[PATCH] D36109: [CodeGen] Provide an advanced shrink-wrapping interface

[Francis Visoiu Mistrih via Phabricator via llvm-commits]

thegameg created this revision.
Herald added a subscriber: mgorny.

Based on Minimizing Register Usage Penalty at Procedure Calls - Fred C. Chow <http://dl.acm.org/citation.cfm?id=53999>, this is an implementation of a shrink-wrapping algorithm supporting multiple save / restore blocks.

The algorithm is meant to provide a shrink-wrapping interface to any kind of input. We can think of callee-saved registers, stack usage, life_range markers, etc. as input data, and run the shrink-wrapper in order to find the most efficient save / restore points.

The `ShrinkWrapInfo` class is meant to be subclassed by the users of this algorithm, in order to provide more details.

- Loops are handled using `scc_iterators` in order to handle irreducible loops and avoid placing any save / restore instruction inside a loop.
- Uses on no-return paths will be ignored and won't affect the final save / restore points.
- A "shrink-wrapping cost" is used to determine if the final placement is better than the default one (entry block, return blocks). This cost is nothing more than `NumberOfSavesAndRestores * BlockFrequency * 100`.
- There is a verifier running only when `EXPENSIVE_CHECKS` is on, checking that all the paths are correctly handled, that there are no saves without restores, no restores without saves, and no nested saves of the same element.

This is a high-level algorithm, without any target-specific implementation.


https://reviews.llvm.org/D36109

Files:
  include/llvm/ADT/PostOrderIterator.h
  include/llvm/CodeGen/ShrinkWrapper.h
  lib/CodeGen/CMakeLists.txt
  lib/CodeGen/ShrinkWrapper.cpp

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