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LLVM backend for 6502
77c1270033
a pair of instructions, one for the used pointer and the second for the user. This simplifies the representation and also makes it more dense. This was noticed because of the miscompile in PR13926. In that case, we were running up against a fundamental "bad idea" in the speculation of PHI and select instructions: the speculation and rewriting are interleaved, which requires phi speculation to also perform load rewriting! This is bad, and causes us to miss opportunities to do (for example) vector rewriting only exposed after PHI speculation, etc etc. It also, in the old system, required us to insert *new* load uses into the current partition's use list, which would then be ignored during rewriting because we had already extracted an end iterator for the use list. The appending behavior (and much of the other oddities) stem from the strange de-duplication strategy in the PartitionUse builder. Amusingly, all this went without notice for so long because it could only be triggered by having *different* GEPs into the same partition of the same alloca, where both different GEPs were operands of a single PHI, and where the GEP which was not encountered first also had multiple uses within that same PHI node... Hence the insane steps required to reproduce. So, step one in fixing this fundamental bad idea is to make the PartitionUse actually contain a Use*, and to make the builder do proper deduplication instead of funky de-duplication. This is enough to remove the appending behavior, and fix the miscompile in PR13926, but there is more work to be done here. Subsequent commits will lift the speculation into its own visitor. It'll be a useful step toward potentially extracting all of the speculation logic into a generic utility transform. The existing PHI test case for repeated operands has been made more extreme to catch even these issues. This test case, run through the old pass, will exactly reproduce the miscompile from PR13926. ;] We were so close here! git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@164925 91177308-0d34-0410-b5e6-96231b3b80d8 |
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