i965/nir: Validate that NIR passes call nir_metadata_preserve().

Failing to call nir_metadata_preserve() can have nasty consequences:
some pass breaks dominance information, but leaves it marked as valid,
causing some subsequent pass to go haywire and probably crash.

This pass adds a simple validation mechanism to ensure passes handle
this properly.  We add a new bogus metadata flag that isn't used for
anything in particular, set it before each pass, and ensure it *isn't*
still set after the pass.  nir_metadata_preserve will reset the flag,
so correct passes will work, and bad passes will assert fail.

(I would have made these functions static inline, but nir.h is included
in C++, so we can't bit-or enums without lots of casting...)

Thanks to Dylan Baker for the idea.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>

src/glsl/nir/nir.h

@@ -1312,6 +1312,7 @@ typedef enum {
    nir_metadata_block_index = 0x1,
    nir_metadata_dominance = 0x2,
    nir_metadata_live_ssa_defs = 0x4,
+   nir_metadata_not_properly_reset = 0x8,
 } nir_metadata;
 
 typedef struct {
@@ -1891,8 +1892,12 @@ void nir_print_instr(const nir_instr *instr, FILE *fp);
 
 #ifdef DEBUG
 void nir_validate_shader(nir_shader *shader);
+void nir_metadata_set_validation_flag(nir_shader *shader);
+void nir_metadata_check_validation_flag(nir_shader *shader);
 #else
 static inline void nir_validate_shader(nir_shader *shader) { (void) shader; }
+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; }
 #endif /* DEBUG */
 
 void nir_calc_dominance_impl(nir_function_impl *impl);

src/glsl/nir/nir_metadata.c

@@ -52,3 +52,39 @@ nir_metadata_preserve(nir_function_impl *impl, nir_metadata preserved)
 {
    impl->valid_metadata &= preserved;
 }
+
+#ifdef DEBUG
+/**
+ * Make sure passes properly invalidate metadata (part 1).
+ *
+ * Call this before running a pass to set a bogus metadata flag, which will
+ * only be preserved if the pass forgets to call nir_metadata_preserve().
+ */
+void
+nir_metadata_set_validation_flag(nir_shader *shader)
+{
+   nir_foreach_overload(shader, overload) {
+      if (overload->impl) {
+         overload->impl->valid_metadata |= nir_metadata_not_properly_reset;
+      }
+   }
+}
+
+/**
+ * Make sure passes properly invalidate metadata (part 2).
+ *
+ * Call this after a pass makes progress to verify that the bogus metadata set by
+ * the earlier function was properly thrown away.  Note that passes may not call
+ * nir_metadata_preserve() if they don't actually make any changes at all.
+ */
+void
+nir_metadata_check_validation_flag(nir_shader *shader)
+{
+   nir_foreach_overload(shader, overload) {
+      if (overload->impl) {
+         assert(!(overload->impl->valid_metadata &
+                  nir_metadata_not_properly_reset));
+      }
+   }
+}
+#endif

src/mesa/drivers/dri/i965/brw_nir.c

@@ -179,8 +179,12 @@ brw_nir_lower_outputs(nir_shader *nir, bool is_scalar)
 }))
 
 #define OPT(pass, ...) _OPT(                   \
+   nir_metadata_set_validation_flag(nir);      \
    this_progress = pass(nir ,##__VA_ARGS__);   \
-   progress = progress || this_progress;     \
+   if (this_progress) {                        \
+      progress = true;                         \
+      nir_metadata_check_validation_flag(nir); \
+   }                                           \
 )
 
 #define OPT_V(pass, ...) _OPT( \