nir: Add some generic helpers for writing lowering passes

Reviewed-by: Eric Anholt <eric@anholt.net>

src/compiler/nir/nir.h

@@ -3277,6 +3277,51 @@ static inline bool should_print_nir(void) { return false; }
 
 #define NIR_SKIP(name) should_skip_nir(#name)
 
+/** An instruction filtering callback
+ *
+ * Returns true if the instruction should be processed and false otherwise.
+ */
+typedef bool (*nir_instr_filter_cb)(const nir_instr *, const void *);
+
+/** A simple instruction lowering callback
+ *
+ * Many instruction lowering passes can be written as a simple function which
+ * takes an instruction as its input and returns a sequence of instructions
+ * that implement the consumed instruction.  This function type represents
+ * such a lowering function.  When called, a function with this prototype
+ * should either return NULL indicating that no lowering needs to be done or
+ * emit a sequence of instructions using the provided builder (whose cursor
+ * will already be placed after the instruction to be lowered) and return the
+ * resulting nir_ssa_def.
+ */
+typedef nir_ssa_def *(*nir_lower_instr_cb)(struct nir_builder *,
+                                           nir_instr *, void *);
+
+/** Iterate over all the instructions in a nir_function_impl and lower them
+ *  using the provided callbacks
+ *
+ * This function implements the guts of a standard lowering pass for you.  It
+ * iterates over all of the instructions in a nir_function_impl and calls the
+ * filter callback on each one.  If the filter callback returns true, it then
+ * calls the lowering call back on the instruction.  (Splitting it this way
+ * allows us to avoid some save/restore work for instructions we know won't be
+ * lowered.)  If the instruction is dead after the lowering is complete, it
+ * will be removed.  If new instructions are added, the lowering callback will
+ * also be called on them in case multiple lowerings are required.
+ *
+ * The metadata for the nir_function_impl will also be updated.  If any blocks
+ * are added (they cannot be removed), dominance and block indices will be
+ * invalidated.
+ */
+bool nir_function_impl_lower_instructions(nir_function_impl *impl,
+                                          nir_instr_filter_cb filter,
+                                          nir_lower_instr_cb lower,
+                                          void *cb_data);
+bool nir_shader_lower_instructions(nir_shader *shader,
+                                   nir_instr_filter_cb filter,
+                                   nir_lower_instr_cb lower,
+                                   void *cb_data);
+
 void nir_calc_dominance_impl(nir_function_impl *impl);
 void nir_calc_dominance(nir_shader *shader);