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LLVM has an @llvm.eh.dwarf.cfa intrinsic, used to lower the GCC-compatible __builtin_dwarf_cfa() builtin. As pointed out in PR26761, this is currently broken on PowerPC (and likely on ARM as well). Currently, @llvm.eh.dwarf.cfa is lowered using: ADD(FRAMEADDR, FRAME_TO_ARGS_OFFSET) where FRAME_TO_ARGS_OFFSET defaults to the constant zero. On x86, FRAME_TO_ARGS_OFFSET is lowered to 2*SlotSize. This setup, however, does not work for PowerPC. Because of the way that the stack layout works, the canonical frame address is not exactly (FRAMEADDR + FRAME_TO_ARGS_OFFSET) on PowerPC (there is a lower save-area offset as well), so it is not just a matter of implementing FRAME_TO_ARGS_OFFSET for PowerPC (unless we redefine its semantics -- We can do that, since it is currently used only for @llvm.eh.dwarf.cfa lowering, but the better to directly lower the CFA construct itself (since it can be easily represented as a fixed-offset FrameIndex)). Mips currently does this, but by using a custom lowering for ADD that specifically recognizes the (FRAMEADDR, FRAME_TO_ARGS_OFFSET) pattern. This change introduces a ISD::EH_DWARF_CFA node, which by default expands using the existing logic, but can be directly lowered by the target. Mips is updated to use this method (which simplifies its implementation, and I suspect makes it more robust), and updates PowerPC to do the same. Fixes PR26761. Differential Revision: https://reviews.llvm.org/D24038 llvm-svn: 280350
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