[PATCH] D140202: [lld][ARM][2/3]Big Endian support - Word invariant support

[Peter Smith via Phabricator via llvm-commits]

peter.smith added a comment.

It will be worth adding to the description of the difference between BE32 and BE8. BE32 is the legacy way of handling big-endian on Arm systems prior to ArmV6. In this model all relocatable object files are BigEndian, and all the instructions and data are BigEndian. In ArmV6 the byte-invariant BE8 mode was added, in the output ELF file (executable/shared library) this is the same model as AArch64 where the ELF file and Data are big-endian but instructions are little-endian. In contrast to AArch64 relocatable object files are identical in BE8 and BE32, the linker is expected to endian reverse big-endian instructions from big-endian to little-endian. In modern Arm systems BE32 support has been removed, leaving only BE8. However BE32 is a necessary pre-requisite to support BE8.

For BE8 support we'll have to endian reverse the instructions. For non-SyntheticSections we'll have to use the mapping symbols to do that during or more likely after writeTo(). For SyntheticSections we have a choice, we can write them big-endian with mapping symbols, and then do the same endian reversal as for non-SyntheticSections; or we can have the SyntheticSections force little-endian writes for instructions.

I notice that ARMErrataFix.cpp hard-codes reading of little-endian instructions. This won't work on a big-endian relocatable object. In practice all the systems that use ARMErrataFix.cpp are likely to be little-endian so it may not be necessary to support it for big-endian at this point. It would be good to error out with an unsupported error message.

I suspect that some reviewers will prefer to have the tests in the same review as the patch as this is making functional changes.

Not had a chance to go over all the small changes, but on a quick scan I couldn't spot anything obviously wrong. The more difficult bit will be spotting anything missing.

I'll be on vacation for three weeks. Happy for others to review in my absence.  Otherwise I can pick it back up next year.


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https://reviews.llvm.org/D140202