Print call preserved registers in comments at call site in assembly files.

[vivekvpandya]

Summary

This patch makes AsmPrinter print call preserved registers in comments at call site in assembly files.

[vivekvpandya]

Updated patch to fetch latest changes in AsmPrinter.cpp. I also feel that it would be good to add an option similar to -asm-show-inst in llc to control this printing. So what are reviewer's thoughts ?

lib/CodeGen/AsmPrinter/AsmPrinter.cpp

 #include "llvm/CodeGen/GCMetadataPrinter.h"
 #include "llvm/CodeGen/MachineConstantPool.h"
 #include "llvm/CodeGen/MachineFrameInfo.h"
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/MachineInstrBundle.h"
 #include "llvm/CodeGen/MachineJumpTableInfo.h"
 #include "llvm/CodeGen/MachineLoopInfo.h"
 #include "llvm/CodeGen/MachineModuleInfoImpls.h"
+#include "llvm/CodeGen/MachineOperand.h"
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/DebugInfo.h"
 #include "llvm/IR/Mangler.h"
 #include "llvm/IR/Module.h"
 #include "llvm/IR/Operator.h"
 #include "llvm/MC/MCAsmInfo.h"
 #include "llvm/MC/MCContext.h"
 #include "llvm/MC/MCExpr.h"
void AsmPrinter::EmitFunctionEntryLabel() {
 
   return OutStreamer->EmitLabel(CurrentFnSym);
 }
 
 /// emitComments - Pretty-print comments for instructions.
 static void emitComments(const MachineInstr &MI, raw_ostream &CommentOS) {
   const MachineFunction *MF = MI.getParent()->getParent();
   const TargetInstrInfo *TII = MF->getSubtarget().getInstrInfo();
+  const TargetRegisterInfo *TRI = MF->getSubtarget().getRegisterInfo();
 
   // Check for spills and reloads
   int FI;
 
   const MachineFrameInfo *FrameInfo = MF->getFrameInfo();
 
   // We assume a single instruction only has a spill or reload, not
   // both.
static void emitComments(const MachineInstr &MI, raw_ostream &CommentOS) {
   } else if (TII->hasStoreToStackSlot(&MI, MMO, FI)) {
     if (FrameInfo->isSpillSlotObjectIndex(FI))
       CommentOS << MMO->getSize() << "-byte Folded Spill\n";
   }
 
   // Check for spill-induced copies
   if (MI.getAsmPrinterFlag(MachineInstr::ReloadReuse))
     CommentOS << " Reload Reuse\n";
+
+  if (MI.isCall()) {
+    const uint32_t *CalleeRegMask;
+    CommentOS << "Call Preserved Registers: ";
+
+    for (const MachineOperand &MO : MI.operands())
+      if (MO.isRegMask()) {
+        CalleeRegMask = MO.getRegMask();
+        break;
+      }
+
+    for (unsigned PReg = 1, PRegE = TRI->getNumRegs(); PReg < PRegE; ++PReg)
+      if (!MachineOperand::clobbersPhysReg(CalleeRegMask, PReg))
+        CommentOS << TRI->getName(PReg) << " ";
+    CommentOS << "\n";
+  }
 }
 
 /// emitImplicitDef - This method emits the specified machine instruction
 /// that is an implicit def.
 void AsmPrinter::emitImplicitDef(const MachineInstr *MI) const {
   unsigned RegNo = MI->getOperand(0).getReg();
 
   SmallString<128> Str;