intel/fs: Set src0 alpha present bit in header when provided in message payload.

Currently the "Source0 Alpha Present to RenderTarget" bit of the RT
write message header is derived from brw_wm_prog_data::replicate_alpha.
However the src0_alpha payload is provided anytime it's specified to
the logical message.  This could theoretically lead to an
inconsistency if somebody provided a src0_alpha value while
brw_wm_prog_data::replicate_alpha was false, as I'm planning to do in
a future commit in order to implement a hardware workaround.

Instead calculate the header bit based on whether a src0_alpha value
was provided to the logical message, which guarantees the same
behavior on pre-ICL and ICL+ (the latter used an extended descriptor
bit for this which didn't suffer from the same issue).  Remove the
brw_wm_prog_data::replicate_alpha flag.

Reviewed-by: Kenneth Graunke <[email protected]>

3 files changed, 6 insertions, 15 deletions

diff --git a/src/intel/compiler/brw_compiler.h b/src/intel/compiler/brw_compiler.h
index 61b7b4f21e6..1b34ed2d6bb 100644
--- a/src/intel/compiler/brw_compiler.h
+++ b/src/intel/compiler/brw_compiler.h
@@ -751,7 +751,6 @@ struct brw_wm_prog_data {
    bool dispatch_16;
    bool dispatch_32;
    bool dual_src_blend;
-   bool replicate_alpha;
    bool persample_dispatch;
    bool uses_pos_offset;
    bool uses_omask;
diff --git a/src/intel/compiler/brw_fs.cpp b/src/intel/compiler/brw_fs.cpp
index 7ebbe47b887..bc60e035b5f 100644
--- a/src/intel/compiler/brw_fs.cpp
+++ b/src/intel/compiler/brw_fs.cpp
@@ -4340,6 +4340,8 @@ lower_fb_write_logical_send(const fs_builder &bld, fs_inst *inst,
    const unsigned components =
       inst->src[FB_WRITE_LOGICAL_SRC_COMPONENTS].ud;
 
+   assert(inst->target != 0 || src0_alpha.file == BAD_FILE);
+
    /* We can potentially have a message length of up to 15, so we have to set
     * base_mrf to either 0 or 1 in order to fit in m0..m15.
     */
@@ -4404,7 +4406,7 @@ lower_fb_write_logical_send(const fs_builder &bld, fs_inst *inst,
       /* Set "Source0 Alpha Present to RenderTarget" bit in message
        * header.
        */
-      if (inst->target > 0 && prog_data->replicate_alpha)
+      if (src0_alpha.file != BAD_FILE)
          g00_bits |= 1 << 11;
 
       /* Set computes stencil to render target */
@@ -4448,8 +4450,6 @@ lower_fb_write_logical_send(const fs_builder &bld, fs_inst *inst,
       length++;
    }
 
-   bool src0_alpha_present = false;
-
    if (src0_alpha.file != BAD_FILE) {
       for (unsigned i = 0; i < bld.dispatch_width() / 8; i++) {
          const fs_builder &ubld = bld.exec_all().group(8, i)
@@ -4459,14 +4459,6 @@ lower_fb_write_logical_send(const fs_builder &bld, fs_inst *inst,
          setup_color_payload(ubld, key, &sources[length], tmp, 1);
          length++;
       }
-      src0_alpha_present = true;
-   } else if (prog_data->replicate_alpha && inst->target != 0) {
-      /* Handle the case when fragment shader doesn't write to draw buffer
-       * zero. No need to call setup_color_payload() for src0_alpha because
-       * alpha value will be undefined.
-       */
-      length += bld.dispatch_width() / 8;
-      src0_alpha_present = true;
    }
 
    if (sample_mask.file != BAD_FILE) {
@@ -4548,7 +4540,7 @@ lower_fb_write_logical_send(const fs_builder &bld, fs_inst *inst,
          /* Set the "Render Target Index" and "Src0 Alpha Present" fields
           * in the extended message descriptor, in lieu of using a header.
           */
-         ex_desc = inst->target << 12 | src0_alpha_present << 15;
+         ex_desc = inst->target << 12 | (src0_alpha.file != BAD_FILE) << 15;
 
          if (key->nr_color_regions == 0)
             ex_desc |= 1 << 20; /* Null Render Target */
diff --git a/src/intel/compiler/brw_fs_visitor.cpp b/src/intel/compiler/brw_fs_visitor.cpp
index 81d0e466cc7..11e0af430d8 100644
--- a/src/intel/compiler/brw_fs_visitor.cpp
+++ b/src/intel/compiler/brw_fs_visitor.cpp
@@ -493,7 +493,7 @@ fs_visitor::emit_fb_writes()
     * so we compute if we need replicate alpha and emit alpha to coverage
     * workaround here.
     */
-   prog_data->replicate_alpha = key->alpha_test_replicate_alpha ||
+   const bool replicate_alpha = key->alpha_test_replicate_alpha ||
       (key->nr_color_regions > 1 && key->alpha_to_coverage &&
        (sample_mask.file == BAD_FILE || devinfo->gen == 6));
 
@@ -506,7 +506,7 @@ fs_visitor::emit_fb_writes()
          ralloc_asprintf(this->mem_ctx, "FB write target %d", target));
 
       fs_reg src0_alpha;
-      if (devinfo->gen >= 6 && prog_data->replicate_alpha && target != 0)
+      if (devinfo->gen >= 6 && replicate_alpha && target != 0)
          src0_alpha = offset(outputs[0], bld, 3);
 
       inst = emit_single_fb_write(abld, this->outputs[target],