[LLVMdev] How to Load a Value?
criswell at uiuc.edu
Thu Apr 8 10:52:59 PDT 2010
You may want to review the LLVM Language Reference Manual
The SSA value %2 is an i32 (32-bit integer). It is an SSA virtual
register; it is defined once, never written, and can be read multiple
times. To read the value, you just use it; no load instruction is
required. For example, to pass it to a function, you would write:
CallInst::Create (Function, pb, "", InsertPt)
... where Function is an SSA value containing the Function to call and
InsertPt is a pointer to an Instruction before which the call should be
Memory, in LLVM IR, is anything that is not in SSA form. This is
essentially global variables, stack allocated objects (the result of an
LLVM alloca instruction), and heap memory (pointers to which are
returned from an allocator function like malloc()). You use load and
store instructions to read and write this memory; load reads the memory
and places the result in a new SSA virtual register, and store places
the value in an SSA virtual register into a memory location.
-- John T.
Zheng Wang wrote:
> I have a problem of generating a load instruction. The LLVM bytecode is:
> %2 = call i32 (...)* @atoi(i8*%1) nounwind
> /*<- Insertpos*/
> %5 = icmp sgt i32 %2, %i.0
> Now I have
> pb: pointer to the Value object of *%2* of bb1.
> Here, I want to generate a load instruction and I did it as:
> new LoadInst(pb, "load_2", InsertPos);
> where InsertPos points to the position immediately after "%2 = call
> i32 (...)* @atoi(i8*%1) nounwind".
> BUT I got a runtime error as:
> include/llvm/Support/Casting.h:199: typename llvm::cast_retty<To,
> From>::ret_type llvm::cast(const Y&) [with X = llvm::PointerType, Y =
> const llvm::Type*]: Assertion `isa<X>(Val) && "cast<Ty>() argument of
> incompatible type!"' failed."
> This is because LoadInst is implemented as:
> LoadInst::LoadInst(Value *Ptr, const Twine &Name, BasicBlock *InsertAE)
> : UnaryInstruction(cast<PointerType>(Ptr->getType())->getElementType(),
> Load, Ptr, InsertAE)
> and pb->getType() is not a "PointerType"!!! WHY? Is it because it is a
> function call? In this situation, how can I load %2 assuming it has
> been load before.
> Can anybody gives me some hints?
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
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