[MC][ELF] Emit a relocation if target is defined in the same section and is non-local

For a target symbol defined in the same section, currently we don't emit
a relocation if VariantKind is VK_None (with few exceptions like RISC-V
relaxation), while GNU as emits one. This causes program behavior
differences with and without -ffunction-sections, and can break intended
symbol interposition in a -shared link.

```
.globl foo
foo:
  call foo      # no relocation. On other targets, may be written as b foo, etc
  call bar      # a relocation if bar is in another section (e.g. -ffunction-sections)
  call foo@plt  # a relocation
```

Unify these cases by always emitting a relocation. If we ever want to
optimize `call foo` in -shared links, we should emit a STB_LOCAL alias
and call via the alias.

ARM/thumb2-beq-fixup.s: we now emit a relocation to global_thumb_fn as GNU as does.
X86/Inputs/align-branch-64-2.s: we now emit R_X86_64_PLT32 to foo as GNU does.

ELF/relax.s: rewrite the test as target-in-same-section.s .
We omitted relocations to `global` and now emit R_X86_64_PLT32.
Note, GNU as does not emit a relocation for `jmp global` (maybe its own
bug). Our new behavior is compatible except `jmp global`.

Reviewed By: peter.smith

Differential Revision: https://reviews.llvm.org/D72197

1 files changed, 1 insertions, 21 deletions

diff --git a/llvm/lib/MC/ELFObjectWriter.cpp b/llvm/lib/MC/ELFObjectWriter.cpp
index f8e93889344..b421d0b2bf6 100644
--- a/llvm/lib/MC/ELFObjectWriter.cpp
+++ b/llvm/lib/MC/ELFObjectWriter.cpp
@@ -569,26 +569,6 @@ void ELFWriter::writeSymbol(SymbolTableWriter &Writer, uint32_t StringIndex,
                      IsReserved);
 }
 
-// True if the assembler knows nothing about the final value of the symbol.
-// This doesn't cover the comdat issues, since in those cases the assembler
-// can at least know that all symbols in the section will move together.
-static bool isWeak(const MCSymbolELF &Sym) {
-  if (Sym.getType() == ELF::STT_GNU_IFUNC)
-    return true;
-
-  switch (Sym.getBinding()) {
-  default:
-    llvm_unreachable("Unknown binding");
-  case ELF::STB_LOCAL:
-    return false;
-  case ELF::STB_GLOBAL:
-    return false;
-  case ELF::STB_WEAK:
-  case ELF::STB_GNU_UNIQUE:
-    return true;
-  }
-}
-
 bool ELFWriter::isInSymtab(const MCAsmLayout &Layout, const MCSymbolELF &Symbol,
                            bool Used, bool Renamed) {
   if (Symbol.isVariable()) {
@@ -1534,7 +1514,7 @@ bool ELFObjectWriter::isSymbolRefDifferenceFullyResolvedImpl(
   const auto &SymA = cast<MCSymbolELF>(SA);
   if (IsPCRel) {
     assert(!InSet);
-    if (isWeak(SymA))
+    if (SymA.getBinding() != ELF::STB_LOCAL)
       return false;
   }
   return MCObjectWriter::isSymbolRefDifferenceFullyResolvedImpl(Asm, SymA, FB,