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gh-112320: Implement on-trace confidence tracking for branches (#112321)

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We track the confidence as a scaled int.
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Guido van Rossum 2023-12-12 13:43:08 -08:00 • committed by GitHub
parent dfaa9e060b
commit 7316dfb0eb
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6 changed files with 56 additions and 3 deletions
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20
Python/optimizer.c
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@ -409,6 +409,9 @@ BRANCH_TO_GUARD[4][2] = {
#define TRACE_STACK_SIZE 5
#define CONFIDENCE_RANGE 1000
#define CONFIDENCE_CUTOFF 333
/* Returns 1 on success,
* 0 if it failed to produce a worthwhile trace,
* and -1 on an error.
@ -431,6 +434,7 @@ translate_bytecode_to_trace(
_Py_CODEUNIT *instr;
} trace_stack[TRACE_STACK_SIZE];
int trace_stack_depth = 0;
int confidence = CONFIDENCE_RANGE; // Adjusted by branch instructions
#ifdef Py_DEBUG
char *python_lltrace = Py_GETENV("PYTHON_LLTRACE");
@ -513,7 +517,6 @@ top: // Jump here after _PUSH_FRAME or likely branches
uint32_t oparg = instr->op.arg;
uint32_t extras = 0;
if (opcode == EXTENDED_ARG) {
instr++;
extras += 1;
@ -543,11 +546,22 @@ top: // Jump here after _PUSH_FRAME or likely branches
int counter = instr[1].cache;
int bitcount = _Py_popcount32(counter);
int jump_likely = bitcount > 8;
if (jump_likely) {
confidence = confidence * bitcount / 16;
}
else {
confidence = confidence * (16 - bitcount) / 16;
}
if (confidence < CONFIDENCE_CUTOFF) {
DPRINTF(2, "Confidence too low (%d)\n", confidence);
OPT_STAT_INC(low_confidence);
goto done;
}
uint32_t uopcode = BRANCH_TO_GUARD[opcode - POP_JUMP_IF_FALSE][jump_likely];
_Py_CODEUNIT *next_instr = instr + 1 + _PyOpcode_Caches[_PyOpcode_Deopt[opcode]];
DPRINTF(4, "%s(%d): counter=%x, bitcount=%d, likely=%d, uopcode=%s\n",
DPRINTF(2, "%s(%d): counter=%x, bitcount=%d, likely=%d, confidence=%d, uopcode=%s\n",
_PyUOpName(opcode), oparg,
counter, bitcount, jump_likely, _PyUOpName(uopcode));
counter, bitcount, jump_likely, confidence, _PyUOpName(uopcode));
ADD_TO_TRACE(uopcode, max_length, 0, target);
if (jump_likely) {
_Py_CODEUNIT *target_instr = next_instr + oparg;

1
Python/specialize.c
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@ -233,6 +233,7 @@ print_optimization_stats(FILE *out, OptimizationStats *stats)
fprintf(out, "Optimization trace too short: %" PRIu64 "\n", stats->trace_too_short);
fprintf(out, "Optimization inner loop: %" PRIu64 "\n", stats->inner_loop);
fprintf(out, "Optimization recursive call: %" PRIu64 "\n", stats->recursive_call);
fprintf(out, "Optimization low confidence: %" PRIu64 "\n", stats->low_confidence);
print_histogram(out, "Trace length", stats->trace_length_hist);
print_histogram(out, "Trace run length", stats->trace_run_length_hist);