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LLVM backend for 6502
86720f7c65
DAGCombiner::GatherAllAliases, which is only used when AA used is enabled during DAGCombine, had a fundamentally incorrect assumption for which this change compensates. GatherAllAliases, which is used to find aliasing predecessor chain nodes (so that a better chain can be selected for a load or store to enable subsequent optimizations) assumed that walking up the chain would always catch all possibly-aliasing loads and stores. This is not true: To really find all aliases, we also need to search for aliases through the value operand of a store, etc. Consider the following situation: Token1 = ... L1 = load Token1, %52 S1 = store Token1, L1, %51 L2 = load Token1, %52+8 S2 = store Token1, L2, %51+8 Token2 = Token(S1, S2) L3 = load Token2, %53 S3 = store Token2, L3, %52 L4 = load Token2, %53+8 S4 = store Token2, L4, %52+8 If we search for aliases of S3 (which loads address %52), and we look only through the chain, then we'll miss the trivial dependence on L1 (which loads from %52). We then might change all loads and stores to use Token1 as their chain operand, which could result in copying %53 into %52 before copying %52 into %51 (which should happen first). The problem is, however, that searching for such data dependencies can become expensive, and the cost is not directly related to the chain depth. Instead, we'll rule out such configurations by insisting that we've visited all chain users (except for users of the original chain, which is not necessary). When doing this, we need to look through nodes we don't care about (otherwise, things like register copies will interfere with trivial use cases). Unfortunately, I don't have a small test case for this problem. Creating the underlying situation is not hard (a pair of memcpys will do it), but arranging for the default instruction schedule to be incorrect is very fragile. This unbreaks self hosting on PPC64 when using -mllvm -combiner-global-alias-analysis -mllvm -combiner-alias-analysis. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200033 91177308-0d34-0410-b5e6-96231b3b80d8 |
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