Make musttail more robust for vector types on x86

Previously I tried to plug musttail into the existing vararg lowering
code. That turned out to be a mistake, because non-vararg calls use
significantly different register lowering, even on x86. For example, AVX
vectors are usually passed in registers to normal functions and memory
to vararg functions.  Now musttail uses a completely separate lowering.

Hopefully this can be used as the basis for non-x86 perfect forwarding.

Reviewers: majnemer

Differential Revision: http://reviews.llvm.org/D6156

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224745 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/CallingConvLower.h

@@ -158,6 +158,16 @@ public:
   }
 };
 
+/// Describes a register that needs to be forwarded from the prologue to a
+/// musttail call.
+struct ForwardedRegister {
+  ForwardedRegister(unsigned VReg, MCPhysReg PReg, MVT VT)
+      : VReg(VReg), PReg(PReg), VT(VT) {}
+  unsigned VReg;
+  MCPhysReg PReg;
+  MVT VT;
+};
+
 /// CCAssignFn - This function assigns a location for Val, updating State to
 /// reflect the change.  It returns 'true' if it failed to handle Val.
 typedef bool CCAssignFn(unsigned ValNo, MVT ValVT,
@@ -470,6 +480,19 @@ public:
     return PendingLocs;
   }
 
+  /// Compute the remaining unused register parameters that would be used for
+  /// the given value type. This is useful when varargs are passed in the
+  /// registers that normal prototyped parameters would be passed in, or for
+  /// implementing perfect forwarding.
+  void getRemainingRegParmsForType(SmallVectorImpl<MCPhysReg> &Regs, MVT VT,
+                                   CCAssignFn Fn);
+
+  /// Compute the set of registers that need to be preserved and forwarded to
+  /// any musttail calls.
+  void analyzeMustTailForwardedRegisters(
+      SmallVectorImpl<ForwardedRegister> &Forwards, ArrayRef<MVT> RegParmTypes,
+      CCAssignFn Fn);
+
 private:
   /// MarkAllocated - Mark a register and all of its aliases as allocated.
   void MarkAllocated(unsigned Reg);