Add GNU attribute 'retain'

For ELF targets, GCC 11 will set SHF_GNU_RETAIN on the section of a
`__attribute__((retain))` function/variable to prevent linker garbage
collection. (See AttrDocs.td for the linker support).

This patch adds `retain` functions/variables to the `llvm.used` list, which has
the desired linker GC semantics. Note: `retain` does not imply `used`,
so an unused function/variable can be dropped by Sema.

Before 'retain' was introduced, previous ELF solutions require inline asm or
linker tricks, e.g.  `asm volatile(".reloc 0, R_X86_64_NONE, target");`
(architecture dependent) or define a non-local symbol in the section and use
`ld -u`. There was no elegant source-level solution.

With D97448, `__attribute__((retain))` will set `SHF_GNU_RETAIN` on ELF targets.

Differential Revision: https://reviews.llvm.org/D97447

1 files changed, 4 insertions, 0 deletions

diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
index 564b07582376..0d499a564039 100644
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -1896,6 +1896,8 @@ void CodeGenModule::setNonAliasAttributes(GlobalDecl GD,
 
   if (D) {
     if (auto *GV = dyn_cast<llvm::GlobalVariable>(GO)) {
+      if (D->hasAttr<RetainAttr>())
+        addUsedGlobal(GV);
       if (auto *SA = D->getAttr<PragmaClangBSSSectionAttr>())
         GV->addAttribute("bss-section", SA->getName());
       if (auto *SA = D->getAttr<PragmaClangDataSectionAttr>())
@@ -1907,6 +1909,8 @@ void CodeGenModule::setNonAliasAttributes(GlobalDecl GD,
     }
 
     if (auto *F = dyn_cast<llvm::Function>(GO)) {
+      if (D->hasAttr<RetainAttr>())
+        addUsedGlobal(F);
       if (auto *SA = D->getAttr<PragmaClangTextSectionAttr>())
         if (!D->getAttr<SectionAttr>())
           F->addFnAttr("implicit-section-name", SA->getName());