CODEWITHSUNDEEP
cllvmllvm-ir
muke
muke

Reputation: 396

LLVM-IR Get Struct Element Pointer

I'm building LLVM-IR with the C API and have a pointer to the following struct type:

{ i8*, i8*, i8*, i8*, i8*, i8*, i8*, [1 x i64] }

That is, several i8 pointers and an array of long integers, currently of size one but this will change in the future.

I want to access the element in the array at index 7, and my current understanding is that I should be able to do this with a call to LLVMBuildStructGEP on the pointer to the struct, passing index 7, and then a call to LLVMBuildGEP2 on the returned address with i64 as the type and 0 as the index.

This method doesn't flag up any problems when passing the module to LLVMAnalyseModule, but if I dump the bitcode of the module and then run llvm-dis on the file, I get the error:

llvm-dis: error: Explicit gep type does not match pointee type of pointer operand

Which I've isolated down to the second GEP on the address of the array returned from the first struct GEP.

For some additional information, I've noticed that if I replace the first struct GEP with a regular GEP taking the struct type as an argument, and then pass the struct type to the second GEP on what should be the i64 array address, llvm-dis will not complain, but then subsequent loads or stores will be working with the wrong types so this is obviously not a real solution.

Upvotes: 1

Views: 833

Answers (1)

CPPCPPCPPCPPCPPCPPCPPCPPCPPCPP
CPPCPPCPPCPPCPPCPPCPPCPPCPPCPP

Reputation: 1025

Okay. So, answering here. As you see, you need to first get zero index, after than indexes 7 and 0. The reason is simple to explain. Let's see an example why extra zero is needed:

char A *a = new A[123];

First index which is zero in your case can be used in arrays, but in your case it is a pointer to single element. In the case of array it can be not zero.

Anyway, I am not so clever about LLVM C APIs. Please don't kill me for that :D

Upvotes: 2

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