[NFC][Target][AsmPrinter] Introduce and use lowerCheriCodeReference

This will allow targets to create a target-specific MCExpr for referring
to code, bypassing any use of trampolines in the presence of a c18n
runtime.

Author: jrtc27

llvm/include/llvm/Target/TargetLoweringObjectFile.h

@@ -215,6 +215,13 @@ class LLVM_ABI TargetLoweringObjectFile : public MCObjectFileInfo {
     return nullptr;
   }
 
+  /// Return an MCExpr to use for a reference to the specified symbol with the
+  /// specified offset, where the symbol is a function symbol but we do not
+  /// want any interposition, as this reference is for a specific instruction
+  /// within the function (e.g. a BlockAddress).
+  virtual const MCExpr *lowerCheriCodeReference(const MCSymbol *Sym,
+                                                const MCExpr *Addend) const;
+
   /// Target supports a PC-relative relocation that references the PLT of a
   /// function.
   bool hasPLTPCRelative() const { return PLTPCRelativeSpecifier; }

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

@@ -4193,12 +4193,14 @@ static void emitGlobalConstantCHERICap(const DataLayout &DL, const Constant *CV,
     return;
   } else if (const MCSymbolRefExpr *SRE = dyn_cast<MCSymbolRefExpr>(Expr)) {
     if (auto BA = dyn_cast<BlockAddress>(CV)) {
-      // For block addresses we emit `.chericap FN+(.LtmpN - FN)`
+      // For block addresses we emit `.chericap FN+(.LtmpN - FN)` as a code
+      // capability.
       // NB: Must use a non-preemptible symbol
       auto FnStart = AP.getSymbolPreferLocal(*BA->getFunction(), true);
       const MCExpr *Start = MCSymbolRefExpr::create(FnStart, Ctx);
       const MCExpr *DiffToStart = MCBinaryExpr::createSub(SRE, Start, Ctx);
-      const MCExpr *CapExpr = MCBinaryExpr::createAdd(Start, DiffToStart, Ctx);
+      const MCExpr *CapExpr =
+          AP.getObjFileLowering().lowerCheriCodeReference(FnStart, DiffToStart);
       AP.OutStreamer->emitCheriCapability(CapExpr, CapWidth);
       return;
     }

llvm/lib/CodeGen/AsmPrinter/AsmPrinterDwarf.cpp

@@ -216,8 +216,8 @@ void AsmPrinter::emitCallSiteCheriCapability(const MCSymbol *Hi,
   // this would result in emitCheriCapability() creating a relocation against
   // section plus offset rather than function + offset. We need the right
   // bounds and permissions info and need to use a non-preemptible alias.
-  const MCExpr *Expr = MCSymbolRefExpr::create(CurrentFnBeginLocal, OutContext);
-  Expr = MCBinaryExpr::createAdd(Expr, DiffToStart, OutContext);
+  const MCExpr *Expr =
+      TLOF.lowerCheriCodeReference(CurrentFnBeginLocal, DiffToStart);
   OutStreamer->emitCheriCapability(Expr, TLOF.getCheriCapabilitySize(TM));
 }
 

llvm/lib/Target/TargetLoweringObjectFile.cpp

@@ -501,6 +501,18 @@ const MCExpr *TargetLoweringObjectFile::getDebugThreadLocalSymbol(const MCSymbol
   return MCSymbolRefExpr::create(Sym, getContext());
 }
 
+const MCExpr *
+TargetLoweringObjectFile::lowerCheriCodeReference(const MCSymbol *Sym,
+                                                  const MCExpr *Addend) const {
+  // Default to a normal expression if the target does not treat these
+  // specially.
+  // TODO: Require every CHERI target to implement this?
+  const MCExpr *Expr = MCSymbolRefExpr::create(Sym, getContext());
+  if (Addend != nullptr)
+    Expr = MCBinaryExpr::createAdd(Expr, Addend, getContext());
+  return Expr;
+}
+
 void TargetLoweringObjectFile::getNameWithPrefix(
     SmallVectorImpl<char> &OutName, const GlobalValue *GV,
     const TargetMachine &TM) const {