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gh-107557: Remove unnecessary SAVE_IP instructions (#108583)

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Also remove NOP instructions.

The "stubs" are not optimized in this fashion (their SAVE_IP should always be preserved since it's where to jump next, and they don't contain NOPs by their nature).
This commit is contained in:
Guido van Rossum 2023-08-29 09:51:51 -07:00 • committed by GitHub
parent 83e191ba76
commit 4f22152713
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GPG key ID: 4AEE18F83AFDEB23
5 changed files with 392 additions and 231 deletions
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140
Python/optimizer.c
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@ -372,6 +372,32 @@ static PyTypeObject UOpExecutor_Type = {
.tp_as_sequence = &uop_as_sequence,
};
static int
move_stubs(
_PyUOpInstruction *trace,
int trace_length,
int stubs_start,
int stubs_end
)
{
memmove(trace + trace_length,
trace + stubs_start,
(stubs_end - stubs_start) * sizeof(_PyUOpInstruction));
// Patch up the jump targets
for (int i = 0; i < trace_length; i++) {
if (trace[i].opcode == _POP_JUMP_IF_FALSE ||
trace[i].opcode == _POP_JUMP_IF_TRUE)
{
int target = trace[i].oparg;
if (target >= stubs_start) {
target += trace_length - stubs_start;
trace[i].oparg = target;
}
}
}
return trace_length + stubs_end - stubs_start;
}
#define TRACE_STACK_SIZE 5
static int
@ -730,36 +756,31 @@ done:
if (trace_length > 3) {
ADD_TO_TRACE(EXIT_TRACE, 0, 0);
DPRINTF(1,
"Created a trace for %s (%s:%d) at byte offset %d -- length %d\n",
"Created a trace for %s (%s:%d) at byte offset %d -- length %d+%d\n",
PyUnicode_AsUTF8(code->co_qualname),
PyUnicode_AsUTF8(code->co_filename),
code->co_firstlineno,
2 * INSTR_IP(initial_instr, code),
trace_length);
if (max_length < buffer_size && trace_length < max_length) {
// Move the stubs back to be immediately after the main trace
// (which ends at trace_length)
DPRINTF(2,
"Moving %d stub uops back by %d\n",
buffer_size - max_length,
max_length - trace_length);
memmove(trace + trace_length,
trace + max_length,
(buffer_size - max_length) * sizeof(_PyUOpInstruction));
// Patch up the jump targets
for (int i = 0; i < trace_length; i++) {
if (trace[i].opcode == _POP_JUMP_IF_FALSE ||
trace[i].opcode == _POP_JUMP_IF_TRUE)
{
int target = trace[i].oparg;
if (target >= max_length) {
target += trace_length - max_length;
trace[i].oparg = target;
}
}
trace_length,
buffer_size - max_length);
if (max_length < buffer_size) {
// There are stubs
if (trace_length < max_length) {
// There's a gap before the stubs
// Move the stubs back to be immediately after the main trace
// (which ends at trace_length)
DPRINTF(2,
"Moving %d stub uops back by %d\n",
buffer_size - max_length,
max_length - trace_length);
trace_length = move_stubs(trace, trace_length, max_length, buffer_size);
}
else {
assert(trace_length == max_length);
// There's no gap
trace_length = buffer_size;
}
}
trace_length += buffer_size - max_length;
return trace_length;
}
else {
@ -779,6 +800,76 @@ done:
#undef DPRINTF
}
static int
remove_unneeded_uops(_PyUOpInstruction *trace, int trace_length)
{
// Stage 1: Replace unneeded SAVE_IP uops with NOP.
// Note that we don't enter stubs, those SAVE_IPs are needed.
int last_save_ip = -1;
int last_instr = 0;
bool need_ip = true;
for (int pc = 0; pc < trace_length; pc++) {
int opcode = trace[pc].opcode;
if (opcode == SAVE_CURRENT_IP) {
// Special case: never remove preceding SAVE_IP
last_save_ip = -1;
}
else if (opcode == SAVE_IP) {
if (!need_ip && last_save_ip >= 0) {
trace[last_save_ip].opcode = NOP;
}
need_ip = false;
last_save_ip = pc;
}
else if (opcode == JUMP_TO_TOP || opcode == EXIT_TRACE) {
last_instr = pc + 1;
break;
}
else {
// If opcode has ERROR or DEOPT, set need_up to true
if (_PyOpcode_opcode_metadata[opcode].flags & (HAS_ERROR_FLAG | HAS_DEOPT_FLAG)) {
need_ip = true;
}
}
}
// Stage 2: Squash NOP opcodes (pre-existing or set above).
int dest = 0;
for (int pc = 0; pc < last_instr; pc++) {
int opcode = trace[pc].opcode;
if (opcode != NOP) {
if (pc != dest) {
trace[dest] = trace[pc];
}
dest++;
}
}
// Stage 3: Move the stubs back.
if (dest < last_instr) {
int new_trace_length = move_stubs(trace, dest, last_instr, trace_length);
#ifdef Py_DEBUG
char *uop_debug = Py_GETENV("PYTHONUOPSDEBUG");
int lltrace = 0;
if (uop_debug != NULL && *uop_debug >= '0') {
lltrace = *uop_debug - '0'; // TODO: Parse an int and all that
}
if (lltrace >= 2) {
printf("Optimized trace (length %d+%d = %d, saved %d):\n",
dest, trace_length - last_instr, new_trace_length,
trace_length - new_trace_length);
for (int pc = 0; pc < new_trace_length; pc++) {
printf("%4d: (%s, %d, %" PRIu64 ")\n",
pc,
uop_name(trace[pc].opcode),
(trace[pc].oparg),
(uint64_t)(trace[pc].operand));
}
}
#endif
trace_length = new_trace_length;
}
return trace_length;
}
static int
uop_optimize(
_PyOptimizerObject *self,
@ -798,6 +889,7 @@ uop_optimize(
if (uop_optimize != NULL && *uop_optimize > '0') {
trace_length = _Py_uop_analyze_and_optimize(code, trace, trace_length, curr_stackentries);
}
trace_length = remove_unneeded_uops(trace, trace_length);
_PyUOpExecutorObject *executor = PyObject_NewVar(_PyUOpExecutorObject, &UOpExecutor_Type, trace_length);
if (executor == NULL) {
return -1;