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compiler-rt has accumulated several regressions that prevent it from building
on ARMv6. It is important to note that there are two major versions of ARMv6:
base ARMv6 and ARMv6K. ARMv6K includes several important new instructions,
such as non-word size atomic operations (ldrexd, strexd, etc.) and the yield
instruction. Most ARMv6 CPUs actually implement ARMv6K, including all those used
in Raspberry Pis, but nixpkgs' "raspberryPi" platform targets base ARMv6.
compiler-rt versions 8-14 fail to build on ARMv6 and ARMv6K. compiler-rt 15 (not
yet in nixpkgs) builds on ARMv6K but not ARMv6. This patch fixes versions 9-14
on both ARMv6 variants. The patches don't apply cleanly to version 8, and I
figured it wasn't worth carrying another version of the patches for such an old
version.
A total of five patches are required to get compiler-rt building on ARMv6:
* armv6-mcr-dmb.patch: use `mcr` to provide the equivalent of `dmb` on ARMv6.
Included in LLVM 15.
* armv6-sync-ops-no-thumb.patch: prevent certain atomic operation functions
from using Thumb mode. Included in LLVM 15.
* armv6-no-ldrexd-strexd.patch: don't use ldrexd or strexd, which are not
available in base ARMv6. Submitted upstream by me.
* armv6-scudo-no-yield.patch: use nop instead of yield on ARMv6 in standalone
scudo. Required by versions >=13, since they enable standalone scudo.
Submitted upstream by me.
* armv6-scudo-libatomic.patch: link standlone scudo to libatomic on ARMv6 (and
any other platforms that need it). Not yet submitted because the backport is
a bit different from the upstream version and I need to test it.
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