MCObjectStreamer: assign MCSymbols in the dummy fragment to offset 0.

In MCObjectStreamer, when there is no current fragment, initially
symbols are created in a "pending" state and assigned to a dummy
empty fragment.

Previously, they were not being assigned an offset, and thus
evaluateAbsolute would fail if trying to evaluate an expression 'a -
b', where both 'a' and 'b' were in this pending state.

Also slightly refactored the EmitLabel overload which takes an
MCFragment for clarity.

Fixes: https://llvm.org/PR41825

Differential Revision: https://reviews.llvm.org/D70062

3 files changed, 21 insertions, 7 deletions

diff --git a/llvm/lib/MC/MCELFStreamer.cpp b/llvm/lib/MC/MCELFStreamer.cpp
index fa2133078bf..c2e2beb23cb 100644
--- a/llvm/lib/MC/MCELFStreamer.cpp
+++ b/llvm/lib/MC/MCELFStreamer.cpp
@@ -106,9 +106,10 @@ void MCELFStreamer::EmitLabel(MCSymbol *S, SMLoc Loc) {
     Symbol->setType(ELF::STT_TLS);
 }
 
-void MCELFStreamer::EmitLabel(MCSymbol *S, SMLoc Loc, MCFragment *F) {
+void MCELFStreamer::EmitLabelAtPos(MCSymbol *S, SMLoc Loc, MCFragment *F,
+                                   uint64_t Offset) {
   auto *Symbol = cast<MCSymbolELF>(S);
-  MCObjectStreamer::EmitLabel(Symbol, Loc, F);
+  MCObjectStreamer::EmitLabelAtPos(Symbol, Loc, F, Offset);
 
   const MCSectionELF &Section =
       static_cast<const MCSectionELF &>(*getCurrentSectionOnly());
diff --git a/llvm/lib/MC/MCObjectStreamer.cpp b/llvm/lib/MC/MCObjectStreamer.cpp
index 83f6ab8fe33..cc33810c7ec 100644
--- a/llvm/lib/MC/MCObjectStreamer.cpp
+++ b/llvm/lib/MC/MCObjectStreamer.cpp
@@ -243,18 +243,32 @@ void MCObjectStreamer::EmitLabel(MCSymbol *Symbol, SMLoc Loc) {
     Symbol->setFragment(F);
     Symbol->setOffset(F->getContents().size());
   } else {
+    // Assign all pending labels to offset 0 within the dummy "pending"
+    // fragment. (They will all be reassigned to a real fragment in
+    // flushPendingLabels())
+    Symbol->setOffset(0);
     PendingLabels.push_back(Symbol);
   }
 }
 
-void MCObjectStreamer::EmitLabel(MCSymbol *Symbol, SMLoc Loc, MCFragment *F) {
+// Emit a label at a previously emitted fragment/offset position. This must be
+// within the currently-active section.
+void MCObjectStreamer::EmitLabelAtPos(MCSymbol *Symbol, SMLoc Loc,
+                                      MCFragment *F, uint64_t Offset) {
+  assert(F->getParent() == getCurrentSectionOnly());
+
   MCStreamer::EmitLabel(Symbol, Loc);
   getAssembler().registerSymbol(*Symbol);
   auto *DF = dyn_cast_or_null<MCDataFragment>(F);
-  if (DF)
+  Symbol->setOffset(Offset);
+  if (DF) {
     Symbol->setFragment(F);
-  else
+  } else {
+    assert(isa<MCDummyFragment>(F) &&
+           "F must either be an MCDataFragment or the pending MCDummyFragment");
+    assert(Offset == 0);
     PendingLabels.push_back(Symbol);
+  }
 }
 
 void MCObjectStreamer::EmitULEB128Value(const MCExpr *Value) {
diff --git a/llvm/lib/Target/ARM/MCTargetDesc/ARMELFStreamer.cpp b/llvm/lib/Target/ARM/MCTargetDesc/ARMELFStreamer.cpp
index 01b98ccf897..f558ca8d2d9 100644
--- a/llvm/lib/Target/ARM/MCTargetDesc/ARMELFStreamer.cpp
+++ b/llvm/lib/Target/ARM/MCTargetDesc/ARMELFStreamer.cpp
@@ -659,11 +659,10 @@ private:
                          uint64_t Offset) {
     auto *Symbol = cast<MCSymbolELF>(getContext().getOrCreateSymbol(
         Name + "." + Twine(MappingSymbolCounter++)));
-    EmitLabel(Symbol, Loc, F);
+    EmitLabelAtPos(Symbol, Loc, F, Offset);
     Symbol->setType(ELF::STT_NOTYPE);
     Symbol->setBinding(ELF::STB_LOCAL);
     Symbol->setExternal(false);
-    Symbol->setOffset(Offset);
   }
 
   void EmitThumbFunc(MCSymbol *Func) override {