[PATCH] [AArch64] fix PR22408: clang no longer bootstraps itself on AArch64 linux

[Kristof Beyls]

Hi t.p.northover,

This patch fixes PR22408.

As is described at http://llvm.org/bugs/show_bug.cgi?id=22408, the GNU linkers
ld.bfd and ld.gold currently only support a subset of the whole range of AArch64
ELF TLS relocations. Furthermore, they assume that some of the code sequences to
access thread-local variables are produced in a very specific sequence.
When the sequence is not as the linker expects, it can silently mis-relaxe/mis-optimize
the instructions.
Even if that wouldn't be the case, it's good to produce the exact sequence,
as that ensures that linkers can perform optimizing relaxations.

This patch:
* implements support for 16MiB TLS area size instead of 4GiB TLS area size.
  Ideally clang would grow an -mtls-size option to allow support for both,
  but that's not part of this patch.
* by default doesn't produce local dynamic access patterns, as even modern
  ld.bfd and ld.gold linkers do not support the associated relocations.
  An option (-aarch64-elf-ldtls-generation) is added to enable generation of
  local dynamic code sequence, but is off by default.
  Alternatively, I think that LLVM could fixup the local dynamic fixups
  itself, instead of producing relocations leaving the work for the linker,
  but that didn't seem straightforward to do.
* makes sure that the exact expected code sequence for local dynamic and
  general dynamic accesses is produced, by making use of a new pseudo instruction.
  The patch also removes two (AArch64ISD::TLSDESC_BLR, AArch64ISD::TLSDESC_CALL)
  pre-existing AArch64-specific pseudo SDNode instructions
  that are superseded by the new one (TLSDESC_CALLSEQ).

REPOSITORY
  rL LLVM

http://reviews.llvm.org/D7677

Files:
  lib/Target/AArch64/AArch64AsmPrinter.cpp
  lib/Target/AArch64/AArch64CleanupLocalDynamicTLSPass.cpp
  lib/Target/AArch64/AArch64ISelLowering.cpp
  lib/Target/AArch64/AArch64ISelLowering.h
  lib/Target/AArch64/AArch64InstrInfo.td
  lib/Target/AArch64/AArch64MCInstLower.cpp
  lib/Target/AArch64/Utils/AArch64BaseInfo.h
  test/CodeGen/AArch64/arm64-tls-dynamics.ll
  test/CodeGen/AArch64/arm64-tls-execs.ll

EMAIL PREFERENCES
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