[LLVMdev] Two questions about creating a new target

[Eli Friedman]

On Tue, Nov 9, 2010 at 6:04 PM, Lorenzo De Carli <lorenzo at cs.wisc.edu> wrote:
> Hi list,
>
> I am in the process of creating a new target in LLVM/Clang for a
> custom 16-bit machine. My initial goal is to configure Clang so that
> when LLVM bytecode (or assembly) is generated, the integer size is 16
> bits.
>
> After defining an appropriate TargetInfo subclass (where integer size
> is set to 16) the generated LLVM assembly looks almost correct,
> however one of the parameters for getelementptr still gets passed as a
> 32-bit integer:
>
> %1 = load i16* getelementptr inbounds ([5 x i16]*
> @myfunc.auto.myvector, i32 0, i16 0)
>
> This fact disturbs me slightly, because it means that Clang somehow
> still "thinks" that the target has the notion of 32-bit variables
> (which is incorrect). Could anyone give me a hint on how to fix it?

AFAICT, the index in question comes from
CodeGenFunction::EmitArraySubscriptExpr in CGExpr.cpp, which I think
was written without really paying attention to the type used for the
index.  In this case, it's harmless; zero is zero no matter its width.
 That said, feel free to submit a patch to cfe-commits.  (And in the
future, please provide C code as an example.)

> Another thing I have noticed... this is more a curiosity about Clang
> internals :-) When I generate code using x86 as target, variables are
> allocated with "alloca", but when I use my target they are simply
> declared as auto-variables. Is there any way to get the same
> allocation that is used for x86?

You'll have to be more specific here; that doesn't sound like
something that should vary by target.  Can you provide a simple C
testcase and the generated IR to show the issue?

On a side note, this is more appropriate for cfe-dev; I'm redirecting
the discussion there.

-Eli