bpo-46724: Use JUMP_ABSOLUTE for all backward jumps. (GH-31326)

Backport of GH-31326, plus supporting code from 3.11.

Author: markshannon

Lib/test/test_dis.py

@@ -1044,7 +1044,7 @@ def jumpy():
   Instruction(opname='COMPARE_OP', opcode=107, arg=4, argval='>', argrepr='>', offset=34, starts_line=None, is_jump_target=False),
   Instruction(opname='POP_JUMP_IF_FALSE', opcode=114, arg=21, argval=42, argrepr='to 42', offset=36, starts_line=None, is_jump_target=False),
   Instruction(opname='POP_TOP', opcode=1, arg=None, argval=None, argrepr='', offset=38, starts_line=8, is_jump_target=False),
-  Instruction(opname='JUMP_ABSOLUTE', opcode=113, arg=26, argval=52, argrepr='to 52', offset=40, starts_line=None, is_jump_target=False),
+  Instruction(opname='JUMP_FORWARD', opcode=110, arg=5, argval=52, argrepr='to 52', offset=40, starts_line=None, is_jump_target=False),
   Instruction(opname='JUMP_ABSOLUTE', opcode=113, arg=4, argval=8, argrepr='to 8', offset=42, starts_line=7, is_jump_target=True),
   Instruction(opname='LOAD_GLOBAL', opcode=116, arg=1, argval='print', argrepr='print', offset=44, starts_line=10, is_jump_target=True),
   Instruction(opname='LOAD_CONST', opcode=100, arg=4, argval='I can haz else clause?', argrepr="'I can haz else clause?'", offset=46, starts_line=None, is_jump_target=False),
@@ -1069,7 +1069,7 @@ def jumpy():
   Instruction(opname='LOAD_CONST', opcode=100, arg=2, argval=4, argrepr='4', offset=84, starts_line=None, is_jump_target=False),
   Instruction(opname='COMPARE_OP', opcode=107, arg=0, argval='<', argrepr='<', offset=86, starts_line=None, is_jump_target=False),
   Instruction(opname='POP_JUMP_IF_FALSE', opcode=114, arg=46, argval=92, argrepr='to 92', offset=88, starts_line=None, is_jump_target=False),
-  Instruction(opname='JUMP_ABSOLUTE', opcode=113, arg=52, argval=104, argrepr='to 104', offset=90, starts_line=17, is_jump_target=False),
+  Instruction(opname='JUMP_FORWARD', opcode=110, arg=6, argval=104, argrepr='to 104', offset=90, starts_line=17, is_jump_target=False),
   Instruction(opname='LOAD_FAST', opcode=124, arg=0, argval='i', argrepr='i', offset=92, starts_line=11, is_jump_target=True),
   Instruction(opname='POP_JUMP_IF_TRUE', opcode=115, arg=28, argval=56, argrepr='to 56', offset=94, starts_line=None, is_jump_target=False),
   Instruction(opname='LOAD_GLOBAL', opcode=116, arg=1, argval='print', argrepr='print', offset=96, starts_line=19, is_jump_target=True),

Misc/NEWS.d/next/Core and Builtins/2022-02-14-14-44-06.bpo-46724.jym_K6.rst

@@ -0,0 +1,2 @@
+Make sure that all backwards jumps use the ``JUMP_ABSOLUTE`` instruction, rather
+than ``JUMP_FORWARD`` with an argument of ``(2**32)+offset``.

Python/compile.c

@@ -128,6 +128,8 @@ typedef struct basicblock_ {
     unsigned b_nofallthrough : 1;
     /* Basic block exits scope (it ends with a return or raise) */
     unsigned b_exit : 1;
+    /* Used by compiler passes to mark whether they have visited a basic block. */
+    unsigned b_visited : 1;
     /* depth of stack upon entry of block, computed by stackdepth() */
     int b_startdepth;
     /* instruction offset for block, computed by assemble_jump_offsets() */
@@ -6701,6 +6703,31 @@ assemble_emit(struct assembler *a, struct instr *i)
     return 1;
 }
 
+static void
+normalize_jumps(struct assembler *a)
+{
+    for (basicblock *b = a->a_entry; b != NULL; b = b->b_next) {
+        b->b_visited = 0;
+    }
+    for (basicblock *b = a->a_entry; b != NULL; b = b->b_next) {
+        b->b_visited = 1;
+        if (b->b_iused == 0) {
+            continue;
+        }
+        struct instr *last = &b->b_instr[b->b_iused-1];
+        if (last->i_opcode == JUMP_ABSOLUTE) {
+            if (last->i_target->b_visited == 0) {
+                last->i_opcode = JUMP_FORWARD;
+            }
+        }
+        if (last->i_opcode == JUMP_FORWARD) {
+            if (last->i_target->b_visited == 1) {
+                last->i_opcode = JUMP_ABSOLUTE;
+            }
+        }
+    }
+}
+
 static void
 assemble_jump_offsets(struct assembler *a, struct compiler *c)
 {
@@ -7137,6 +7164,10 @@ assemble(struct compiler *c, int addNone)
     }
     propagate_line_numbers(&a);
     guarantee_lineno_for_exits(&a, c->u->u_firstlineno);
+
+    /* Order of basic blocks must have been determined by now */
+    normalize_jumps(&a);
+
     /* Can't modify the bytecode after computing jump offsets. */
     assemble_jump_offsets(&a, c);