[all-commits] [llvm/llvm-project] 28cb62: Change some addUsedGlobal to addUsedOrCompilerUsed...

[Fangrui Song via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 28cb620321f5461255423f84c85e6891b5174c13
      https://github.com/llvm/llvm-project/commit/28cb620321f5461255423f84c85e6891b5174c13
  Author: Fangrui Song <i at maskray.me>
  Date:   2021-02-26 (Fri, 26 Feb 2021)

  Changed paths:
    M clang/lib/CodeGen/CGDecl.cpp
    M clang/lib/CodeGen/CodeGenModule.cpp
    M clang/lib/CodeGen/CodeGenModule.h
    M clang/test/CodeGen/2005-12-04-AttributeUsed.c
    M clang/test/CodeGen/attr-msp430.c
    M clang/test/CodeGen/attr-target-mv.c
    M clang/test/CodeGen/attr-used.c
    M clang/test/CodeGen/attr-x86-interrupt.c
    M clang/test/CodeGen/keep-static-consts.cpp
    M clang/test/CodeGenCUDA/llvm-used.cu
    M clang/test/CodeGenCXX/attr-x86-interrupt.cpp
    M clang/test/CodeGenCXX/extern-c.cpp

  Log Message:
  -----------
  Change some addUsedGlobal to addUsedOrCompilerUsedGlobal

An global value in the `llvm.used` list does not have GC root semantics on ELF targets.
This will be changed in a subsequent backend patch.

Change some `llvm.used` in the ELF code path to use `llvm.compiler.used` to
prevent undesired GC root semantics.

Change one extern "C" alias (due to `__attribute__((used))` in extern "C") to use `llvm.compiler.used` on all targets.

GNU ld has a rule "`__start_/__stop_` references from a live input section retain the associated C identifier name sections",
which LLD may drop entirely (currently refined to exclude SHF_LINK_ORDER/SHF_GROUP) in a future release (the rule makes it clumsy to GC metadata sections; D96914 added a way to try the potential future behavior).
For `llvm.used` global values defined in a C identifier name section, keep using `llvm.used` so that
the future LLD change will not affect them.

rnk kindly categorized the changes:
```
ObjC/blocks: this wants GC root semantics, since ObjC mainly runs on Mac.
MS C++ ABI stuff: wants GC root semantics, no change
OpenMP: unsure, but GC root semantics probably don't hurt
CodeGenModule: affected in this patch to *not* use GC root semantics so that __attribute__((used)) behavior remains the same on ELF, plus two other minor use cases that don't want GC semantics
Coverage: Probably want GC root semantics
CGExpr.cpp: refers to LTO, wants GC root
CGDeclCXX.cpp: one is MS ABI specific, so yes GC root, one is some other C++ init functionality, which should form GC roots (C++ initializers can have side effects and must run)
CGDecl.cpp: Changed in this patch for __attribute__((used))
```

Differential Revision: https://reviews.llvm.org/D97446