py/emit: Remove logic to detect last-emit-was-return-value.

This optimisation to remove dead code is not as good as it could be.

Signed-off-by: Damien George <damien@micropython.org>

4 files changed, 7 insertions, 43 deletions

diff --git a/py/compile.c b/py/compile.c
index 9cca5df40..ff3e5ffc6 100644
--- a/py/compile.c
+++ b/py/compile.c
@@ -1330,12 +1330,8 @@ STATIC void compile_if_stmt(compiler_t *comp, mp_parse_node_struct_t *pns) {
             goto done;
         }
 
-        if (
-            // optimisation: don't jump over non-existent elif/else blocks
-            !(MP_PARSE_NODE_IS_NULL(pns->nodes[2]) && MP_PARSE_NODE_IS_NULL(pns->nodes[3]))
-            // optimisation: don't jump if last instruction was return
-            && !EMIT(last_emit_was_return_value)
-            ) {
+        // optimisation: don't jump over non-existent elif/else blocks
+        if (!(MP_PARSE_NODE_IS_NULL(pns->nodes[2]) && MP_PARSE_NODE_IS_NULL(pns->nodes[3]))) {
             // jump over elif/else blocks
             EMIT_ARG(jump, l_end);
         }
@@ -1362,10 +1358,7 @@ STATIC void compile_if_stmt(compiler_t *comp, mp_parse_node_struct_t *pns) {
                 goto done;
             }
 
-            // optimisation: don't jump if last instruction was return
-            if (!EMIT(last_emit_was_return_value)) {
-                EMIT_ARG(jump, l_end);
-            }
+            EMIT_ARG(jump, l_end);
             EMIT_ARG(label_assign, l_fail);
         }
     }
@@ -1580,9 +1573,7 @@ STATIC void compile_for_stmt(compiler_t *comp, mp_parse_node_struct_t *pns) {
     EMIT_ARG(for_iter, pop_label);
     c_assign(comp, pns->nodes[0], ASSIGN_STORE); // variable
     compile_node(comp, pns->nodes[2]); // body
-    if (!EMIT(last_emit_was_return_value)) {
-        EMIT_ARG(jump, continue_label);
-    }
+    EMIT_ARG(jump, continue_label);
     EMIT_ARG(label_assign, pop_label);
     EMIT(for_iter_end);
 
@@ -3048,11 +3039,8 @@ STATIC bool compile_scope(compiler_t *comp, scope_t *scope, pass_kind_t pass) {
         }
 
         compile_node(comp, pns->nodes[3]); // 3 is function body
-        // emit return if it wasn't the last opcode
-        if (!EMIT(last_emit_was_return_value)) {
-            EMIT_ARG(load_const_tok, MP_TOKEN_KW_NONE);
-            EMIT(return_value);
-        }
+        EMIT_ARG(load_const_tok, MP_TOKEN_KW_NONE);
+        EMIT(return_value);
     } else if (scope->kind == SCOPE_LAMBDA) {
         assert(MP_PARSE_NODE_IS_STRUCT(scope->pn));
         mp_parse_node_struct_t *pns = (mp_parse_node_struct_t *)scope->pn;
diff --git a/py/emit.h b/py/emit.h
index 608734552..4e8a55e77 100644
--- a/py/emit.h
+++ b/py/emit.h
@@ -115,7 +115,6 @@ typedef struct _emit_method_table_t {
 
     void (*start_pass)(emit_t *emit, pass_kind_t pass, scope_t *scope);
     bool (*end_pass)(emit_t *emit);
-    bool (*last_emit_was_return_value)(emit_t *emit);
     void (*adjust_stack_size)(emit_t *emit, mp_int_t delta);
     void (*set_source_line)(emit_t *emit, mp_uint_t line);
 
@@ -227,7 +226,6 @@ void emit_native_xtensawin_free(emit_t *emit);
 
 void mp_emit_bc_start_pass(emit_t *emit, pass_kind_t pass, scope_t *scope);
 bool mp_emit_bc_end_pass(emit_t *emit);
-bool mp_emit_bc_last_emit_was_return_value(emit_t *emit);
 void mp_emit_bc_adjust_stack_size(emit_t *emit, mp_int_t delta);
 void mp_emit_bc_set_source_line(emit_t *emit, mp_uint_t line);
 
diff --git a/py/emitbc.c b/py/emitbc.c
index 2007975c5..9c0d78d79 100644
--- a/py/emitbc.c
+++ b/py/emitbc.c
@@ -47,7 +47,6 @@ struct _emit_t {
     byte dummy_data[DUMMY_DATA_SIZE];
 
     pass_kind_t pass : 8;
-    mp_uint_t last_emit_was_return_value : 8;
 
     int stack_size;
 
@@ -272,7 +271,6 @@ STATIC void emit_write_bytecode_byte_label(emit_t *emit, int stack_adj, byte b1,
 void mp_emit_bc_start_pass(emit_t *emit, pass_kind_t pass, scope_t *scope) {
     emit->pass = pass;
     emit->stack_size = 0;
-    emit->last_emit_was_return_value = false;
     emit->scope = scope;
     emit->last_source_line_offset = 0;
     emit->last_source_line = 1;
@@ -397,10 +395,6 @@ bool mp_emit_bc_end_pass(emit_t *emit) {
     return true;
 }
 
-bool mp_emit_bc_last_emit_was_return_value(emit_t *emit) {
-    return emit->last_emit_was_return_value;
-}
-
 void mp_emit_bc_adjust_stack_size(emit_t *emit, mp_int_t delta) {
     if (emit->pass == MP_PASS_SCOPE) {
         return;
@@ -410,7 +404,6 @@ void mp_emit_bc_adjust_stack_size(emit_t *emit, mp_int_t delta) {
     if (emit->stack_size > emit->scope->stack_size) {
         emit->scope->stack_size = emit->stack_size;
     }
-    emit->last_emit_was_return_value = false;
 }
 
 void mp_emit_bc_set_source_line(emit_t *emit, mp_uint_t source_line) {
@@ -773,7 +766,6 @@ void mp_emit_bc_call_method(emit_t *emit, mp_uint_t n_positional, mp_uint_t n_ke
 
 void mp_emit_bc_return_value(emit_t *emit) {
     emit_write_bytecode_byte(emit, -1, MP_BC_RETURN_VALUE);
-    emit->last_emit_was_return_value = true;
 }
 
 void mp_emit_bc_raise_varargs(emit_t *emit, mp_uint_t n_args) {
@@ -806,7 +798,6 @@ const emit_method_table_t emit_bc_method_table = {
 
     mp_emit_bc_start_pass,
     mp_emit_bc_end_pass,
-    mp_emit_bc_last_emit_was_return_value,
     mp_emit_bc_adjust_stack_size,
     mp_emit_bc_set_source_line,
 
diff --git a/py/emitnative.c b/py/emitnative.c
index 6683ea420..5b695a22a 100644
--- a/py/emitnative.c
+++ b/py/emitnative.c
@@ -276,8 +276,6 @@ struct _emit_t {
     uint16_t n_info;
     uint16_t n_cell;
 
-    bool last_emit_was_return_value;
-
     scope_t *scope;
 
     ASM_T *as;
@@ -370,7 +368,6 @@ STATIC void emit_native_start_pass(emit_t *emit, pass_kind_t pass, scope_t *scop
     emit->pass = pass;
     emit->do_viper_types = scope->emit_options == MP_EMIT_OPT_VIPER;
     emit->stack_size = 0;
-    emit->last_emit_was_return_value = false;
     emit->scope = scope;
 
     // allocate memory for keeping track of the types of locals
@@ -733,10 +730,6 @@ STATIC bool emit_native_end_pass(emit_t *emit) {
     return true;
 }
 
-STATIC bool emit_native_last_emit_was_return_value(emit_t *emit) {
-    return emit->last_emit_was_return_value;
-}
-
 STATIC void ensure_extra_stack(emit_t *emit, size_t delta) {
     if (emit->stack_size + delta > emit->stack_info_alloc) {
         size_t new_alloc = (emit->stack_size + delta + 8) & ~3;
@@ -793,7 +786,7 @@ STATIC void emit_native_set_source_line(emit_t *emit, mp_uint_t source_line) {
 
 // this must be called at start of emit functions
 STATIC void emit_native_pre(emit_t *emit) {
-    emit->last_emit_was_return_value = false;
+    (void)emit;
 }
 
 // depth==0 is top, depth==1 is before top, etc
@@ -917,7 +910,6 @@ STATIC void emit_fold_stack_top(emit_t *emit, int reg_dest) {
 // If stacked value is in a register and the register is not r1 or r2, then
 // *reg_dest is set to that register.  Otherwise the value is put in *reg_dest.
 STATIC void emit_pre_pop_reg_flexible(emit_t *emit, vtype_kind_t *vtype, int *reg_dest, int not_r1, int not_r2) {
-    emit->last_emit_was_return_value = false;
     stack_info_t *si = peek_stack(emit, 0);
     if (si->kind == STACK_REG && si->data.u_reg != not_r1 && si->data.u_reg != not_r2) {
         *vtype = si->vtype;
@@ -930,12 +922,10 @@ STATIC void emit_pre_pop_reg_flexible(emit_t *emit, vtype_kind_t *vtype, int *re
 }
 
 STATIC void emit_pre_pop_discard(emit_t *emit) {
-    emit->last_emit_was_return_value = false;
     adjust_stack(emit, -1);
 }
 
 STATIC void emit_pre_pop_reg(emit_t *emit, vtype_kind_t *vtype, int reg_dest) {
-    emit->last_emit_was_return_value = false;
     emit_access_stack(emit, 1, vtype, reg_dest);
     adjust_stack(emit, -1);
 }
@@ -2771,7 +2761,6 @@ STATIC void emit_native_return_value(emit_t *emit) {
 
         // Do the unwinding jump to get to the return handler
         emit_native_unwind_jump(emit, emit->exit_label, emit->exc_stack_size);
-        emit->last_emit_was_return_value = true;
         return;
     }
 
@@ -2809,7 +2798,6 @@ STATIC void emit_native_return_value(emit_t *emit) {
         ASM_MOV_LOCAL_REG(emit->as, LOCAL_IDX_RET_VAL(emit), REG_PARENT_RET);
     }
     emit_native_unwind_jump(emit, emit->exit_label, emit->exc_stack_size);
-    emit->last_emit_was_return_value = true;
 }
 
 STATIC void emit_native_raise_varargs(emit_t *emit, mp_uint_t n_args) {
@@ -2928,7 +2916,6 @@ const emit_method_table_t EXPORT_FUN(method_table) = {
 
     emit_native_start_pass,
     emit_native_end_pass,
-    emit_native_last_emit_was_return_value,
     emit_native_adjust_stack_size,
     emit_native_set_source_line,