http://llvm.org/bugs/show_bug.cgi?id=16402
Bug ID: 16402
Summary: BlockFrequency underflow leads to slow code for 7zip
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Global Analyses
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected],
[email protected]
Classification: Unclassified
In LzmaEnc_CodeOneBlock in LzmaEnc.c from MultiSouce/Benchmarks/7zip the block
frequency analysis returns 0 frequencies for blocks in the inner loop. The
frequency calculation hits 0 at some point and never recovers. This made
r184105, the change that made spill weight depend on BlockFrequencyInfo,
regress 7zip by 10% due to excessive spilling. Simply raising
BlockFrequency::ENTRY_FREQ recovers the regression.
Can we do something about this? Increasing ENTRY_FREQ looks like a band aid to
me, we could hit this again any time.
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