[LLVMdev] Operations on constant array value?

James Williams junk at giantblob.com
Mon Jan 11 11:11:32 PST 2010

2010/1/11 Garrison Venn <gvenn.cfe.dev at gmail.com>

> Does the C API have an equivalent of stack storage? Via the C++ APIs one
> can shove the string constant on the stack via
> a store instruction operation on an alloca instruction--the address needed
> is the alloca. For example:
> llvm::Value* stringVar = builder.CreateAlloca(stringConstant->getType());
> builder.CreateStore(stringConstant, stringVar);
> The stringVar is your address.
> Garrison

Thanks but I want something I can use to generate code for a pointer to a C
string in any context so I should avoid alloca. Otherwise I'll risk massive
stack growth if the generated code is within a loop for example. It looks
like using a global variable is the canonical way to do this so I'll stick
with it.

-- James

> On Jan 11, 2010, at 10:07, James Williams wrote:
> Hi,
> I've read http://llvm.org/docs/LangRef.html#t_array and
> http://llvm.org/docs/GetElementPtr.html and if I've understood right there
> are no operations that act directly on arrays - instead I need to use
> getelementptr on a pointer to an array to get a pointer to an array element.
> I also understand that there is no 'address of' operation.
> As a result I can't figure out how to use constant derived types without
> assigning them to a global. Say I want to use the C bindings function
> LLVMValueRef LLVMConstString(char *, int, int) to get an int8* pointer to a
> C string constant - there doesn't seem to be any way to directly use the
> resulting [N x i8] value directly and there's no operator that gives me its
> address.
> The only way I can see to get a pointer to the string constant array is to
> go through a global variable, for example:
> g = LLVMAddGlobal(module, LLVMTypeOf(v), "__string_" +
> string_literal_number);
> string_literal_number = string_literal_number + 1;
> v = LLVMConstString(string_literal, string_literal.Length, 0);
> LLVMSetInitializer(g, v);
> elements = { LLVMConstInt(LLVMInt32Type(), 0L, 0),
> LLVMConstInt(LLVMInt32Type(), 0L, 0) };
> return LLVMConstInBoundsGEP(g, elements, 2);
> Is it possible to get the address of an element of a constant array or
> struct without first initializing a global variable to the constant?
> Thanks in advance,
> -- James Williams
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