[LLVMbugs] [Bug 16402] New: BlockFrequency underflow leads to slow code for 7zip

[bugzilla-daemon at llvm.org]

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: unassignedbugs at nondot.org
          Reporter: benny.kra at gmail.com
                CC: atrick at apple.com, llvmbugs at cs.uiuc.edu,
                    stoklund at 2pi.dk
    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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