[PATCH 12/12] mode-switching: Add a backprop hook

[Jeff Law]

On 11/5/23 11:50, Richard Sandiford wrote:
> This patch adds a way for targets to ask that selected mode changes
> be brought forward, through a combination of:
> 
> (1) requiring a mode in blocks where the entity was previously
>      transparent
> 
> (2) pushing the transition at the head of a block onto incomging edges
> 
> SME has two uses for this:
> 
> - A "one-shot" entity that, for any given path of execution,
>    either stays off or makes exactly one transition from off to on.
>    This relies only on (1) above; see the hook description for more info.
> 
>    The main purpose of using mode-switching for this entity is to
>    shrink-wrap the code that requires it.
> 
> - A second entity for which all transitions must be from known
>    modes, which is enforced using a combination of (1) and (2).
>    More specifically, (1) looks for edges B1->B2 for which:
> 
>    - B2 requires a specific mode and
>    - B1 does not guarantee a specific starting mode
> 
>    In this system, such an edge is only possible if the entity is
>    transparent in B1.  (1) then forces B1 to require some safe common
>    mode.  Applying this inductively means that all incoming edges are
>    from known modes.  If different edges give different starting modes,
>    (2) pushes the transitions onto the edges themselves; this only
>    happens if the entity is not transparent in some predecessor block.
> 
> The patch also uses the back-propagation as an excuse to do a simple
> on-the-fly optimisation.
> 
> Hopefully the comments in the patch explain things a bit better.
> 
> gcc/
> 	* target.def (mode_switching.backprop): New hook.
> 	* doc/tm.texi.in (TARGET_MODE_BACKPROP): New @hook.
> 	* doc/tm.texi: Regenerate.
> 	* mode-switching.cc (struct bb_info): Add single_succ.
> 	(confluence_info): Add transp field.
> 	(single_succ_confluence_n, single_succ_transfer): New functions.
> 	(backprop_confluence_n, backprop_transfer): Likewise.
> 	(optimize_mode_switching): Use them.  Push mode transitions onto
> 	a block's incoming edges, if the backprop hook requires it.
OK.  Really curious if we might be able to use this to improve the 
vsetvl bits in the RISC-V backend.

Jeff