[llvm][IR] Do not place constants with static relocations in a mergeable section

This patch provides two major changes:

1. Add getRelocationInfo to check if a constant will have static, dynamic, or
   no relocations. (Also rename the original needsRelocation to needsDynamicRelocation.)
2. Only allow a constant with no relocations (static or dynamic) to be placed
   in a mergeable section.

This will allow unused symbols that contain static relocations and happen to
fit in mergeable constant sections (.rodata.cstN) to instead be placed in
unique-named sections if -fdata-sections is used and subsequently garbage collected
by --gc-sections.

See https://lists.llvm.org/pipermail/llvm-dev/2021-February/148281.html.

Differential Revision: https://reviews.llvm.org/D95960

1 files changed, 2 insertions, 1 deletions

diff --git a/llvm/lib/Target/TargetLoweringObjectFile.cpp b/llvm/lib/Target/TargetLoweringObjectFile.cpp
index 239f12c..4efac23 100644
--- a/llvm/lib/Target/TargetLoweringObjectFile.cpp
+++ b/llvm/lib/Target/TargetLoweringObjectFile.cpp
@@ -290,7 +290,8 @@ SectionKind TargetLoweringObjectFile::getKindForGlobal(const GlobalObject *GO,
       // consideration when it tries to merge entries in the section.
       Reloc::Model ReloModel = TM.getRelocationModel();
       if (ReloModel == Reloc::Static || ReloModel == Reloc::ROPI ||
-          ReloModel == Reloc::RWPI || ReloModel == Reloc::ROPI_RWPI)
+          ReloModel == Reloc::RWPI || ReloModel == Reloc::ROPI_RWPI ||
+          !C->needsDynamicRelocation())
         return SectionKind::getReadOnly();
 
       // Otherwise, the dynamic linker needs to fix it up, put it in the