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LLVM backend for 6502
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------------------------------------------------------------------------ r214481 | hfinkel | 2014-07-31 22:20:41 -0700 (Thu, 31 Jul 2014) | 38 lines [PowerPC] Generate unaligned vector loads using intrinsics instead of regular loads Altivec vector loads on PowerPC have an interesting property: They always load from an aligned address (by rounding down the address actually provided if necessary). In order to generate an actual unaligned load, you can generate two load instructions, one with the original address, one offset by one vector length, and use a special permutation to extract the bytes desired. When this was originally implemented, I generated these two loads using regular ISD::LOAD nodes, now marked as aligned. Unfortunately, there is a problem with this: The alignment of a load does not contribute to its identity, and SDNodes are uniqued. So, imagine that we have some unaligned load, L1, that is not aligned. The routine will create two loads, L1(aligned) and (L1+16)(aligned). Further imagine that there had already existed a load (L1+16)(unaligned) with the same chain operand as the load L1. When (L1+16)(aligned) is created as part of the lowering of L1, this load *is* also the (L1+16)(unaligned) node, just now marked as aligned (because the new alignment overwrites the old). But the original users of (L1+16)(unaligned) now get the data intended for the permutation yielding the data for L1, and (L1+16)(unaligned) no longer exists to get its own permutation-based expansion. This was PR19991. A second potential problem has to do with the MMOs on these loads, which can be used by AA during instruction scheduling to break chain-based dependencies. If the new "aligned" loads get the MMO from the original unaligned load, this does not represent the fact that it will load data from below the original address. Normally, this would not matter, but this load might be combined with another load pair for a previous vector, and then the dependency on the otherwise- ignored lower bytes can matter. To fix both problems, instead of generating the necessary loads using regular ISD::LOAD instructions, ppc_altivec_lvx intrinsics are used instead. These are provided with MMOs with a conservative address range. Unfortunately, I no longer have a failing test case (since PR19991 was reported, other changes in CodeGen have forced this bug back into hiding it again). Nevertheless, this should fix the underlying problem. ------------------------------------------------------------------------ git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_35@215058 91177308-0d34-0410-b5e6-96231b3b80d8 |
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