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[ARM] Fix handling of zero offsets in LOB instructions.
The BF and WLS/WLSTP instructions have various branch-offset fields occupying different positions and lengths in the instruction encoding, and all of them were decoded at disassembly time by the function DecodeBFLabelOffset() which returned SoftFail if the offset was zero. In fact, it's perfectly fine and not even a SoftFail for most of those offset fields to be zero. The only one that can't be zero is the 4-bit field labelled `boff` in the architecture spec, occupying bits {26-23} of the BF instruction family. If that one is zero, the encoding overlaps other instructions (WLS, DLS, LETP, VCTP), so it ought to be a full Fail. Fixed by adding an extra template parameter to DecodeBFLabelOffset which controls whether a zero offset is accepted or rejected. Adjusted existing tests (only in error messages for bad disassemblies); added extra tests to demonstrate zero offsets being accepted in all the right places, and a few demonstrating rejection of zero `boff`. Reviewers: DavidSpickett, ostannard Subscribers: javed.absar, kristof.beyls, hiraditya, llvm-commits Tags: #llvm Differential Revision: https://reviews.llvm.org/D63864 llvm-svn: 364533
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