Re: [PATCH 6/6] Add a late-combine pass [PR106594]

[Richard Biener <richard.guenther@gmail.com>]

On Fri, Jun 21, 2024 at 10:21 AM Richard Sandiford
<richard.sandiford@arm.com> wrote:
>
> Richard Biener <richard.guenther@gmail.com> writes:
> > [...]
> > I wonder if you can amend doc/passes.texi, specifically noting differences
> > between fwprop, combine and late-combine?
>
> Ooh, we have a doc/passes.texi? :)  Somehow missed that.

Yeah, I also usually forget this.

> How about the patch below?

Thanks - looks good to me.

Richard.

> Thanks,
> Richard
>
>
> diff --git a/gcc/doc/passes.texi b/gcc/doc/passes.texi
> index 5746d3ec636..4ac7a2306a1 100644
> --- a/gcc/doc/passes.texi
> +++ b/gcc/doc/passes.texi
> @@ -991,6 +991,25 @@ RTL expressions for the instructions by substitution, simplifies the
>  result using algebra, and then attempts to match the result against
>  the machine description.  The code is located in @file{combine.cc}.
>
> +@item Late instruction combination
> +
> +This pass attempts to do further instruction combination, on top of
> +that performed by @file{combine.cc}.  Its current purpose is to
> +substitute definitions into all uses simultaneously, so that the
> +definition can be removed.  This differs from the forward propagation
> +pass, whose purpose is instead to simplify individual uses on the
> +assumption that the definition will remain.  It differs from
> +@file{combine.cc} in that there is no hard-coded limit on the number
> +of instructions that can be combined at once.  It also differs from
> +@file{combine.cc} in that it can move instructions, where necessary.
> +
> +However, the pass is not in principle limited to this form of
> +combination.  It is intended to be a home for other, future
> +combination approaches as well.
> +
> +The pass runs twice, once before register allocation and once after
> +register allocation.  The code is located in @file{late-combine.cc}.
> +
>  @item Mode switching optimization
>
>  This pass looks for instructions that require the processor to be in a