[LLVMdev] Passing ConstantDataArray to GetElementPtrInst

[Nick Lewycky]

On 28 May 2015 at 16:52, jsrduck <jsrduck at gmail.com> wrote:

> Hi, I'm having a hard time finding an answer to what I assume is a very
> basic
> idea. I'm trying to produce this llvm code using the LLVM api:
>
>   %myString = alloca [13 x i8], i32 13
>   store [13 x i8] c"Hello world.\00", [13 x i8]* %myString
>   %tmp1 = getelementptr [13 x i8]* %myString, i32 0, i32 0
>   %tmp2 = call i32 (i8*, ...)* @printf( i8* %tmp1 ) nounwind
>
> A simple Hello World example. But I can't seem to figure out what I'm
> supposed to pass to the GetElementPtrInst. For example, this snippet:
>
>    std::vector<llvm::Value*> args;
>    auto strValue =
> llvm::ConstantDataArray::getString(llvm::getGlobalContext(),
> llvm::StringRef("Hello world."));
>

Note that this is the actual sequence of bytes, it is not a pointer to a
place where that sequence of bytes is stored. %myString is a pointer to 13
bytes, and your store instruction put those 13 bytes in it.

   std::vector<llvm::Value*> index_vector;
>    index_vector.push_back(Builder.getInt32(0));
>    index_vector.push_back(Builder.getInt32(0));
>    auto valueAsPtr = Builder.CreateGEP(strValue, index_vector, "tmp1");
>

This is trying to create something like:

  %tmp1 = getelementptr [13 x i8] c"Hello world.\00", i32 0, i32 0

which is not valid IR (as the error message says, c"Hello world.\00" is not
a pointer). You want to create what you wrote in your snippet of IR, where
the first argument is the AllocaInst you created.

Nick


>    args.push_back(valueAsPtr);
>    Builder.CreateCall(TheModule->getFunction("printf"), args,
> "callprintf");
>
> Note that this is summarized, I already have the IRBuilder, etc
> initialized.
> I've also tried this with a LoadInstr that loads the ConstantDataArray (I
> just inserted strValue into the example code to simplify things, I get the
> same error either way).
>
> If I run this code, LLVM asserts in the CreateGEP call with the error
> message "Non-pointer type for constant GetElementPtr expression" since the
> type I passed to CreateGEP is an Array type. Callstack:
>
> > Lexer.exe!llvm::ConstantExpr::getGetElementPtr(llvm::Constant * C,
> > llvm::ArrayRef<llvm::Value *> Idxs, bool InBounds, llvm::Type *
> > OnlyIfReducedTy) Line 2005 C++
>   Lexer.exe!llvm::ConstantFolder::CreateGetElementPtr(llvm::Constant * C,
> llvm::ArrayRef<llvm::Value *> IdxList) Line 133 C++
>
>
> Lexer.exe!llvm::IRBuilder<1,llvm::ConstantFolder,llvm::IRBuilderDefaultInserter<1>
> >::CreateGEP(llvm::Value * Ptr, llvm::ArrayRef<llvm::Value *> IdxList,
> const
> llvm::Twine & Name) Line 1021 C++
>
>
> Shockingly, I have been unable to find examples of anyone else doing this.
> Everyone else seems to create a global variable to hold the string value,
> and then passes that Global variable to CreateGEP. I know the llvm code I'm
> trying to create works correctly, I just can't seem to produce it using the
> tools, and I'm not sure why.
>
> Thanks,
> Jordan
>
>
>
> --
> View this message in context:
> http://llvm.1065342.n5.nabble.com/Passing-ConstantDataArray-to-GetElementPtrInst-tp81889.html
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