Add TRI::getCallPreservedMask() hook.

The hook returns a bit-mask of call-preserved registers that will
eventually replace the current list of implicit defs on call
instructions.  This will make it possible to support multiple calling
conventions without duplicating call instruction descriptors.

The call-preserved mask is slightly different from the list returned by
the getCalleeSavedRegs() hook, it includes all aliases that are
preserved by calls.

The hook takes a CallingConv::ID argument instead of a MachineFunction
pointer, so it can provide information about calls to extern functions,
and even indirect function calls.

TRI::getCalleeSavedRegs() returns information about the function
currently being compiled. TRI::getCallPreservedMask() returns
information about the functions it is calling.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@148165 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Target/TargetRegisterInfo.h

@@ -20,6 +20,7 @@
 #include "llvm/CodeGen/MachineBasicBlock.h"
 #include "llvm/CodeGen/ValueTypes.h"
 #include "llvm/ADT/ArrayRef.h"
+#include "llvm/CallingConv.h"
 #include <cassert>
 #include <functional>
 
@@ -357,10 +358,30 @@ public:
   /// getCalleeSavedRegs - Return a null-terminated list of all of the
   /// callee saved registers on this target. The register should be in the
   /// order of desired callee-save stack frame offset. The first register is
-  /// closed to the incoming stack pointer if stack grows down, and vice versa.
+  /// closest to the incoming stack pointer if stack grows down, and vice versa.
+  ///
   virtual const unsigned* getCalleeSavedRegs(const MachineFunction *MF = 0)
                                                                       const = 0;
 
+  /// getCallPreservedMask - Return a mask of call-preserved registers for the
+  /// given calling convention on the current sub-target.  The mask should
+  /// include all call-preserved aliases.  This is used by the register
+  /// allocator to determine which registers can be live across a call.
+  ///
+  /// The mask is an array containing (TRI::getNumRegs()+31)/32 entries.
+  /// A set bit indicates that all bits of the corresponding register are
+  /// preserved across the function call.  The bit mask is expected to be
+  /// sub-register complete, i.e. if A is preserved, so are all its
+  /// sub-registers.
+  ///
+  /// Bits are numbered from the LSB, so the bit for physical register Reg can
+  /// be found as (Mask[Reg / 32] >> Reg % 32) & 1.
+  /// NULL pointer is equivalent to an all-zero mask.
+  ///
+  virtual const uint32_t *getCallPreservedMask(CallingConv::ID) const {
+    // The default mask clobbers everything.  All targets should override.
+    return 0;
+  }
 
   /// getReservedRegs - Returns a bitset indexed by physical register number
   /// indicating if a register is a special register that has particular uses