[PATCH] D23299: [PM] Port the always inliner to the new pass manager in a much more minimal and boring form than the old pass manager's version.

[Chandler Carruth via llvm-commits]

chandlerc created this revision.
chandlerc added a subscriber: llvm-commits.
Herald added a reviewer: tstellarAMD.
Herald added subscribers: mcrosier, arsenm.

This pass does the very minimal amount of work necessary to inline
functions declared as always-inline. It doesn't support a wide array of
things that the legacy pass manager did support, but is alse ... about
20 lines of code. So it has that going for it. Notably things this
doesn't support:

- Array alloca merging
  - To support the above, bottom-up inlining with careful history
    tracking and call graph updates
- DCE of the functions that become dead after this inlining.
- Inlining through call instructions with the always_inline attribute.
  Instead, it focuses on inlining functions with that attribute.

The first I've omitted because I'm hoping to just turn it off for the
primary pass manager. If that doesn't pan out, I can add it here but it
will be reasonably expensive to do so.

The second should really be handled by running global-dce after the
inliner. I don't want to re-implement the non-trivial logic necessary to
do comdat-correct DCE of functions. This means the -O0 pipeline will
have to be at least 'always-inline,global-dce', but that seems
reasonable to me. If others are seriously worried about this I'd like to
heard and understand why. Again, this is all solveable by factoring that
logic into a utility and calling it here, but I'd like to wait to do
that until there is a clear reason why the existing pass-based factoring
won't work.

The final point is a serious one. I can fairly easily add support for
this, but it seems both costly and a confusing construct for the use
case of the always inliner running at O0. This attribute can of course
still impact the normal inliner easily (although I find that
a questionable re-use of the same attribute). I've started a discussion
to sort out what semantics we want here and based on that can figure out
if it makes sense ta have this complexity at O0 or not.

One other advantage of this design is that it should be quite a bit
faster due to checking for whether the function is a viable candidate
for inlining exactly once per function instead of doing it for each call
site.

Anyways, hopefully a reasonable starting point for this pass.

https://reviews.llvm.org/D23299

Files:
  include/llvm/InitializePasses.h
  include/llvm/LinkAllPasses.h
  include/llvm/Transforms/IPO.h
  include/llvm/Transforms/IPO/AlwaysInliner.h
  lib/Passes/PassBuilder.cpp
  lib/Passes/PassRegistry.def
  lib/Target/AMDGPU/AMDGPUTargetMachine.cpp
  lib/Transforms/IPO/AlwaysInliner.cpp
  lib/Transforms/IPO/CMakeLists.txt
  lib/Transforms/IPO/IPO.cpp
  lib/Transforms/IPO/InlineAlways.cpp
  test/Transforms/Inline/always-inline.ll
  tools/bugpoint/bugpoint.cpp
  tools/opt/opt.cpp

-------------- next part --------------
A non-text attachment was scrubbed...
Name: D23299.67299.patch
Type: text/x-patch
Size: 14565 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20160809/c2afc799/attachment.bin>