Use absoluteSymbolDiff, add target hook for whether to prefer using relocs

Specifically: bbea642 took away a hook for whether a target wanted
symbol differences to be expressed in relocations or not, and here I've
found a use case for it.

Author: jmorse

llvm/include/llvm/MC/MCAsmBackend.h

@@ -135,6 +135,10 @@ class MCAsmBackend {
                           uint64_t Value, bool IsResolved,
                           const MCSubtargetInfo *STI) const = 0;
 
+  /// Check whether the given target requires emitting differences of two
+  /// symbols as a set of relocations.
+  virtual bool requiresDiffExpressionRelocations() const { return false; }
+
   /// @}
 
   /// \name Target Relaxation Interfaces

llvm/lib/MC/MCObjectStreamer.cpp

@@ -469,17 +469,14 @@ void MCObjectStreamer::emitDwarfAdvanceLineAddr(int64_t LineDelta,
   // fragment, then the address-offset is already a fixed constant and is not
   // relaxable. Emit the advance-line-addr data immediately to save time and
   // memory.
-  // As per commit bbea64250f6548, RISCV always desires symbolic relocations.
-  MCFragment *LastFrag = LastLabel->getFragment();
-  if (!getAssembler().getContext().getTargetTriple().isRISCV() &&
-      LastFrag->getKind() == MCFragment::FT_Data &&
-      Label->getFragment() == LastFrag) {
-    uint64_t AddrDelta = Label->getOffset() - LastLabel->getOffset();
-    SmallString<16> Tmp;
-    MCDwarfLineAddr::encode(getContext(), Assembler->getDWARFLinetableParams(),
-                            LineDelta, AddrDelta, Tmp);
-    emitBytes(Tmp);
-    return;
+  if (!Assembler->getBackend().requiresDiffExpressionRelocations()) {
+    if (auto OptAddrDelta = absoluteSymbolDiff(Label, LastLabel)) {
+      SmallString<16> Tmp;
+      MCDwarfLineAddr::encode(getContext(), Assembler->getDWARFLinetableParams(),
+                              LineDelta, *OptAddrDelta, Tmp);
+      emitBytes(Tmp);
+      return;
+    }
   }
 
   const MCExpr *AddrDelta = buildSymbolDiff(*this, Label, LastLabel, SMLoc());

llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.h

@@ -35,6 +35,20 @@ class RISCVAsmBackend : public MCAsmBackend {
 
   void setForceRelocs() { ForceRelocs = true; }
 
+  // Returns true if relocations will be forced for shouldForceRelocation by
+  // default. This will be true if relaxation is enabled or had previously
+  // been enabled.
+  bool willForceRelocations() const {
+    return ForceRelocs || STI.getFeatureBits()[RISCV::FeatureRelax];
+  }
+
+  // Generate diff expression relocations if the relax feature is enabled or had
+  // previously been enabled, otherwise it is safe for the assembler to
+  // calculate these internally.
+  bool requiresDiffExpressionRelocations() const override {
+    return willForceRelocations();
+  }
+
   // Return Size with extra Nop Bytes for alignment directive in code section.
   bool shouldInsertExtraNopBytesForCodeAlign(const MCAlignFragment &AF,
                                              unsigned &Size) override;