bpo-39320: Handle unpacking of **values in compiler (GH-18141)

* Add DICT_UPDATE and DICT_MERGE bytecodes. Use them for ** unpacking.

* Remove BUILD_MAP_UNPACK and BUILD_MAP_UNPACK_WITH_CALL, as they are now unused.

* Update magic number for ** unpacking opcodes.

* Update dis.rst to incorporate new bytecodes.

* Add blurb entry.

1 files changed, 58 insertions, 32 deletions

diff --git a/Python/compile.c b/Python/compile.c
index 9ed29f4a1f..6776df54d4 100644
--- a/Python/compile.c
+++ b/Python/compile.c
@@ -1007,9 +1007,6 @@ stack_effect(int opcode, int oparg, int jump)
         case BUILD_SET:
         case BUILD_STRING:
             return 1-oparg;
-        case BUILD_MAP_UNPACK:
-        case BUILD_MAP_UNPACK_WITH_CALL:
-            return 1 - oparg;
         case BUILD_MAP:
             return 1 - 2*oparg;
         case BUILD_CONST_KEY_MAP:
@@ -1125,6 +1122,8 @@ stack_effect(int opcode, int oparg, int jump)
             return 0;
         case LIST_EXTEND:
         case SET_UPDATE:
+        case DICT_MERGE:
+        case DICT_UPDATE:
             return -1;
         default:
             return PY_INVALID_STACK_EFFECT;
@@ -3868,37 +3867,58 @@ static int
 compiler_dict(struct compiler *c, expr_ty e)
 {
     Py_ssize_t i, n, elements;
-    int containers;
+    int have_dict;
     int is_unpacking = 0;
     n = asdl_seq_LEN(e->v.Dict.values);
-    containers = 0;
+    have_dict = 0;
     elements = 0;
     for (i = 0; i < n; i++) {
         is_unpacking = (expr_ty)asdl_seq_GET(e->v.Dict.keys, i) == NULL;
-        if (elements == 0xFFFF || (elements && is_unpacking)) {
-            if (!compiler_subdict(c, e, i - elements, i))
-                return 0;
-            containers++;
-            elements = 0;
-        }
         if (is_unpacking) {
+            if (elements) {
+                if (!compiler_subdict(c, e, i - elements, i)) {
+                    return 0;
+                }
+                if (have_dict) {
+                    ADDOP_I(c, DICT_UPDATE, 1);
+                }
+                have_dict = 1;
+                elements = 0;
+            }
+            if (have_dict == 0) {
+                ADDOP_I(c, BUILD_MAP, 0);
+                have_dict = 1;
+            }
             VISIT(c, expr, (expr_ty)asdl_seq_GET(e->v.Dict.values, i));
-            containers++;
+            ADDOP_I(c, DICT_UPDATE, 1);
         }
         else {
-            elements++;
+            if (elements == 0xFFFF) {
+                if (!compiler_subdict(c, e, i - elements, i)) {
+                    return 0;
+                }
+                if (have_dict) {
+                    ADDOP_I(c, DICT_UPDATE, 1);
+                }
+                have_dict = 1;
+                elements = 0;
+            }
+            else {
+                elements++;
+            }
         }
     }
-    if (elements || containers == 0) {
-        if (!compiler_subdict(c, e, n - elements, n))
+    if (elements) {
+        if (!compiler_subdict(c, e, n - elements, n)) {
             return 0;
-        containers++;
+        }
+        if (have_dict) {
+            ADDOP_I(c, DICT_UPDATE, 1);
+        }
+        have_dict = 1;
     }
-    /* If there is more than one dict, they need to be merged into a new
-     * dict.  If there is one dict and it's an unpacking, then it needs
-     * to be copied into a new dict." */
-    if (containers > 1 || is_unpacking) {
-        ADDOP_I(c, BUILD_MAP_UNPACK, containers);
+    if (!have_dict) {
+        ADDOP_I(c, BUILD_MAP, 0);
     }
     return 1;
 }
@@ -4263,8 +4283,8 @@ ex_call:
     }
     /* Then keyword arguments */
     if (nkwelts) {
-        /* the number of dictionaries on the stack */
-        Py_ssize_t nsubkwargs = 0;
+        /* Has a new dict been pushed */
+        int have_dict = 0;
 
         nseen = 0;  /* the number of keyword arguments on the stack following */
         for (i = 0; i < nkwelts; i++) {
@@ -4272,13 +4292,18 @@ ex_call:
             if (kw->arg == NULL) {
                 /* A keyword argument unpacking. */
                 if (nseen) {
-                    if (!compiler_subkwargs(c, keywords, i - nseen, i))
+                    if (!compiler_subkwargs(c, keywords, i - nseen, i)) {
                         return 0;
-                    nsubkwargs++;
+                    }
+                    have_dict = 1;
                     nseen = 0;
                 }
+                if (!have_dict) {
+                    ADDOP_I(c, BUILD_MAP, 0);
+                    have_dict = 1;
+                }
                 VISIT(c, expr, kw->value);
-                nsubkwargs++;
+                ADDOP_I(c, DICT_MERGE, 1);
             }
             else {
                 nseen++;
@@ -4286,14 +4311,15 @@ ex_call:
         }
         if (nseen) {
             /* Pack up any trailing keyword arguments. */
-            if (!compiler_subkwargs(c, keywords, nkwelts - nseen, nkwelts))
+            if (!compiler_subkwargs(c, keywords, nkwelts - nseen, nkwelts)) {
                 return 0;
-            nsubkwargs++;
-        }
-        if (nsubkwargs > 1) {
-            /* Pack it all up */
-            ADDOP_I(c, BUILD_MAP_UNPACK_WITH_CALL, nsubkwargs);
+            }
+            if (have_dict) {
+                ADDOP_I(c, DICT_MERGE, 1);
+            }
+            have_dict = 1;
         }
+        assert(have_dict);
     }
     ADDOP_I(c, CALL_FUNCTION_EX, nkwelts > 0);
     return 1;