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limited batch updates. Specifically, allow removing multiple reference edges starting from a common source node. There are a few constraints that play into supporting this form of batching: 1) The way updates occur during the CGSCC walk, about the most we can functionally batch together are those with a common source node. This also makes the batching simpler to implement, so it seems a worthwhile restriction. 2) The far and away hottest function for large C++ files I measured (generated code for protocol buffers) showed a huge amount of time was spent removing ref edges specifically, so it seems worth focusing there. 3) The algorithm for removing ref edges is very amenable to this restricted batching. There are just both API and implementation special casing for the non-batch case that gets in the way. Once removed, supporting batches is nearly trivial. This does modify the API in an interesting way -- now, we only preserve the target RefSCC when the RefSCC structure is unchanged. In the face of any splits, we create brand new RefSCC objects. However, all of the users were OK with it that I could find. Only the unittest needed interesting updates here. How much does batching these updates help? I instrumented the compiler when run over a very large generated source file for a protocol buffer and found that the majority of updates are intrinsically updating one function at a time. However, nearly 40% of the total ref edges removed are removed as part of a batch of removals greater than one, so these are the cases batching can help with. When compiling the IR for this file with 'opt' and 'O3', this patch reduces the total time by 8-9%. Differential Revision: https://reviews.llvm.org/D36352 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@310450 91177308-0d34-0410-b5e6-96231b3b80d8