[LLVMdev] know if individual LLVM's Instruction has a result, and how to obtain it?

[Duncan Sands]

Hi Chuck,

> Most LLVM IR instructions have a result field, according to the Language Reference.
>
> I want to know, for all LLVM Instructions, is there an easy and consistent way
> to know if the current Inst has a result field?
> And if yes, what is the best way to obtain it?

the instruction is the result!  So everywhere you though you needed to use
Instruction->getResult(), just use Instruction.  Thus your question about
whether there is a result field doesn't make sense.  The closest thing is
probably: the instruction does not have void type.

Ciao, Duncan.

>
> E.g.:
>
>    <result>  = add<ty>  <op1>,<op2>           /; yields {ty}:result
>
> /
>
> All ADD instruction will have a result field, regardless of whether its result
> is used.
>
> I checked the source code for Instruction.h/.cpp, getOperand(int) is for
> obtaining the operands only. There is no dedicated method to obtain the result.
> {I was expecting something like Instruction->getResult().}
>
>
> Instruction * Inst;
> ...
> Value * Res = Inst;
>
> Is the above the right way to do the work?
>
> For an other example:
>
>   <result>  = [tail] call [cconv  <http://llvm.org/docs/LangRef.html#callingconv>] [ret attrs  <http://llvm.org/docs/LangRef.html#paramattrs>]<ty>  [<fnty>*]<fnptrval>(<function args>) [fn attrs  <http://llvm.org/docs/LangRef.html#fnattrs>]
>
>
> According to the LLVM Language reference, CallInst should always have a result.
> But for void bar(int) types, there will be no return. So the generated IR will
> be similar to:
>
>    call void @bar(i32 %2) nounwind
>
> instead of
>
>    %3 = call void @bar(i32 %2) nounwind
>
> How can I handle this or similar case?
>
> Are there special handling for certain Instructions? (E.g. LoadInst, StoreInst,
> etc.)
>
> Thank you very much
>
> Chuck
>
>
>
>
>
>
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