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LLVM backend for 6502
bfed08e41f
these DAG combines. The DAG auto-CSE thing is truly terrible. Due to it, when RAUW-ing a node with its operand, you can cause its uses to CSE to itself, which then causes their uses to become your uses which causes them to be picked up by the RAUW. For nodes that are determined to be "no-ops", this is "fine". But if the RAUW is one of several steps to enact a transformation, this causes the DAG to really silently eat an discard nodes that you would never expect. It took days for me to actually pinpoint a test case triggering this and a really frustrating amount of time to even comprehend the bug because I never even thought about the ability of RAUW to iteratively consume nodes due to CSE-ing them into itself. To fix this, we have to build up a brand-new chain of operations any time we are combining across (potentially) intervening nodes. But once the logic is added to do this, another issue surfaces: CombineTo eagerly deletes the one node combined, *but no others*. This is... really frustrating. If deleting it makes its operands become dead, those operand nodes often won't go onto the worklist in the order you would want -- they're already on it and not near the top. That means things higher on the worklist will get combined prior to these dead nodes being GCed out of the worklist, and if the chain is long, the immediate users won't be enough to re-detect where the root of the chain is that became single-use again after deleting the dead nodes. The better way to do this is to never immediately delete nodes, and instead to just enqueue them so we can recursively delete them. The combined-from node is typically not on the worklist anyways by virtue of having been popped off.... But that in turn breaks other tests that *require* CombineTo to delete unused nodes. :: sigh :: Fortunately, there is a better way. This whole routine should have been returning the replacement rather than using CombineTo which is quite hacky. Switch to that, and all the pieces fall together. I suspect the same kind of miscompile is possible in the half-shuffle folding code, and potentially the recursive folding code. I'll be switching those over to a pattern more like this one for safety's sake even though I don't immediately have any test cases for them. Note that the only way I got a test case for this instance was with *heavily* DAG combined 256-bit shuffle sequences generated by my fuzzer. ;] git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216319 91177308-0d34-0410-b5e6-96231b3b80d8 |
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