Re: [PATCH 3/6] rtl-ssa: Fix ICE when deleting memory clobbers

[Jeff Law <jeffreyalaw@gmail.com>]

On 10/24/23 04:50, Richard Sandiford wrote:
> Sometimes an optimisation can remove a clobber of scratch registers
> or scratch memory.  We then need to update the DU chains to reflect
> the removed clobber.
> 
> For registers this isn't a problem.  Clobbers of registers are just
> momentary blips in the register's lifetime.  They act as a barrier for
> moving uses later or defs earlier, but otherwise they have no effect on
> the semantics of other instructions.  Removing a clobber is therefore a
> cheap, local operation.
> 
> In contrast, clobbers of memory are modelled as full sets.
> This is because (a) a clobber of memory does not invalidate
> *all* memory and (b) it's a common idiom to use (clobber (mem ...))
> in stack barriers.  But removing a set and redirecting all uses
> to a different set is a linear operation.  Doing it for potentially
> every optimisation could lead to quadratic behaviour.
> 
> This patch therefore refrains from removing sets of memory that appear
> to be redundant.  There's an opportunity to clean this up in linear time
> at the end of the pass, but as things stand, nothing would benefit from
> that.
> 
> This is also a very rare event.  Usually we should try to optimise the
> insn before the scratch memory has been allocated.
> 
> gcc/
> 	* rtl-ssa/changes.cc (function_info::finalize_new_accesses):
> 	If a change describes a set of memory, ensure that that set
> 	is kept, regardless of the insn pattern.
OK
jeff