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72017e9b16
The ABI for big-endian AArch32, as specified by AAELF32, is above- averagely complicated. Relocatable object files are expected to store instruction encodings in byte order matching the ELF file's endianness (so, big-endian for a BE ELF file). But executable images can //either// do that //or// store instructions little-endian regardless of data and ELF endianness (to support BE32 and BE8 platforms respectively). They signal the latter by setting the EF_ARM_BE8 flag in the ELF header. (In the case of the Thumb instruction set, this all means that each 16-bit halfword of a Thumb instruction is stored in one or other endianness. The two halfwords of a 32-bit Thumb instruction must appear in the same order no matter what, because the first halfword is the one that must avoid overlapping the encoding of any 16-bit Thumb instruction.) llvm-objdump was unconditionally expecting Arm instructions to be stored little-endian. So it would correctly disassemble a BE8 image, but if you gave it a BE32 image or a BE object file, it would retrieve every instruction in byte-swapped form and disassemble it to nonsense. (Even an object file output by LLVM itself, because ARMMCCodeEmitter outputs instructions big-endian in big-endian mode, which is correct for writing an object file.) This patch allows llvm-objdump to correctly disassemble all three of those classes of Arm ELF file. It does it by introducing a new SubtargetFeature for big-endian instructions, setting it from the ELF image type and flags during llvm-objdump setup, and teaching both ARMDisassembler and llvm-objdump itself to pay attention to it when retrieving instruction data from a section being disassembled. Differential Revision: https://reviews.llvm.org/D130902