[PATCH] D24038: Add ISD::EH_DWARF_CFA, simplify @llvm.eh.dwarf.cfa on Mips, fix on PowerPC

Hal Finkel via llvm-commits llvm-commits at lists.llvm.org
Tue Aug 30 09:20:59 PDT 2016

hfinkel created this revision.
hfinkel added reviewers: dsanders, echristo, jmolloy.
hfinkel added a subscriber: llvm-commits.
Herald added subscribers: nemanjai, mehdi_amini, sdardis, mcrosier, aemerson.

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:


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 patch introduces a ISD::EH_DWARF_CFA node, which by default expands using the existing logic, but can be directly lowered by the target. It Mips to use this method (which simplifies its implementation, and I suspect makes it more robust), and updates PowerPC to do the same.

Fixes PR26761.



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