How to forbid register allocator to overlap bewteen DEST and SOURCE

[jiejie_rong@c-sky.com]

Hi,

	Thanks for your reply :)

	Maybe there are some register limits in reload pass with my case like <multiply-add>

	We need to expand to ‘mov’ and ‘madd’ first to get suitable registers.

—Jojo
在 2020年7月3日 +0800 AM5:35，Jim Wilson <jimw@sifive.com>，写道：
> On Wed, Jul 1, 2020 at 8:40 PM <jiejie_rong@c-sky.com> wrote:
> > GCC seems to overlap register bewteen DEST and SOURCE in different machine mode,
> > Is there any target hooks to control this feature ?
> > I use <Constraint Modifier> ‘&’ to forbid register allocator to overlap bewteen DEST and SOURCE,
> > but there are some redundancy instructions in the result code :(
>
> & is the correct solution in general.
>
> Presumably this is about your draft v0.7.1 vector port. This port
> uses an unspec in every pattern. This limits the compiler's ability
> to optimize code. You might get better results if you eliminated as
> many of the unspecs as you can.
>
> You might want to check TARGET_MODES_TIEABLE_P though this is mostly
> about casts and moves not register allocation.
>
> Jim