[LLVMdev] Custom lowering of load with extension

[Damien Vincent]

  Thank you Eli, it seems to fix my bug.

  Damien

On Mon, Jul 18, 2011 at 7:25 PM, Eli Friedman <eli.friedman at gmail.com>wrote:

> 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
>
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