[PATCH] D36749: [LLD][ELF][AArch64] Complete implementation of -fix-cortex-a53-843419

[Peter Smith via Phabricator via llvm-commits]

peter.smith created this revision.
Herald added subscribers: kristof.beyls, javed.absar, emaste, rengolin, aemerson.

This patch provides the mechanism to fix instances of the instruction sequence that may trigger the cortex-a53 843419 erratum. The fix is provided by replacing one of the instructions in the erratum sequence with a branch to a replacement patch sequence. The patch sequence is just the same instruction, followed by a branch back to the instruction following the one we replaced. The act of adding the branch is sufficient to prevent the erratum.

The implementation of the patching is similar to Thunks. Once we have discovered an instance of the erratum sequence we create a patch that we will insert into a PatchSection in the same way that we insert a Thunk into a ThunkSection. Once we have a location and a symbol defined for the patch we can modify or add a relocation to the InputSection we are patching (dependent on https://reviews.llvm.org/D36745 to overwrite all bits of an instruction when doing relocation) to point to the patch.

This is patch 3 of 3 to fix pr33463 https://bugs.llvm.org/show_bug.cgi?id=33463 . It depends on https://reviews.llvm.org/D36745, https://reviews.llvm.org/D36742 and https://reviews.llvm.org/D36739

There are a number of design decisions that I've taken with the fix:

- I've chosen to follow the branch to patch fix in the same way as gold and ld.bfd. I've left a comment in pr33463 https://bugs.llvm.org/show_bug.cgi?id=33463#c15 with the reasons why.
- In theory Thunks and Patches can be merged or at least more tightly integrated. I've chosen not to do this as AArch64 does not currently support Thunks and no other Target needs patching, and merging the implementations would complicate both. If AArch64 needs to support range-thunks in the future some tighter integration work may be required.
- I've chosen to follow ld.bfd and round the size of the PatchSection up to the next 0x1000 boundary, this means that if there are InputSectionDescriptions or OutputSections following an inserted PatchSection the addresses of the instructions modulo 0x1000 won't change so that we can do all the scanning and patching in one pass.

Full details of the erratum sequence can be found in http://infocenter.arm.com/help/topic/com.arm.doc.epm048406/Cortex_A53_MPCore_Software_Developers_Errata_Notice.pdf


https://reviews.llvm.org/D36749

Files:
  ELF/SectionPatcher.cpp
  ELF/SectionPatcher.h
  ELF/SyntheticSections.cpp
  ELF/SyntheticSections.h
  test/ELF/aarch64-cortex-a53-843419-address.s
  test/ELF/aarch64-cortex-a53-843419-recognize.s

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