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py: Modify bytecode "with" behaviour so it doesn't use any heap.

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Before this patch a "with" block needed to create a bound method object
on the heap for the __exit__ call.  Now it doesn't because we use
load_method instead of load_attr, and save the method+self on the stack.
This commit is contained in:
Damien George 2015-04-24 01:52:28 +01:00
parent ede0f3ab3d
commit 8c1d23a0e2
3 changed files with 50 additions and 33 deletions
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7
py/emitbc.c
View File

@ -680,12 +680,15 @@ void mp_emit_bc_unwind_jump(emit_t *emit, mp_uint_t label, mp_uint_t except_dept
}
void mp_emit_bc_setup_with(emit_t *emit, mp_uint_t label) {
emit_bc_pre(emit, 7);
// TODO We can probably optimise the amount of needed stack space, since
// we don't actually need 4 slots during the entire with block, only in
// the cleanup handler in certain cases. It needs some thinking.
emit_bc_pre(emit, 4);
emit_write_bytecode_byte_unsigned_label(emit, MP_BC_SETUP_WITH, label);
}
void mp_emit_bc_with_cleanup(emit_t *emit) {
emit_bc_pre(emit, -7);
emit_bc_pre(emit, -4);
emit_write_bytecode_byte(emit, MP_BC_WITH_CLEANUP);
}

74
py/vm.c
View File

@ -548,68 +548,82 @@ dispatch_loop:
ENTRY(MP_BC_SETUP_WITH): {
MARK_EXC_IP_SELECTIVE();
// stack: (..., ctx_mgr)
mp_obj_t obj = TOP();
SET_TOP(mp_load_attr(obj, MP_QSTR___exit__));
mp_load_method(obj, MP_QSTR___enter__, sp + 1);
mp_obj_t ret = mp_call_method_n_kw(0, 0, sp + 1);
mp_load_method(obj, MP_QSTR___exit__, sp);
mp_load_method(obj, MP_QSTR___enter__, sp + 2);
mp_obj_t ret = mp_call_method_n_kw(0, 0, sp + 2);
sp += 1;
PUSH_EXC_BLOCK(1);
PUSH(ret);
// stack: (..., __exit__, ctx_mgr, as_value)
DISPATCH();
}
ENTRY(MP_BC_WITH_CLEANUP): {
MARK_EXC_IP_SELECTIVE();
// Arriving here, there's "exception control block" on top of stack,
// and __exit__ bound method underneath it. Bytecode calls __exit__,
// and __exit__ method (with self) underneath it. Bytecode calls __exit__,
// and "deletes" it off stack, shifting "exception control block"
// to its place.
static const mp_obj_t no_exc[] = {mp_const_none, mp_const_none, mp_const_none};
if (TOP() == mp_const_none) {
sp--;
mp_obj_t obj = TOP();
// stack: (..., __exit__, ctx_mgr, None)
sp[1] = mp_const_none;
sp[2] = mp_const_none;
sp -= 2;
mp_call_method_n_kw(3, 0, sp);
SET_TOP(mp_const_none);
mp_call_function_n_kw(obj, 3, 0, no_exc);
} else if (MP_OBJ_IS_SMALL_INT(TOP())) {
mp_int_t cause_val = MP_OBJ_SMALL_INT_VALUE(TOP());
if (cause_val == UNWIND_RETURN) {
mp_call_function_n_kw(sp[-2], 3, 0, no_exc);
// stack: (..., __exit__, ctx_mgr, ret_val, UNWIND_RETURN)
mp_obj_t ret_val = sp[-1];
sp[-1] = mp_const_none;
sp[0] = mp_const_none;
sp[1] = mp_const_none;
mp_call_method_n_kw(3, 0, sp - 3);
sp[-3] = ret_val;
sp[-2] = MP_OBJ_NEW_SMALL_INT(UNWIND_RETURN);
} else {
assert(cause_val == UNWIND_JUMP);
mp_call_function_n_kw(sp[-3], 3, 0, no_exc);
// Pop __exit__ boundmethod at sp[-3]
sp[-3] = sp[-2];
// stack: (..., __exit__, ctx_mgr, dest_ip, num_exc, UNWIND_JUMP)
mp_obj_t dest_ip = sp[-2];
mp_obj_t num_exc = sp[-1];
sp[-2] = mp_const_none;
sp[-1] = mp_const_none;
sp[0] = mp_const_none;
mp_call_method_n_kw(3, 0, sp - 4);
sp[-4] = dest_ip;
sp[-3] = num_exc;
sp[-2] = MP_OBJ_NEW_SMALL_INT(UNWIND_JUMP);
}
sp[-2] = sp[-1]; // copy retval down
sp[-1] = sp[0]; // copy cause down
sp--; // discard top value (was cause)
sp -= 2; // we removed (__exit__, ctx_mgr)
} else {
assert(mp_obj_is_exception_type(TOP()));
// stack: (..., __exit__, ctx_mgr, traceback, exc_val, exc_type)
// Need to pass (sp[0], sp[-1], sp[-2]) as arguments so must reverse the
// order of these on the value stack (don't want to create a temporary
// array because it increases stack footprint of the VM).
mp_obj_t obj = sp[-2];
sp[-2] = sp[0];
sp[0] = obj;
mp_obj_t ret_value = mp_call_function_n_kw(sp[-3], 3, 0, &sp[-2]);
mp_obj_t ret_value = mp_call_method_n_kw(3, 0, sp - 4);
if (mp_obj_is_true(ret_value)) {
// This is what CPython does
//PUSH(MP_OBJ_NEW_SMALL_INT(UNWIND_SILENCED));
// But what we need to do is - pop exception from value stack...
// We need to silence/swallow the exception. This is done
// by popping the exception and the __exit__ handler and
// replacing it with None, which signals END_FINALLY to just
// execute the finally handler normally.
sp -= 4;
// ... pop "with" exception handler, and signal END_FINALLY
// to just execute finally handler normally (by pushing None
// on value stack)
SET_TOP(mp_const_none);
assert(exc_sp >= exc_stack);
POP_EXC_BLOCK();
PUSH(mp_const_none);
} else {
// Pop __exit__ boundmethod at sp[-3], remembering that top 3 values
// are reversed.
sp[-3] = sp[0];
obj = sp[-2];
sp[-2] = sp[-1];
sp[-1] = obj;
sp--;
// We need to re-raise the exception. We pop __exit__ handler
// and copy the 3 exception values down (remembering that they
// are reversed due to above code).
sp[-4] = sp[0];
sp[-3] = sp[-1];
sp -= 2;
}
}
DISPATCH();

2
tests/cmdline/cmd_showbc.py.exp
View File

@ -29,7 +29,7 @@ Raw bytecode (code_info_size=\\d\+, bytecode_size=\\d\+):
########
\.\+5b
arg names:
(N_STATE 25)
(N_STATE 22)
(N_EXC_STACK 2)
(INIT_CELL 14)
(INIT_CELL 15)