[PATCH] D15178: 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: dexonsmith, llvm-commits.

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.

The trick to process callees is to materialize function in the source
module when building the list of function to import, and inspect them
in their source module, collecting the list of callees for each
callee.

When we move the the actual import, we will import from each source
module exactly once. Each source module is loaded exactly once.
The only drawback it that it requires to have all the lazy-loaded
source Module in memory at the same time.

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  0m40.697s  user  2m10.237s sys  0m4.375s

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

http://reviews.llvm.org/D15178

Files:
  include/llvm/Transforms/IPO/FunctionImport.h
  lib/Transforms/IPO/FunctionImport.cpp

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