[llvm-dev] Optimisation passes introducing address space casts

[Mehdi Amini via llvm-dev]

Hi,

> On Nov 9, 2016, at 10:13 AM, James Price via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hi,
> 
> I’ve recently encountered an issue where the `instcombine` pass replaces an `llvm.memcpy` between two distinct address spaces with an `addrspacecast` instruction.
> 
> As an example, see the trivial OpenCL kernel attached. I’m compiling like this:
> 
>    clang -cc1 -triple spir64-unknown-unknown -x cl -O0 -emit-llvm array_init.cl -o before.ll
> 
> This yields an `llvm.memcpy` to copy the array initialiser data from the global variable (in `addrspace(2)`) to the `alloca` result (in `addrspace(0)`).
> 
> I then apply the `instcombine` pass via:
> 
>    opt -S -instcombine before.ll -o after.ll
> 
> This results in the memcpy being nuked, and the `addrspace(2)` data is now accessed directly via an `addrspacecast` to `addrspace(0)`.
> 
> 
> It seems to me that this sort of optimisation is only valid if it is guaranteed that the two address spaces alias for the given target triple (which for SPIR, they do not).

I’m not sure “alias” is the right consideration here.

> 
> 
> This particular optimisation is coming from lines ~290-300 of InstCombineLoadStoreAlloca.cpp, although I suspect this isn’t the only case where this might happen.
> 
> Adding a check to only perform this replacement if the two address spaces are equal fixes the issue for me, but this is probably too conservative since many targets with flat address spaces will probably benefit from this optimisation.
> It feels like passes should query the target about whether two address spaces alias before introducing an `addrspacecast`, but I’m not familiar enough with LLVM internals to know if this is information that is easy to make available (if it isn’t already).
> 
> Is there something we can do here to avoid this sort of optimisation causing problems for targets with segmented address spaces?

This is a bug, we can’t assume any memory layout I believe.

The memcpy is supposed to be equivalent to a sequence of load and store. Here we are just failing to keep the property that the load is performed through addrspace(2). The fix is not “trivial” though, the transformation can only be performed if the address of the alloca does not escape, and some rewriting of the uses is needed to propagate the address space through GEPs for instance.

— 
Mehdi