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py: Fix optimised for-loop compiler so it follows proper semantics. 

You can now assign to the range end variable and the for-loop still
works correctly.  This fully addresses issue #565.

Also fixed a bug with the stack not being fully popped when breaking out
of an optimised for-loop (and it's actually impossible to write a test
for this case!).
This commit is contained in:
Damien George 2014-12-12 17:19:56 +00:00
parent 7764f163fa
commit e181c0dc07
2 changed files with 43 additions and 19 deletions
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54
py/compile.c
View File

@ -1776,26 +1776,40 @@ STATIC void compile_while_stmt(compiler_t *comp, mp_parse_node_struct_t *pns) {
}
#if !MICROPY_EMIT_CPYTHON
// TODO preload end and step onto stack if they are not constants
// Note that, as per semantics of for .. range, the final failing value should not be stored in the loop variable
// And, if the loop never runs, the loop variable should never be assigned
// This function compiles an optimised for-loop of the form:
// for <var> in range(<start>, <end>, <step>):
// <body>
// else:
// <else>
// <var> must be an identifier and <step> must be a small-int.
//
// Semantics of for-loop require:
// - final failing value should not be stored in the loop variable
// - if the loop never runs, the loop variable should never be assigned
// - assignments to <var>, <end> or <step> in the body do not alter the loop
// (<step> is a constant for us, so no need to worry about it changing)
//
// If <end> is a small-int, then the stack during the for-loop contains just
// the current value of <var>. Otherwise, the stack contains <end> then the
// current value of <var>.
STATIC void compile_for_stmt_optimised_range(compiler_t *comp, mp_parse_node_t pn_var, mp_parse_node_t pn_start, mp_parse_node_t pn_end, mp_parse_node_t pn_step, mp_parse_node_t pn_body, mp_parse_node_t pn_else) {
START_BREAK_CONTINUE_BLOCK
// note that we don't need to pop anything when breaking from an optimise for loop
uint top_label = comp_next_label(comp);
uint entry_label = comp_next_label(comp);
// compile: start, duplicated on stack
// put the end value on the stack if it's not a small-int constant
bool end_on_stack = !MP_PARSE_NODE_IS_SMALL_INT(pn_end);
if (end_on_stack) {
compile_node(comp, pn_end);
}
// compile: start
compile_node(comp, pn_start);
EMIT(dup_top);
EMIT_ARG(jump, entry_label);
EMIT_ARG(label_assign, top_label);
// at this point we actually have 1 less element on the stack
EMIT_ARG(adjust_stack_size, -1);
// duplicate next value and store it to var
EMIT(dup_top);
c_assign(comp, pn_var, ASSIGN_STORE);
@ -1805,15 +1819,20 @@ STATIC void compile_for_stmt_optimised_range(compiler_t *comp, mp_parse_node_t p
EMIT_ARG(label_assign, continue_label);
// compile: var + step, duplicated on stack
// compile: var + step
compile_node(comp, pn_step);
EMIT_ARG(binary_op, MP_BINARY_OP_INPLACE_ADD);
EMIT(dup_top);
EMIT_ARG(label_assign, entry_label);
// compile: if var <cond> end: goto top
compile_node(comp, pn_end);
if (end_on_stack) {
EMIT(dup_top_two);
EMIT(rot_two);
} else {
EMIT(dup_top);
compile_node(comp, pn_end);
}
assert(MP_PARSE_NODE_IS_SMALL_INT(pn_step));
if (MP_PARSE_NODE_LEAF_SMALL_INT(pn_step) >= 0) {
EMIT_ARG(binary_op, MP_BINARY_OP_LESS);
@ -1822,15 +1841,20 @@ STATIC void compile_for_stmt_optimised_range(compiler_t *comp, mp_parse_node_t p
}
EMIT_ARG(pop_jump_if_true, top_label);
// discard final value of var that failed the loop condition
EMIT(pop_top);
// break/continue apply to outer loop (if any) in the else block
END_BREAK_CONTINUE_BLOCK
compile_node(comp, pn_else);
EMIT_ARG(label_assign, break_label);
// discard final value of var that failed the loop condition
EMIT(pop_top);
// discard <end> value if it's on the stack
if (end_on_stack) {
EMIT(pop_top);
}
}
#endif

8
tests/basics/for3.py
View File

@ -5,7 +5,7 @@ for i in range(2):
# test assigning to range parameter within the loop
# (since we optimise for loops, this needs checking, currently it fails)
#n = 2
#for i in range(n):
# print(i)
# n = 0
n = 2
for i in range(n):
print(i)
n = 0