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LLVM backend for 6502
48f7a2389e
The way that stack coloring updated MMOs when merging stack slots, while correct, is suboptimal, and is incompatible with the use of AA during instruction scheduling. The solution, which involves the use of const_cast (and more importantly, updating the IR from within an MI-level pass), obviously requires some explanation: When the stack coloring pass was originally committed, the code in ScheduleDAGInstrs::buildSchedGraph tracked possible alias sets by using GetUnderlyingObject, and all load/store and store/store memory control dependencies where added between SUs at the object level (where only one object, that returned by GetUnderlyingObject, was used to identify the object associated with each MMO). When stack coloring merged stack slots, it would replace MMOs derived from the remapped alloca with the alloca with which the remapped alloca was being replaced. Because ScheduleDAGInstrs only used single objects, and tracked alias sets at the object level, this was a fine solution. In r169744, (Andy and) I updated the code in ScheduleDAGInstrs to use GetUnderlyingObjects, and track alias sets using, potentially, multiple underlying objects for each MMO. This was done, primarily, to provide the ability to look through PHIs, and provide better scheduling for induction-variable-dependent loads and stores inside loops. At this point, the MMO-updating code in stack coloring became suboptimal, because it would clear the MMOs for (i.e. completely pessimize) all instructions for which r169744 might help in scheduling. Updating the IR directly is the simplest fix for this (and the one with, by far, the least compile-time impact), but others are possible (we could give each MMO a small vector of potential values, or make use of a remapping table, constructed from MFI, inside ScheduleDAGInstrs). Unfortunately, replacing all MMO values derived from the remapped alloca with the base replacement alloca fundamentally breaks our ability to use AA during instruction scheduling (which is critical to performance on some targets). The reason is that the original MMO might have had an offset (either constant or dynamic) from the base remapped alloca, and that offset is not present in the updated MMO. One possible way around this would be to use GetPointerBaseWithConstantOffset, and update not only the MMO's value, but also its offset based on the original offset. Unfortunately, this solution would only handle constant offsets, and for safety (because AA is not completely restricted to deducing relationships with constant offsets), we would need to clear all MMOs without constant offsets over the entire function. This would be an even worse pessimization than the current single-object restriction. Any other solution would involve passing around a vector of remapped allocas, and teaching AA to use it, introducing additional complexity and overhead into AA. Instead, when remapping an alloca, we replace all IR uses of that alloca as well (optionally inserting a bitcast as necessary). This is even more efficient that the old MMO-updating code in the stack coloring pass (because it removes the need to call GetUnderlyingObject on all MMO values), removes the single-object pessimization in the default configuration, and enables the correct use of AA during instruction scheduling (all without any additional overhead). LLVM now no longer miscompiles itself on x86_64 when using -enable-misched -enable-aa-sched-mi -misched-bottomup=0 -misched-topdown=0 -misched=shuffle! Fixed PR18497. Because the alloca replacement is now done at the IR level, unless the MMO directly refers to the remapped alloca, the change cannot be seen at the MI level. As a result, there is no good way to fix test/CodeGen/X86/pr14090.ll. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199658 91177308-0d34-0410-b5e6-96231b3b80d8 |
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