https://bugs.llvm.org/show_bug.cgi?id=47746
Bug ID: 47746
Summary: MergeBlockIntoPredecessor in UnrollLoop is very slow
for sufficiently complicated loops
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Transformation Utilities
Assignee: unassignedb...@nondot.org
Reporter: ctetr...@quicinc.com
CC: llvm-bugs@lists.llvm.org
UnrollLoop in Transforms/Utils/LoopUnroll.cpp calls MergeBlockIntoPredecessor
for each of the latch basic blocks. MergeBlockIntoPredecessor calls
RemoveRedundantDbgInstrs, which iterates over the instructions of the basic
block twice. For sufficiently complicated loops, this can cause extremely long
compile times.
We're seeing > 10 minute compile times on internal codebases. Unfortunately, I
cannot share any code for these cases. Running in a profiler, we're seeing ~98%
of the runtime inside of RemoveRedundantDbgInstrs.
I believe it might be possible add a flag to MergeBlockIntoPredecessor skip the
call to RemoveRedundantDbgInstrs, then call RemoveRedundantDbgInstrs for each
basic block inside of simplifyLoopAfterUnroll, however I am not familiar enough
with this code to know for certain there will be no unintended consequences. I
experimented with this in my downstream, and it reduced the runtime from 10
minutes to 4 seconds.
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