[Mesa-dev] [PATCH] i965/fs: Disable CSE optimization for untyped & typed surface reads

[Jordan Justen]

An untyped surface read is volatile because it might be affected by a
write.

In the ES31-CTS.compute_shader.resources-max test, two back to back
read/modify/writes of an SSBO variable looked something like this:

  r1 = untyped_surface_read(ssbo_float)
  r2 = r1 + 1
  untyped_surface_write(ssbo_float, r2)
  r3 = untyped_surface_read(ssbo_float)
  r4 = r3 + 1
  untyped_surface_write(ssbo_float, r4)

And after CSE, we had:

  r1 = untyped_surface_read(ssbo_float)
  r2 = r1 + 1
  untyped_surface_write(ssbo_float, r2)
  r4 = r1 + 1
  untyped_surface_write(ssbo_float, r4)

Signed-off-by: Jordan Justen <jordan.l.justen at intel.com>
---
 src/mesa/drivers/dri/i965/brw_fs_cse.cpp |  3 ++-
 src/mesa/drivers/dri/i965/brw_shader.cpp | 14 ++++++++++++++
 src/mesa/drivers/dri/i965/brw_shader.h   |  6 ++++++
 3 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/src/mesa/drivers/dri/i965/brw_fs_cse.cpp b/src/mesa/drivers/dri/i965/brw_fs_cse.cpp
index c7628dc..3a28c8d 100644
--- a/src/mesa/drivers/dri/i965/brw_fs_cse.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs_cse.cpp
@@ -93,7 +93,8 @@ is_expression(const fs_visitor *v, const fs_inst *const inst)
    case SHADER_OPCODE_LOAD_PAYLOAD:
       return !inst->is_copy_payload(v->alloc);
    default:
-      return inst->is_send_from_grf() && !inst->has_side_effects();
+      return inst->is_send_from_grf() && !inst->has_side_effects() &&
+         !inst->is_volatile();
    }
 }
 
diff --git a/src/mesa/drivers/dri/i965/brw_shader.cpp b/src/mesa/drivers/dri/i965/brw_shader.cpp
index 2324b56..be911ed 100644
--- a/src/mesa/drivers/dri/i965/brw_shader.cpp
+++ b/src/mesa/drivers/dri/i965/brw_shader.cpp
@@ -969,6 +969,20 @@ backend_instruction::has_side_effects() const
    }
 }
 
+bool
+backend_instruction::is_volatile() const
+{
+   switch (opcode) {
+   case SHADER_OPCODE_UNTYPED_SURFACE_READ:
+   case SHADER_OPCODE_UNTYPED_SURFACE_READ_LOGICAL:
+   case SHADER_OPCODE_TYPED_SURFACE_READ:
+   case SHADER_OPCODE_TYPED_SURFACE_READ_LOGICAL:
+      return true;
+   default:
+      return false;
+   }
+}
+
 #ifndef NDEBUG
 static bool
 inst_is_in_block(const bblock_t *block, const backend_instruction *inst)
diff --git a/src/mesa/drivers/dri/i965/brw_shader.h b/src/mesa/drivers/dri/i965/brw_shader.h
index b33b08f..35ee210 100644
--- a/src/mesa/drivers/dri/i965/brw_shader.h
+++ b/src/mesa/drivers/dri/i965/brw_shader.h
@@ -115,6 +115,12 @@ struct backend_instruction : public exec_node {
     * optimize these out unless you know what you are doing.
     */
    bool has_side_effects() const;
+
+   /**
+    * True if the instruction might be affected by side effects of other
+    * instructions.
+    */
+   bool is_volatile() const;
 #else
 struct backend_instruction {
    struct exec_node link;
-- 
2.5.1