bpo-43950: optimize column table assembling with pre-sizing object (GH-26997)

The new resizing system works like this; ``` $ cat t.py a + a + a + b + c + a + a + a + b + c + a + a + a + b + c + a + a + a + b + c [repeated 99 more times] $ ./python t.py RESIZE: prev len = 32, new len = 66 FINAL SIZE: 56 ----------------------------------------------------- RESIZE: prev len = 32, new len = 66 RESIZE: prev len = 66, new len = 134 RESIZE: prev len = 134, new len = 270 RESIZE: prev len = 270, new len = 542 RESIZE: prev len = 542, new len = 1086 RESIZE: prev len = 1086, new len = 2174 RESIZE: prev len = 2174, new len = 4350 RESIZE: prev len = 4350, new len = 8702 FINAL SIZE: 8004 ``` So now we do considerably lower number of `_PyBytes_Resize` calls. Automerge-Triggered-By: GH:isidentical

Author: isidentical

Python/compile.c

@@ -40,7 +40,7 @@
 #define DEFAULT_BLOCKS 8
 #define DEFAULT_CODE_SIZE 128
 #define DEFAULT_LNOTAB_SIZE 16
-#define DEFAULT_CNOTAB_SIZE 0
+#define DEFAULT_CNOTAB_SIZE 32
 
 #define COMP_GENEXP 0
 #define COMP_LISTCOMP 1
@@ -6587,6 +6587,7 @@ struct assembler {
  PyObject* a_cnotab; /* bytes containing cnotab */
  int a_lnotab_off; /* offset into lnotab */
  int a_enotab_off; /* offset into enotab */
+ int a_cnotab_off; /* offset into cnotab */
  PyObject *a_except_table; /* bytes containing exception table */
  int a_except_table_off; /* offset into exception table */
  int a_prevlineno; /* lineno of last emitted line in line table */
@@ -6696,6 +6697,7 @@ assemble_init(struct assembler *a, int nblocks, int firstlineno)
  a->a_lnotab = NULL;
  a->a_enotab = NULL;
  a->a_cnotab = NULL;
+ a->a_cnotab_off = 0;
  a->a_except_table = NULL;
  a->a_bytecode = PyBytes_FromStringAndSize(NULL, DEFAULT_CODE_SIZE);
  if (a->a_bytecode == NULL) {
@@ -7106,14 +7108,16 @@ static int
 assemble_cnotab(struct assembler* a, struct instr* i, int instr_size)
 {
  Py_ssize_t len = PyBytes_GET_SIZE(a->a_cnotab);
- // TODO: Allocate more memory than just what we immediately need
- // like a_lnotab does.
- if (_PyBytes_Resize(&a->a_cnotab, len + (instr_size * 2)) < 0) {
- return 0;
+ int difference = instr_size * 2;
+ if (a->a_cnotab_off + difference >= len) {
+ if (_PyBytes_Resize(&a->a_cnotab, difference + (len * 2)) < 0) {
+ return 0;
+ }
  }
 
  unsigned char* cnotab = (unsigned char*)PyBytes_AS_STRING(a->a_cnotab);
- cnotab += len;
+ cnotab += a->a_cnotab_off;
+ a->a_cnotab_off += difference;
 
  for (int j = 0; j < instr_size; j++) {
  if (i->i_col_offset > 255 || i->i_end_col_offset > 255) {
@@ -7855,6 +7859,9 @@ assemble(struct compiler *c, int addNone)
  if (!merge_const_one(c, &a.a_enotab)) {
  goto error;
  }
+ if (_PyBytes_Resize(&a.a_cnotab, a.a_cnotab_off) < 0) {
+ goto error;
+ }
  if (_PyBytes_Resize(&a.a_bytecode, a.a_offset * sizeof(_Py_CODEUNIT)) < 0) {
  goto error;
  }