forked from OSchip/llvm-project
aaad9f84be
disturbing the graph or having to update edges. This is motivated by porting argument promotion to the new pass manager. Because of how LLVM IR Function objects work, in order to change their signature a new object needs to be created. This is efficient and straight forward in the IR but previously was very hard to implement in LCG. We could easily replace the function a node in the graph represents. The challenging part is how to handle updating the edges in the graph. LCG previously used an edge to a raw function to represent a node that had not yet been scanned for calls and references. This was the core of its laziness. However, that model causes this kind of update to be very hard: 1) The keys to lookup an edge need to be `Function*`s that would all need to be updated when we update the node. 2) There will be some unknown number of edges that haven't transitioned from `Function*` edges to `Node*` edges. All of this complexity isn't necessary. Instead, we can always build a node around any function, always pointing edges at it and always using it as the key to lookup an edge. To maintain the laziness, we need to sink the *edges* of a node into a secondary object and explicitly model transitioning a node from empty to populated by scanning the function. This design seems much cleaner in a number of ways, but importantly there is now exactly *one* place where the `Function*` has to be updated! Some other cleanups that fall out of this include having something to model the *entry* edges more accurately. Rather than hand rolling parts of the node in the graph itself, we have an explicit `EdgeSequence` object that gives us exactly the functionality needed. We also have a consistent place to define the edge iterators and can use them for both the entry edges and the internal edges of the graph. The API used to model the separation between a node and its edges is intentionally very thin as most clients are expected to deal with nodes that have populated edges. We model this exactly as an optional does with an additional method to populate the edges when that is a reasonable thing for a client to do. This is based on API design suggestions from Richard Smith and David Blaikie, credit goes to them for helping pick how to model this without it being either too explicit or too implicit. The patch is somewhat noisy due to shifting around iterator types and new syntax for walking the edges of a node, but most of the functionality change is in the `Edge`, `EdgeSequence`, and `Node` types. Differential Revision: https://reviews.llvm.org/D29577 llvm-svn: 294653 |
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README.txt
Low Level Virtual Machine (LLVM) ================================ This directory and its subdirectories contain source code for LLVM, a toolkit for the construction of highly optimized compilers, optimizers, and runtime environments. LLVM is open source software. You may freely distribute it under the terms of the license agreement found in LICENSE.txt. Please see the documentation provided in docs/ for further assistance with LLVM, and in particular docs/GettingStarted.rst for getting started with LLVM and docs/README.txt for an overview of LLVM's documentation setup. If you are writing a package for LLVM, see docs/Packaging.rst for our suggestions.