[llvm-bugs] [Bug 48368] New: Support the AArch64 variant PCS symbol st_other value

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https://bugs.llvm.org/show_bug.cgi?id=48368

            Bug ID: 48368
           Summary: Support the AArch64 variant PCS symbol st_other value
           Product: lld
           Version: unspecified
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: ELF
          Assignee: unassignedbugs at nondot.org
          Reporter: smithp352 at googlemail.com
                CC: llvm-bugs at lists.llvm.org, smithp352 at googlemail.com

This is an enhancement request for lld to support the AArch64 variant procedure
call standard (PCS) in LLD. This is needed to support SVE functions that follow
the vector procedure calling standard (a variant procedure call standard that
requires more registers to be saved than normal).

Functions that follow a variant PCS need special handling by a dynamic linker.
To mark such functions the AArch64 ABI defines the st_other value
STO_AARCH64_VARIANT_PCS in ELF for the 64 bit Arm Architecture. This is
documented in:
https://github.com/ARM-software/abi-aa/blob/master/aaelf64/aaelf64.rst#551st_other-values

The static linker's responsibilities with respect to STO_AARCH64_VARIANT_PCS
are small. To quote from the ABI:

"Static linkers must preserve the marking and propagate it to the dynamic
symbol table if any reference or definition of the symbol is marked with
STO_AARCH64_VARIANT_PCS, and add a DT_AARCH64_VARIANT_PCS dynamic tag if
required by the Dynamic Section section."

Message describing motivation and design:
https://sourceware.org/legacy-ml/binutils/2019-05/msg00294.html
LLVM patch that adds STO_AARCH64_VARIANT_PCS to LLVM
https://reviews.llvm.org/D89138
binutils patch https://sourceware.org/pipermail/binutils/2019-May/106963.html
glibc patch to support STO_AARCH64_VARIANT_PCS
https://sourceware.org/pipermail/glibc-cvs/2020q1/069075.html

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