Currently, in PhaseCFG::hoist_to_cheaper_block(), StressGCM follows the GCM heuristic when the candidate block has strictly better frequency and only only randomizes tie-breaking among equally frequent blocks:
if (LCA_freq < least_freq || // Better Frequency
(StressGCM && C->randomized_select(cand_cnt)) || // Should be randomly accepted in stress mode
...
The randomization scope can be significantly extended by allowing GCM to move instructions to blocks with worse frequency at random:
if ((StressGCM && C->randomized_select(cand_cnt)) || // Should be randomly accepted in stress mode
(!StressGCM && LCA_freq < least_freq) || // Better Frequency
...
This would improve StressGCM's ability to expose bugs where C2 relies on GCM heuristics for correctness.
if (LCA_freq < least_freq || // Better Frequency
(StressGCM && C->randomized_select(cand_cnt)) || // Should be randomly accepted in stress mode
...
The randomization scope can be significantly extended by allowing GCM to move instructions to blocks with worse frequency at random:
if ((StressGCM && C->randomized_select(cand_cnt)) || // Should be randomly accepted in stress mode
(!StressGCM && LCA_freq < least_freq) || // Better Frequency
...
This would improve StressGCM's ability to expose bugs where C2 relies on GCM heuristics for correctness.
- duplicates
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JDK-8271118 C2: StressGCM should have higher priority than frequency-based policy
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- Resolved
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- is blocked by
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JDK-8258894 C2: Forbid GCM to move stores into loops
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- Resolved
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- Resolved
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- Resolved
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- relates to
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- Open
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- links to
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Review
openjdk/jdk/1469
(1 links to)