[PATCH] [CaptureTracking] Avoid long compilation time on large basic blocks

[Bruno Cardoso Lopes]

Hi nicholas, hfinkel,


=== Problem
For large basic blocks CaptureTracking may become very expensive: for each memory related instruction considered, walks top down the BB to check ordering and dominance among two instructions. In my testcase there are 81782 instructions in a BB, which leads to compilation time of ~12min w/ 'opt -O1'. This triggers in the flow: DeadStoreElimination -> MepDepAnalysis -> CaptureTracking.

=== Solution
To fix the compile time bloat, the patch changes 'shouldExplore' to do two things:
1) Add a special case to compute the ordering between instructions when both are in the same basic block. It avoids the use of two expensive functions in this scenario: 'dominates' and 'isPotentiallyReachable'. 
2) Limit the search by using a threshold=1000 on the number of instructions to search. This limit presented no measurable regressions on the test-suite.

This lead to a compile time reduction of 'opt -O1' from ~12min to 1s.

REPOSITORY
  rL LLVM

http://reviews.llvm.org/D7010

Files:
  include/llvm/Analysis/CaptureTracking.h
  lib/Analysis/CaptureTracking.cpp
  lib/Transforms/IPO/FunctionAttrs.cpp

EMAIL PREFERENCES
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