Look through variables when computing relocations.

Given

bar = foo + 4
	.long bar

MC would eat the 4. GNU as includes it in the relocation. The rule seems to be
that a variable that defines a symbol is used in the relocation and one that
does not define a symbol is evaluated and the result included in the relocation.

Fixing this unfortunately required some other changes:

* Since the variable is now evaluated, it would prevent the ELF writer from
  noticing the weakref marker the elf streamer uses. This patch then replaces
  that with a VariantKind in MCSymbolRefExpr.

* Using VariantKind then requires us to look past other VariantKind to see

	.weakref	bar,foo
	call	bar@PLT

  doing this also fixes

	zed = foo +2
	call zed@PLT

  so that is a good thing.

* Looking past VariantKind means that the relocation selection has to use
  the fixup instead of the target.

This is a reboot of the previous fixes for MC. I will watch the sanitizer
buildbot and wait for a build before adding back the previous fixes.

llvm-svn: 204294

1 files changed, 9 insertions, 5 deletions

diff --git a/llvm/lib/MC/ELFObjectWriter.cpp b/llvm/lib/MC/ELFObjectWriter.cpp
index 909ea800830..1dae2f467d3 100644
--- a/llvm/lib/MC/ELFObjectWriter.cpp
+++ b/llvm/lib/MC/ELFObjectWriter.cpp
@@ -778,7 +778,7 @@ void ELFObjectWriter::RecordRelocation(const MCAssembler &Asm,
         Index = 0;
       }
     } else {
-      if (Asm.getSymbolData(Symbol).getFlags() & ELF_Other_Weakref)
+      if (Target.getSymA()->getKind() == MCSymbolRefExpr::VK_WEAKREF)
         WeakrefUsedInReloc.insert(RelocSymbol);
       else
         UsedInReloc.insert(RelocSymbol);
@@ -823,8 +823,14 @@ ELFObjectWriter::getSymbolIndexInSymbolTable(const MCAssembler &Asm,
 bool ELFObjectWriter::isInSymtab(const MCAssembler &Asm,
                                  const MCSymbolData &Data,
                                  bool Used, bool Renamed) {
-  if (Data.getFlags() & ELF_Other_Weakref)
-    return false;
+  const MCSymbol &Symbol = Data.getSymbol();
+  if (Symbol.isVariable()) {
+    const MCExpr *Expr = Symbol.getVariableValue();
+    if (const MCSymbolRefExpr *Ref = dyn_cast<MCSymbolRefExpr>(Expr)) {
+      if (Ref->getKind() == MCSymbolRefExpr::VK_WEAKREF)
+        return false;
+    }
+  }
 
   if (Used)
     return true;
@@ -832,8 +838,6 @@ bool ELFObjectWriter::isInSymtab(const MCAssembler &Asm,
   if (Renamed)
     return false;
 
-  const MCSymbol &Symbol = Data.getSymbol();
-
   if (Symbol.getName() == "_GLOBAL_OFFSET_TABLE_")
     return true;