8000 gh-115727: Reduce confidence even on 100% predicted jumps by gvanrossum · Pull Request #115748 · python/cpython · GitHub
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gh-115727: Reduce confidence even on 100% predicted jumps #115748

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Merged
merged 3 commits into from
Feb 22, 2024
Merged

gh-115727: Reduce confidence even on 100% predicted jumps #115748

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c1583b1
Improve debug logging for confidence
gvanrossum Feb 20, 2024
2537f75
Be less confident about jumps always/never taken
gvanrossum Feb 20, 2024
cdf96e2
Improve trace construction debug output
gvanrossum Feb 21, 2024
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20 changes: 13 additions & 7 deletions Python/optimizer.c
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Original file line number Diff line number Diff line change
Expand Up @@ -546,6 +546,8 @@ translate_bytecode_to_trace(
uint32_t oparg = instr->op.arg;
uint32_t extended = 0;

DPRINTF(3, "%d: %s(%d)\n", target, _PyOpcode_OpName[opcode], oparg);

if (opcode == ENTER_EXECUTOR) {
assert(oparg < 256);
_PyExecutorObject *executor = code->co_executors->executors[oparg];
Expand Down Expand Up @@ -593,21 +595,25 @@ translate_bytecode_to_trace(
int counter = instr[1].cache;
int bitcount = _Py_popcount32(counter);
int jump_likely = bitcount > 8;
/* If bitcount is 8 (half the jumps were taken), adjust confidence by 50%.
If it's 16 or 0 (all or none were taken), adjust by 10%
(since the future is still somewhat uncertain).
For values in between, adjust proportionally. */
if (jump_likely) {
confidence = confidence * bitcount / 16;
confidence = confidence * (bitcount + 2) / 20;
}
else {
confidence = confidence * (16 - bitcount) / 16;
confidence = confidence * (18 - bitcount) / 20;
}
uint32_t uopcode = BRANCH_TO_GUARD[opcode - POP_JUMP_IF_FALSE][jump_likely];
DPRINTF(2, "%d: %s(%d): counter=%x, bitcount=%d, likely=%d, confidence=%d, uopcode=%s\n",
target, _PyOpcode_OpName[opcode], oparg,
counter, bitcount, jump_likely, confidence, _PyUOpName(uopcode));
if (confidence < CONFIDENCE_CUTOFF) {
DPRINTF(2, "Confidence too low (%d)\n", confidence);
DPRINTF(2, "Confidence too low (%d < %d)\n", confidence, CONFIDENCE_CUTOFF);
OPT_STAT_INC(low_confidence);
goto done;
}
uint32_t uopcode = BRANCH_TO_GUARD[opcode - POP_JUMP_IF_FALSE][jump_likely];
DPRINTF(2, "%s(%d): counter=%x, bitcount=%d, likely=%d, confidence=%d, uopcode=%s\n",
_PyOpcode_OpName[opcode], oparg,
counter, bitcount, jump_likely, confidence, _PyUOpName(uopcode));
_Py_CODEUNIT *next_instr = instr + 1 + _PyOpcode_Caches[_PyOpcode_Deopt[opcode]];
_Py_CODEUNIT *target_instr = next_instr + oparg;
if (jump_likely) {
Expand Down
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