i965/fs: Disable CSE optimization for untyped & typed surface reads

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 <[email protected]>
Reviewed-by: Iago Toral Quiroga <[email protected]>

1 files changed, 6 insertions, 0 deletions

diff --git a/src/mesa/drivers/dri/i965/brw_shader.h b/src/mesa/drivers/dri/i965/brw_shader.h
index 51b059fcaa1..6a2dfc9bbb6 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;