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

[John Criswell via cfe-dev]

On 12/11/17 11:10 AM, Seth Goldstein wrote:
> 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?

Yes.

You can create an inline assembly code "value" by creating an InlineAsm 
object.  You then make that "value" an argument to an LLVM CallInst object.

For more information, look at the InlineAsm and CallInst classes in the 
LLVM Doxygen documentation.

Regards,

John Criswell

>
> -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 
> <mailto: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 <tel:%28412%29%20268-3828>
>>
>>
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>
>     -- 
>     John Criswell
>     Assistant Professor
>     Department of Computer Science, University of Rochester
>     http://www.cs.rochester.edu/u/criswell
>     <http://www.cs.rochester.edu/u/criswell>
>
-- 
John Criswell
Assistant Professor
Department of Computer Science, University of Rochester
http://www.cs.rochester.edu/u/criswell
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