Re: [Patch combine] Don't create vector mode ZERO_EXTEND from subregs

[Marc Glisse <marc.glisse@inria.fr>]

I was happily going to close PR 55549, but it looks like we are trying the 
same transformation in a second place in that file :-(

On Mon, 11 Dec 2017, James Greenhalgh wrote:

> Hi,
>
> In simplify_set we try transforming the paradoxical subreg expression:
>
>  (set FOO (subreg:M (mem:N BAR) 0))
>
> in to:
>
>  (set FOO (zero_extend:M (mem:N BAR)))
>
> However, this code does not consider the case where M is a vector
> mode, allowing it to construct (for example):
>
>  (zero_extend:V4SI (mem:SI))
>
> This would clearly have the wrong semantics, but fortunately we fail long
> before then in expand_compound_operation. As we really don't want a vector
> zero_extend of a scalar value.
>
> We need to explicitly reject vector modes from this transformation.
>
> This fixes a failure I'm seeing on a branch in which I'm trying to
> tackle some performance regressions, so I have no live testcase for
> this, but it is wrong by observation.
>
> Tested on aarch64-none-elf and bootstrapped on aarch64-none-linux-gnu with
> no issues.
>
> OK?
>
> Thanks,
> James
>
> ---
> 2017-12-11  James Greenhalgh  <james.greenhalgh@arm.com>
>
> 	* combine.c (simplify_set): Do not transform subregs to zero_extends
> 	if the destination mode is a vector mode.
>
>

-- 
Marc Glisse