Implememting named register intrinsics

This patch implements the infrastructure to use named register constructs in
programs that need access to specific registers (bare metal, kernels, etc).

So far, only the stack pointer is supported as a technology preview, but as it
is, the intrinsic can already support all non-allocatable registers from any
architecture.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208104 91177308-0d34-0410-b5e6-96231b3b80d8

test/CodeGen/X86/stackpointer.ll

@@ -0,0 +1,25 @@
+; RUN: llc < %s -mtriple=x86_64-apple-darwin  | FileCheck %s
+; RUN: llc < %s -mtriple=x86_64-linux-gnueabi | FileCheck %s
+
+define i64 @get_stack() nounwind {
+entry:
+; CHECK-LABEL: get_stack:
+; CHECK: movq	%rsp, %rax
+	%sp = call i64 @llvm.read_register.i64(metadata !0)
+  ret i64 %sp
+}
+
+define void @set_stack(i64 %val) nounwind {
+entry:
+; CHECK-LABEL: set_stack:
+; CHECK: movq	%rdi, %rsp
+  call void @llvm.write_register.i64(metadata !0, i64 %val)
+  ret void
+}
+
+declare i64 @llvm.read_register.i64(metadata) nounwind
+declare void @llvm.write_register.i64(metadata, i64) nounwind
+
+; register unsigned long current_stack_pointer asm("rsp");
+; CHECK-NOT: .asciz  "rsp"
+!0 = metadata !{metadata !"rsp\00"}