nir: Add a dominance validation pass

We don't do full dominance validation of SSA values in nir_validate
because it requires generating valid dominance information and, while
that's not extremely expensive, it's probably more than we want to do on
every pass.  Also, dominance information is generated through the
metadata system so if we ran it by default in nir_validate, we would get
different beavior of the metadata system based on whether or not you
have a debug build and metadata bugs would be very hard to find.

However, having a pass for it that can be run occasionally, should help
detect and expose bugs.  For ease of use, we add a NIR_VALIDATE_SSA_DOMINANCE
environment variable which can be set to manually enable dominance
validation as a standard part of nir_validate.

Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5288>

src/compiler/nir/nir.h

@@ -3886,6 +3886,7 @@ void nir_shader_serialize_deserialize(nir_shader *s);
 
 #ifndef NDEBUG
 void nir_validate_shader(nir_shader *shader, const char *when);
+void nir_validate_ssa_dominance(nir_shader *shader, const char *when);
 void nir_metadata_set_validation_flag(nir_shader *shader);
 void nir_metadata_check_validation_flag(nir_shader *shader);
 
@@ -3937,6 +3938,7 @@ should_print_nir(void)
 }
 #else
 static inline void nir_validate_shader(nir_shader *shader, const char *when) { (void) shader; (void)when; }
+static inline void nir_validate_ssa_dominance(nir_shader *shader, const char *when) { (void) shader; (void)when; }
 static inline void nir_metadata_set_validation_flag(nir_shader *shader) { (void) shader; }
 static inline void nir_metadata_check_validation_flag(nir_shader *shader) { (void) shader; }
 static inline bool should_skip_nir(UNUSED const char *pass_name) { return false; }

src/compiler/nir/nir_validate.c

@@ -1206,6 +1206,60 @@ validate_var_decl(nir_variable *var, nir_variable_mode valid_modes,
    state->var = NULL;
 }
 
+static bool
+validate_ssa_def_dominance(nir_ssa_def *def, void *_state)
+{
+   validate_state *state = _state;
+
+   validate_assert(state, def->index < state->impl->ssa_alloc);
+   validate_assert(state, !BITSET_TEST(state->ssa_defs_found, def->index));
+   BITSET_SET(state->ssa_defs_found, def->index);
+
+   return true;
+}
+
+static bool
+validate_src_dominance(nir_src *src, void *_state)
+{
+   validate_state *state = _state;
+   if (!src->is_ssa)
+      return true;
+
+   if (src->ssa->parent_instr->block == src->parent_instr->block) {
+      validate_assert(state, src->ssa->index < state->impl->ssa_alloc);
+      validate_assert(state, BITSET_TEST(state->ssa_defs_found,
+                                         src->ssa->index));
+   } else {
+      validate_assert(state, nir_block_dominates(src->ssa->parent_instr->block,
+                                                 src->parent_instr->block));
+   }
+   return true;
+}
+
+static void
+validate_ssa_dominance(nir_function_impl *impl, validate_state *state)
+{
+   nir_metadata_require(impl, nir_metadata_dominance);
+
+   nir_foreach_block(block, impl) {
+      state->block = block;
+      nir_foreach_instr(instr, block) {
+         state->instr = instr;
+         if (instr->type == nir_instr_type_phi) {
+            nir_phi_instr *phi = nir_instr_as_phi(instr);
+            nir_foreach_phi_src(src, phi) {
+               validate_assert(state,
+                  nir_block_dominates(src->src.ssa->parent_instr->block,
+                                      src->pred));
+            }
+         } else {
+            nir_foreach_src(instr, validate_src_dominance, state);
+         }
+         nir_foreach_ssa_def(instr, validate_ssa_def_dominance, state);
+      }
+   }
+}
+
 static void
 validate_function_impl(nir_function_impl *impl, validate_state *state)
 {
@@ -1257,6 +1311,14 @@ validate_function_impl(nir_function_impl *impl, validate_state *state)
 
    validate_assert(state, state->ssa_srcs->entries == 0);
    _mesa_set_clear(state->ssa_srcs, NULL);
+
+   static int validate_dominance = -1;
+   if (validate_dominance < 0) {
+      validate_dominance =
+         env_var_as_boolean("NIR_VALIDATE_SSA_DOMINANCE", false);
+   }
+   if (validate_dominance)
+      validate_ssa_dominance(impl, state);
 }
 
 static void
@@ -1365,4 +1427,38 @@ nir_validate_shader(nir_shader *shader, const char *when)
    destroy_validate_state(&state);
 }
 
+void
+nir_validate_ssa_dominance(nir_shader *shader, const char *when)
+{
+   static int should_validate = -1;
+   if (should_validate < 0)
+      should_validate = env_var_as_boolean("NIR_VALIDATE", true);
+   if (!should_validate)
+      return;
+
+   validate_state state;
+   init_validate_state(&state);
+
+   state.shader = shader;
+
+   nir_foreach_function(func, shader) {
+      if (func->impl == NULL)
+         continue;
+
+      state.ssa_defs_found = reralloc(state.mem_ctx, state.ssa_defs_found,
+                                      BITSET_WORD,
+                                      BITSET_WORDS(func->impl->ssa_alloc));
+      memset(state.ssa_defs_found, 0, BITSET_WORDS(func->impl->ssa_alloc) *
+                                      sizeof(BITSET_WORD));
+
+      state.impl = func->impl;
+      validate_ssa_dominance(func->impl, &state);
+   }
+
+   if (_mesa_hash_table_num_entries(state.errors) > 0)
+      dump_errors(&state, when);
+
+   destroy_validate_state(&state);
+}
+
 #endif /* NDEBUG */