[LLVMdev] replacing GetElementPtrConstantExpr with GetElementPtrInst ... sometimes

[Eli Friedman]

On Fri, Aug 2, 2013 at 10:56 AM, Robert Lytton <robert at xmos.com> wrote:
> Hi
>
> During a pass, the XCore target lowers thread local global variables by
> turning them into global variable arrays indexed by the (max 8) thread ID.
> (see XCoreLowerThreadLocal.cpp)
>
> This works fine for instructions e.g. GetElementPtrInst
> But can't be done for constants e.g. GetElementPtrConstantExpr
>
> Thus I would like to replace GetElementPtrConstantExpr with
> GetElementPtrInst when it is accessing a thread local global variable.
> (Other constant expression ignored for now).
>
> My attempt (example code below) has revealed my lack of understanding of
> llvm.
> For one thing I need to distinguish when the GetElementPtrConstantExpr is a
> child of an instruction and hence can be changed into an instruction itself.
>
> When offered:
>     @tl = external thread_local global [1 x i32]
>     define i32* @tli () {
>         ret i32* getelementptr([1 x i32]* @tl, i32 0, i32 0)
>     }
> the example code prints:
>     @tl = external thread_local global [1 x i32]
>     ConstantExpr ops...
>     @tl = external thread_local global [1 x i32]
>     i32 0
>     i32 0
>     Replace...
>     i32* getelementptr inbounds ([1 x i32]* @tl, i32 0, i32 0)
>     with...
>       <badref> = getelementptr inbounds [1 x i32]* @tl, i32 0, i32 0
>     leave for backend error
>     Instruction does not dominate all uses!
>       <badref> = getelementptr inbounds [1 x i32]* @tl, i32 0, i32 0
>       ret i32* <badref>
>     Broken module found, compilation aborted!
>
>
> Q: Is my approach generally correct?
> I am also concerned that:
>     @i = constant i32* getelementptr ([1 x i32]* @tl, i32 0, i32 0)
> must be skipped and not replaced.
> Hence 'replaceAllUsesWith' is a bad idea!

This isn't legal IR given that tl is thread-local: what would it even mean?

> I plan to explore the parent node next to verify its type and replace its
> GetElementPtrConstantExpr operand if it is an instruction.
> Q: Is this the right direction?

You're going to need something a lot more complicated: in the general
case, you need to split critical edges if there's a reference to the
global in an operand to a PHI nodes.  ISel already has a bunch of
utilities for handling that sort of thing; you might want to consider
putting your lowering there instead.

-Eli