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

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* 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
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Om G 2020-12-16 17:48:05 +05:30 • committed by GitHub
parent 8203c73f3b
commit c71581c7a4
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5 changed files with 174 additions and 101 deletions
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42
Python/compile.c
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@ -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;
}