PrologEpilogInserter: Rewrite API to determine callee save regsiters.

This changes TargetFrameLowering::processFunctionBeforeCalleeSavedScan():

- Rename the function to determineCalleeSaves()
- Pass a bitset of callee saved registers by reference, thus avoiding
  the function-global PhysRegUsed bitset in MachineRegisterInfo.
- Without PhysRegUsed the implementation is fine tuned to not save
  physcial registers which are only read but never modified.

Related to rdar://21539507

Differential Revision: http://reviews.llvm.org/D10909

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242165 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/TargetFrameLoweringImpl.cpp

@@ -11,9 +11,12 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "llvm/ADT/BitVector.h"
 #include "llvm/Target/TargetFrameLowering.h"
 #include "llvm/CodeGen/MachineFrameInfo.h"
 #include "llvm/CodeGen/MachineFunction.h"
+#include "llvm/CodeGen/MachineModuleInfo.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
 #include "llvm/IR/Function.h"
 #include "llvm/Target/TargetRegisterInfo.h"
 #include "llvm/Target/TargetSubtargetInfo.h"
@@ -54,3 +57,30 @@ bool TargetFrameLowering::needsFrameIndexResolution(
     const MachineFunction &MF) const {
   return MF.getFrameInfo()->hasStackObjects();
 }
+
+void TargetFrameLowering::determineCalleeSaves(MachineFunction &MF,
+                                               BitVector &SavedRegs,
+                                               RegScavenger *RS) const {
+  // Get the callee saved register list...
+  const TargetRegisterInfo &TRI = *MF.getSubtarget().getRegisterInfo();
+  const MCPhysReg *CSRegs = TRI.getCalleeSavedRegs(&MF);
+
+  // Early exit if there are no callee saved registers.
+  if (!CSRegs || CSRegs[0] == 0)
+    return;
+
+  SavedRegs.resize(TRI.getNumRegs());
+
+  // In Naked functions we aren't going to save any registers.
+  if (MF.getFunction()->hasFnAttribute(Attribute::Naked))
+    return;
+
+  // Functions which call __builtin_unwind_init get all their registers saved.
+  bool CallsUnwindInit = MF.getMMI().callsUnwindInit();
+  const MachineRegisterInfo &MRI = MF.getRegInfo();
+  for (unsigned i = 0; CSRegs[i]; ++i) {
+    unsigned Reg = CSRegs[i];
+    if (CallsUnwindInit || MRI.isPhysRegModified(Reg))
+      SavedRegs.set(Reg);
+  }
+}