[AARCH64][FIX] Emit data symbol for constant pool data

[dnsampaio]

Summary

The ARM64 elf emitter would omit printing data
symbol for zero filled constant data. This patch
overrides the emitFill method as to enforce that
the symbol is correctly printed.

[olista01]

LGTM, thanks!

[olista01]

Actually, the test uses cortex-a76, but I don't think that's been committed yet. This isn't CPU-dependent, so you should be able to just remove the option.

[dnsampaio]

Removed processor and specific triple target from test.

[olista01]

LGTM

lib/Target/AArch64/MCTargetDesc/AArch64ELFStreamer.cpp

public:
   /// This is one of the functions used to emit data into an ELF section, so the
   /// AArch64 streamer overrides it to add the appropriate mapping symbol ($d)
   /// if necessary.
   void EmitValueImpl(const MCExpr *Value, unsigned Size, SMLoc Loc) override {
     EmitDataMappingSymbol();
     MCELFStreamer::EmitValueImpl(Value, Size, Loc);
   }
 
+  void emitFill(const MCExpr &NumBytes, uint64_t FillValue,
+                                  SMLoc Loc) override {
+    EmitDataMappingSymbol();
+    MCObjectStreamer::emitFill(NumBytes, FillValue, Loc);
+  }
 private:
   enum ElfMappingSymbol {
     EMS_None,
     EMS_A64,
     EMS_Data
   };
 
   void EmitDataMappingSymbol() {

test/MC/AArch64/CheckDataSymbol.s

+# RUN: llvm-mc -filetype=obj -assemble \
+# RUN: -triple=aarch64- %s -o - \
+# RUN: | llvm-readobj -s -t - | FileCheck %s
+# CHECK:     Name: $d.1 ({{[1-9][0-9]+}})
+# CHECK-NEXT:     Value: 0x4
+# CHECK-NEXT:     Size: 0
+# CHECK-NEXT:     Binding: Local (0x0)
+# CHECK-NEXT:     Type: None (0x0)
+# CHECK-NEXT:     Other: 0
+# CHECK-NEXT:     Section: .text (0x2)
+# CHECK-NEXT:   }
+
+.text
+nop
+.zero 4