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GH-118093: Don't lose confidence when tracing through 100% biased branches #124813

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merged 5 commits into from
Oct 2, 2024
Merged

GH-118093: Don't lose confidence when tracing through 100% biased branches #124813

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8b40e19
Use Laplace's rule of succession for trace confidence
brandtbucher Sep 6, 2024
130eede
Clarify exact value
brandtbucher Sep 6, 2024
18cf547
Don't assume the future is uncertain
brandtbucher Sep 7, 2024
6c156af
blurb add
brandtbucher Sep 30, 2024
4e14b5c
Remove weird comment
brandtbucher Oct 2, 2024
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2024-09-30-16-39-37.gh-issue-118093.J2A3gz.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Improve the experimental JIT compiler's ability to stay "on trace" when
encountering highly-biased branches.
6 changes: 2 additions & 4 deletions Python/optimizer.c
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Original file line number Diff line number Diff line change
Expand Up @@ -643,14 +643,12 @@ translate_bytecode_to_trace(
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 + 2) / 20;
confidence = confidence * bitcount / 16;
}
else {
confidence = confidence * (18 - bitcount) / 20;
confidence = confidence * (16 - bitcount) / 16;
}
uint32_t uopcode = BRANCH_TO_GUARD[opcode - POP_JUMP_IF_FALSE][jump_likely];
DPRINTF(2, "%d: %s(%d): counter=%04x, bitcount=%d, likely=%d, confidence=%d, uopcode=%s\n",
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