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[ARM] Fix endianness of Thumb .inst.w directive

Authored by olista01 on Dec 13 2017, 7:33 AM.



Wide Thumb2 instructions should be emitted into the object file as pairs of 16-bit words of the appropriate endianness, not one 32-bit word.

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olista01 created this revision.Dec 13 2017, 7:33 AM
fhahn added a subscriber: fhahn.Jan 4 2018, 1:52 AM

Could you provide a pointer to where this behavior is stated?

This is described in the "Instruction endianness" section in the architecture reference manual, which is section A3.3.1 in the latest version of the v7-A/R manual. Here's the relevant bit:

A3.3.1 Instruction endianness

In ARMv7-A, the mapping of instruction memory is always little-endian. In ARMv7-R, instruction endianness can
be controlled at the system level, see Instruction endianness static configuration, ARMv7-R only on page A3-112.

Note: For information about data memory endianness control, see Endianness mapping register, ENDIANSTATE on
page A2-53.

Before ARMv7, the ARM architecture included legacy support for an alternative big-endian memory model,
described as BE-32 and controlled by SCTLR.B bit, bit[7] of the register, see Endian configuration and control on
page AppxL-2516. ARMv7 does not support BE-32 operation, and bit SCTLR[7] is RAZ/SBZP.

Where legacy object code for ARM processors contains instructions with a big-endian byte order, the removal of
support for BE-32 operation requires the instructions in the object files to have their bytes reversed for the code to
be executed on an ARMv7 processor. This means that:

- each Thumb instruction, whether a 32-bit Thumb instruction or a 16-bit Thumb instruction, must have the
  byte order of each halfword of instruction reversed
- each ARM instruction must have the byte order of each word of instruction reversed.

For most situations, this can be handled in the link stage of a tool-flow, provided the object files include sufficient
information to permit this to happen. In practice, this is the situation for all applications with the ARMv7-A profile.
fhahn accepted this revision.Jan 4 2018, 3:33 AM

Thanks for providing the relevant context!

LGTM, according to the spec

This revision is now accepted and ready to land.Jan 4 2018, 3:33 AM
This revision was automatically updated to reflect the committed changes.