7 can't be written when this one is written. Enabled by
+    * nir_io_use_frag_result_dual_src_blend.
+    */
+   FRAG_RESULT_DUAL_SRC_BLEND,
+   FRAG_RESULT_MAX,
 } gl_frag_result;
 
 const char *gl_frag_result_name(gl_frag_result result);
-
+int mesa_frag_result_get_color_index(gl_frag_result result);
-#define FRAG_RESULT_MAX		(FRAG_RESULT_DATA0 + MAX_DRAW_BUFFERS)
 
 /**
  * \brief Layout qualifiers for gl_FragDepth.

src/mesa/program/prog_print.c

@@ -327,7 +327,8 @@ arb_output_attrib_string(GLuint index, unsigned stage)
       "result.color[4]",
       "result.color[5]",
       "result.color[6]",
-      "result.color[7]" /* MAX_DRAW_BUFFERS = 8 */
+      "result.color[7]", /* MAX_DRAW_BUFFERS = 8 */
+      "result.dual_src_blend",
    };
 
    /* sanity checks */