[all-commits] [llvm/llvm-project] e2e82c: [CodeGenModule] Drop dso_local on function declara...

[Fangrui Song via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: e2e82c9983f0dff671dd47d3e256d80faa40e892
      https://github.com/llvm/llvm-project/commit/e2e82c9983f0dff671dd47d3e256d80faa40e892
  Author: Fangrui Song <i at maskray.me>
  Date:   2021-01-09 (Sat, 09 Jan 2021)

  Changed paths:
    M clang/lib/CodeGen/CodeGenModule.cpp
    M clang/test/CodeGen/dso-local-executable.c

  Log Message:
  -----------
  [CodeGenModule] Drop dso_local on function declarations for ELF -fno-pic -fno-direct-access-external-data

ELF -fno-pic sets dso_local on a function declaration to allow direct accesses
when taking its address (similar to a data symbol). The emitted code follows the
traditional GCC/Clang -fno-pic behavior: an absolute relocation is produced.

If the function is not defined in the executable, a canonical PLT entry will be
needed at link time. This is similar to a copy relocation and is incompatible
with (-Bsymbolic or --dynamic-list linked shared objects / protected symbols in
a shared object).

This patch gives -fno-pic code a way to avoid such a canonical PLT entry.

The FIXME was about a generalization for -fpie -mpie-copy-relocations (now -fpie
-fdirect-access-external-data). While we could set dso_local to avoid GOT when
taking the address of a function declaration (there is an ignorable difference
about R_386_PC32 vs R_386_PLT32 on i386), it likely does not provide any benefit
and can just cause trouble, so we don't make the generalization.