Define MCAsmBackend::shouldForceRelocation

Return true if the MCValue has a specifier. When a relocation specifier
is specified, GNU Assembler will generate a relocation unless the
specifier can be optimized due to target-specific reasons (e.g. PPC `@l`
`@ha`).

This reduces targets' reliance on a MCAssembler::evaluateFixup hack
`if (Target.SymSpecifier || SA.isUndefined()) {`, previosuly
`if (A->getKind() != MCSymbolRefExpr::VK_None || SA.isUndefined()) {`

llvm/test/MC/SystemZ/fixups.s is known to rely on this hack.

Author: MaskRay

llvm/include/llvm/MC/MCAsmBackend.h

@@ -89,13 +89,10 @@ class MCAsmBackend {
   /// Get information on a fixup kind.
   virtual const MCFixupKindInfo &getFixupKindInfo(MCFixupKind Kind) const;
 
-  /// Hook to check if a relocation is needed for some target specific reason.
-  virtual bool shouldForceRelocation(const MCAssembler &Asm,
-                                     const MCFixup &Fixup,
-                                     const MCValue &Target,
-                                     const MCSubtargetInfo *STI) {
-    return false;
-  }
+  // Hook to check if a relocation is needed. The default implementation tests
+  // whether the MCValue has a relocation specifier.
+  virtual bool shouldForceRelocation(const MCAssembler &, const MCFixup &,
+                                     const MCValue &, const MCSubtargetInfo *);
 
   /// Hook to check if extra nop bytes must be inserted for alignment directive.
   /// For some targets this may be necessary in order to support linker

llvm/lib/MC/MCAsmBackend.cpp

@@ -109,6 +109,12 @@ const MCFixupKindInfo &MCAsmBackend::getFixupKindInfo(MCFixupKind Kind) const {
   return Builtins[Kind];
 }
 
+bool MCAsmBackend::shouldForceRelocation(const MCAssembler &, const MCFixup &,
+                                         const MCValue &Target,
+                                         const MCSubtargetInfo *) {
+  return Target.getSpecifier();
+}
+
 bool MCAsmBackend::fixupNeedsRelaxationAdvanced(const MCAssembler &Asm,
                                                 const MCFixup &Fixup,
                                                 bool Resolved, uint64_t Value,