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

Seth Goldstein via cfe-dev cfe-dev at lists.llvm.org
Mon Dec 11 08:10:18 PST 2017


The issue is that I need to insert this code as part of a transformation.
Is there a way to insert assembly code in a pass?

-seth


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

On Mon, Dec 11, 2017 at 10:51 AM, John Criswell <jtcriswel at gmail.com> wrote:

> 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 <(412)%20268-3828>
>
>
> _______________________________________________
> cfe-dev mailing listcfe-dev at lists.llvm.orghttp://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>
>
>
> --
> John Criswell
> Assistant Professor
> Department of Computer Science, University of Rochesterhttp://www.cs.rochester.edu/u/criswell
>
>
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