bpo-42615: Delete redundant jump instructions that only bypass empty blocks (GH-23733)

* Delete jump instructions that bypass empty blocks

* Add news entry

* Explicitly check for unconditional jump opcodes

Using the is_jump function results in the inclusion of instructions like
returns for which this optimization is not really valid. So, instead
explicitly check that the instruction is an unconditional jump.

* Handle conditional jumps, delete jumps gracefully

* Ensure b_nofallthrough and b_reachable are valid

* Add test for redundant jumps

* Regenerate importlib.h and edit Misc/ACKS

* Fix bad whitespace

1 files changed, 42 insertions, 0 deletions

diff --git a/Python/compile.c b/Python/compile.c
index ae92869..4792733 100644
--- a/Python/compile.c
+++ b/Python/compile.c
@@ -6437,8 +6437,50 @@ optimize_cfg(struct assembler *a, PyObject *consts)
     for (basicblock *b = a->a_entry; b != NULL; b = b->b_next) {
        if (b->b_reachable == 0) {
             b->b_iused = 0;
+            b->b_nofallthrough = 0;
        }
     }
+    /* Delete jump instructions made redundant by previous step. If a non-empty
+       block ends with a jump instruction, check if the next non-empty block
+       reached through normal flow control is the target of that jump. If it
+       is, then the jump instruction is redundant and can be deleted.
+    */
+    for (basicblock *b = a->a_entry; b != NULL; b = b->b_next) {
+        if (b->b_iused > 0) {
+            struct instr *b_last_instr = &b->b_instr[b->b_iused - 1];
+            if (b_last_instr->i_opcode == POP_JUMP_IF_FALSE ||
+                b_last_instr->i_opcode == POP_JUMP_IF_TRUE ||
+                b_last_instr->i_opcode == JUMP_ABSOLUTE ||
+                b_last_instr->i_opcode == JUMP_FORWARD) {
+                basicblock *b_next_act = b->b_next;
+                while (b_next_act != NULL && b_next_act->b_iused == 0) {
+                    b_next_act = b_next_act->b_next;
+                }
+                if (b_last_instr->i_target == b_next_act) {
+                    b->b_nofallthrough = 0;
+                    switch(b_last_instr->i_opcode) {
+                        case POP_JUMP_IF_FALSE:
+                        case POP_JUMP_IF_TRUE:
+                            b_last_instr->i_opcode = POP_TOP;
+                            b_last_instr->i_target = NULL;
+                            b_last_instr->i_oparg = 0;
+                            break;
+                        case JUMP_ABSOLUTE:
+                        case JUMP_FORWARD:
+                            b_last_instr->i_opcode = NOP;
+                            clean_basic_block(b);
+                            break;
+                    }
+                    /* The blocks after this one are now reachable through it */
+                    b_next_act = b->b_next;
+                    while (b_next_act != NULL && b_next_act->b_iused == 0) {
+                        b_next_act->b_reachable = 1;
+                        b_next_act = b_next_act->b_next;
+                    }
+                }
+            }
+        }
+    }
     minimize_lineno_table(a);
     return 0;
 }