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

[Jeff Law <law@redhat.com>]

On 12/11/2017 07:18 AM, 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.
> 
OK.  Ideally you'd have a test for the testsuite as well, but I won't
stress without it :-)


jeff