Re: [PATCH 2/3] rtl-ssa: Extend make_uses_available

[Jeff Law <jeffreyalaw@gmail.com>]

On 10/24/23 11:58, Richard Sandiford wrote:
> The first in-tree use of RTL-SSA was fwprop, and one of the goals
> was to make the fwprop rewrite preserve the old behaviour as far
> as possible.  The switch to RTL-SSA was supposed to be a pure
> infrastructure change.  So RTL-SSA has various FIXMEs for things
> that were artifically limited to faciliate the old-fwprop vs.
> new-fwprop comparison.
> 
> One of the things that fwprop wants to do is extend live ranges, and
> function_info::make_use_available tried to keep within the cases that
> old fwprop could handle.
> 
> Since the information is built in extended basic blocks, it's easy
> to handle intra-EBB queries directly.  This patch does that, and
> removes the associated FIXME.
> 
> To get a flavour for how much difference this makes, I tried compiling
> the testsuite at -Os for at least one target per supported CPU and OS.
> For most targets, only a handful of tests changed, but the vast majority
> of changes were positive.  The only target that seemed to benefit
> significantly was i686-apple-darwin.
> 
> The main point of the patch is to remove the FIXME and to enable
> the upcoming post-RA late-combine pass to handle more cases.
> 
> gcc/
> 	* rtl-ssa/functions.h (function_info::remains_available_at_insn):
> 	New member function.
> 	* rtl-ssa/accesses.cc (function_info::remains_available_at_insn):
> 	Likewise.
> 	(function_info::make_use_available): Avoid false negatives for
> 	queries within an EBB.
OK
jeff