[LLVMdev] Custom lowering of load with extension

[Eli Friedman]

On Mon, Jul 18, 2011 at 6:35 PM, Damien Vincent <damien.llvm at gmail.com> wrote:
>
>   Hi,
>
> The target I am working on does not support i8 loading: it supports only 32
> bit aligned loads.
> I had to custom lower i8 load so that only i32 loads are generated.
>
> My issue is the following:
> In a very specific case, the lowering stage generates the following pattern
> where the load is a i32 load:
> (and (load x), 255)
> This pattern is somehow converted by the DAG combiner back to:
>  (zextload x, i8)
> And then, during the instruction selection phase, I get:
> error in backend: Cannot select: 0xb3a5558: i32,ch = load 0xb3823c4,
> 0xb3a1890, 0xb3a2440<LD1[%4](align=4), zext from i8>
> because the instruction is simply not supported by the backend (that's the
> initial reason for implementing the custom lowering !)
>
> How come the DAG combiner converts my lowering back to an i8 loading ?
> Looking at the DAGCombiner.cpp code, this transformation is done because
> isLoadExtLegal returns true in the case of custom lowering of load which
> does not seem logical to me (do you have an explanation for this ?).
>
> Well, I assume I didn't do sth the right way... Can you point a mistake in
> my reasoning ?

Looks like a bug to me too. :)  r135462 should fix it.

-Eli