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

[John Criswell via cfe-dev]

On 12/10/17 9:58 PM, Seth Goldstein via cfe-dev wrote:
> 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.

Is there a reason that you cannot insert some inline assembly code to 
read the stack pointer?  The only downside to it (as far as I can tell) 
is that you would need to have different assembly code for each platform 
on which you want to do this.

Regards,

John Criswell
>
> 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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-- 
John Criswell
Assistant Professor
Department of Computer Science, University of Rochester
http://www.cs.rochester.edu/u/criswell

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