[PATCH] D15176: FunctionImporter: implement bulk function importing for efficiency

[Mehdi AMINI via llvm-commits]

joker.eph created this revision.
joker.eph added a reviewer: tejohnson.
joker.eph added subscribers: llvm-commits, dexonsmith.

The current importing scheme is processing one function at a time,
loading the source Module, linking the function in the destination
module, and destroying the source Module before repeating with the
next function to import (potentially from the same Module).

Ideally we would keep the source Module alive and import the next
Function needed from this Module. Unfortunately this is not possible
because the linker does not leave it in a usable state.

However we can do better by first computing the list of all candidates
per Module, and only then load the source Module and import all the
function we need for it.

We still need to repeat the process for callees of the imported
Function. This is avoidable with another alternative scheme where
we would load the source Module, materialize the Function, and
add the callees to the Worklist without actually importing the
Function. The import would take place in the end when we're done
with computing the import set.

Currently this patch already improves considerably the link time,
a multithreaded link of llvm-dis on my laptop was:

  real  1m12.175s  user  6m32.430s sys  0m10.529s

and is now:

  real  0m47.400s  user  3m1.551s  sys  0m5.825s

Note: this is the full link time (linker+Import+Optimizer+CodeGen)

http://reviews.llvm.org/D15176

Files:
  lib/Transforms/IPO/FunctionImport.cpp

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