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

[mats petersson via cfe-dev]

I believe clang does support getting the stack-pointer via the "gcc
register variable extension". The syntax and behaviour would follow this:
https://gcc.gnu.org/onlinedocs/gcc/Global-Register-Variables.html#Global-Register-Variables

however, it is restricted specifically to the stack-pointer.

#include <stdio.h>

register size_t stack asm("rsp");

int main()
{
    printf("%zx\n", stack);
}

It compiles to this LLVM IR:
define i32 @main() #0 {
entry:
  %0 = call i64 @llvm.read_register.i64(metadata !0)
  %call = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([5 x i8],
[5 x i8]* @.str, i32 0, i32 0), i64 %0)
  ret i32 0
}

--
Mats



On 11 December 2017 at 02:58, Seth Goldstein via cfe-dev <
cfe-dev at lists.llvm.org> 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.
>
> 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>
>
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> cfe-dev at lists.llvm.org
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>
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