nir/dead_write_vars: Handle volatile

We can't remove volatile writes and we can't combine them with other
volatile writes so all we can do is clear the unused bits.

Reviewed-by: Caio Marcelo de Oliveira Filho <[email protected]>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4767>

2 files changed, 61 insertions, 0 deletions

diff --git a/src/compiler/nir/nir_opt_dead_write_vars.c b/src/compiler/nir/nir_opt_dead_write_vars.c
index 6bd20db07ca..5a8e2ffaaa0 100644
--- a/src/compiler/nir/nir_opt_dead_write_vars.c
+++ b/src/compiler/nir/nir_opt_dead_write_vars.c
@@ -176,6 +176,19 @@ remove_dead_write_vars_local(void *mem_ctx, nir_block *block)
 
       case nir_intrinsic_store_deref: {
          nir_deref_instr *dst = nir_src_as_deref(intrin->src[0]);
+
+         if (nir_intrinsic_access(intrin) & ACCESS_VOLATILE) {
+            /* Consider a volatile write to also be a sort of read.  This
+             * prevents us from deleting a non-volatile write just before a
+             * volatile write thanks to a non-volatile write afterwards.  It's
+             * quite the corner case, but this should be safer and more
+             * predictable for the programmer than allowing two non-volatile
+             * writes to be combined with a volatile write between them.
+             */
+            clear_unused_for_read(&unused_writes, dst);
+            break;
+         }
+
          nir_component_mask_t mask = nir_intrinsic_write_mask(intrin);
          progress |= update_unused_writes(&unused_writes, intrin, dst, mask);
          break;
@@ -185,6 +198,12 @@ remove_dead_write_vars_local(void *mem_ctx, nir_block *block)
          nir_deref_instr *src = nir_src_as_deref(intrin->src[1]);
          nir_deref_instr *dst = nir_src_as_deref(intrin->src[0]);
 
+         if (nir_intrinsic_dst_access(intrin) & ACCESS_VOLATILE) {
+            clear_unused_for_read(&unused_writes, src);
+            clear_unused_for_read(&unused_writes, dst);
+            break;
+         }
+
          /* Self-copy is removed. */
          if (nir_compare_derefs(src, dst) & nir_derefs_equal_bit) {
             nir_instr_remove(instr);
diff --git a/src/compiler/nir/tests/vars_tests.cpp b/src/compiler/nir/tests/vars_tests.cpp
index c1b82791e7e..6079eb7d4e6 100644
--- a/src/compiler/nir/tests/vars_tests.cpp
+++ b/src/compiler/nir/tests/vars_tests.cpp
@@ -1345,6 +1345,48 @@ TEST_F(nir_dead_write_vars_test, no_dead_writes_different_components_in_block)
    ASSERT_FALSE(progress);
 }
 
+TEST_F(nir_dead_write_vars_test, volatile_write)
+{
+   nir_variable *v = create_int(nir_var_mem_ssbo, "v");
+
+   nir_store_var(b, v, nir_imm_int(b, 0), 0x1);
+   nir_store_var_volatile(b, v, nir_imm_int(b, 1), 0x1);
+   nir_store_var(b, v, nir_imm_int(b, 2), 0x1);
+
+   /* Our approach here is a bit scorched-earth.  We expect the volatile store
+    * in the middle to cause both that store and the one before it to be kept.
+    * Technically, volatile only prevents combining the volatile store with
+    * another store and one could argue that the store before the volatile and
+    * the one after it could be combined.  However, it seems safer to just
+    * treat a volatile store like an atomic and prevent any combining across
+    * it.
+    */
+   bool progress = nir_opt_dead_write_vars(b->shader);
+   ASSERT_FALSE(progress);
+}
+
+TEST_F(nir_dead_write_vars_test, volatile_copies)
+{
+   nir_variable **v = create_many_int(nir_var_mem_ssbo, "v", 2);
+
+   nir_copy_var(b, v[0], v[1]);
+   nir_copy_deref_with_access(b, nir_build_deref_var(b, v[0]),
+                                 nir_build_deref_var(b, v[1]),
+                                 ACCESS_VOLATILE, (gl_access_qualifier)0);
+   nir_copy_var(b, v[0], v[1]);
+
+   /* Our approach here is a bit scorched-earth.  We expect the volatile store
+    * in the middle to cause both that store and the one before it to be kept.
+    * Technically, volatile only prevents combining the volatile store with
+    * another store and one could argue that the store before the volatile and
+    * the one after it could be combined.  However, it seems safer to just
+    * treat a volatile store like an atomic and prevent any combining across
+    * it.
+    */
+   bool progress = nir_opt_dead_write_vars(b->shader);
+   ASSERT_FALSE(progress);
+}
+
 TEST_F(nir_dead_write_vars_test, no_dead_writes_in_if_statement)
 {
    nir_variable **v = create_many_int(nir_var_mem_ssbo, "v", 6);