[cfe-dev] Adding an intrinsic to read the stack pointer, e.g., RSP in x86

[Seth Goldstein via cfe-dev]

I need to use the stack pointer, e.g., RSP on X86, to get some
information off the stack.  I am not sure what the best way to do this is.

My plan was to create an intrinsic, e.g.,

  let TargetPrefix = "x86" in {
     def int_x86_read_sp : GCCBuiltin<"__builtin_read_sp">,
      Intrinsic<[llvm_i64_ty], [], [IntrNoMem]>;
  }

I thought I would define a PseudoI, e.g.,

  let Uses = [RSP] in
      def READSP64 : PseudoI<(outs GR64:$dst), (ins),
            [(set GR64:$dst, RSP)]>,
                Requires<[In64BitMode]>;
But, actually I am not sure how this would come into play.  (Do I need
this?  If so, how do I use it?)

To test it out I added a Builtin to BuiltinsX86.def, e.g.,

  BUILTIN(__builtin_read_sp, "ULLi", "")

When I try and compile, I get a proper .ll file from clang with the
instruction

    %0 = call i64 @llvm.x86.read.sp()

However, it fails, with

llc: ./llvm/lib/CodeGen/SelectionDAG/InstrEmitter.cpp:303: unsigned int
llvm::InstrEmitter::getVR(llvm::SDValue, llvm::DenseMap<llvm::SDValue,
unsigned int>&): Assertion `I != VRBaseMap.end() && "Node emitted out of
order - late"' failed.

I am not sure how to introduce the IR to read the RSP so I can create
code to do a calculation on the RSP to retrieve a value buried in the
stack.  I figure I need to lower the `call i64 @llvm.x86.read.sp` in
X86TargetLowering::LowerOperation.  I am not sure if this would be
ISD::INTRINSIC_WO_CHAIN or ISD::INTRINSIC_W_CHAIN (and I don't
actually understand what would go in the intrinsic definition to make
it one or the other.  Any pointers would be appreciated.  Thanks,

seth


--------------
Seth Copen Goldstein
Carnegie Mellon University
Computer Science Dept
7111 GHC
412-268-3828
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