Re: [PATCH v2] cselib: add function to check if SET is redundant [PR106187]

[Jeff Law <jeffreyalaw@gmail.com>]

On 7/29/2022 7:52 AM, Richard Earnshaw via Gcc-patches wrote:
> A SET operation that writes memory may have the same value as an 
> earlier store but if the alias sets of the new and earlier store do 
> not conflict then the set is not truly redundant.  This can happen, 
> for example, if objects of different types share a stack slot.
>
> To fix this we define a new function in cselib that first checks for
> equality and if that is successful then finds the earlier store in the
> value history and checks the alias sets.
>
> The routine is used in two places elsewhere in the compiler. Firstly
> in cfgcleanup and secondly in postreload.
>
> gcc/ChangeLog:
>     * alias.h (mems_same_for_tbaa_p): Declare.
>     * alias.cc (mems_same_for_tbaa_p): New function.
>     * dse.cc (record_store): Use it instead of open-coding
>     alias check.
>     * cselib.h (cselib_redundant_set_p): Declare.
>     * cselib.cc: Include alias.h
>     (cselib_redundant_set_p): New function.
>     * cfgcleanup.cc: (mark_effect): Use cselib_redundant_set_p instead
>     of rtx_equal_for_cselib_p.
>     * postreload.c (reload_cse_simplify): Use cselib_redundant_set_p.
>     (reload_cse_noop_set_p): Delete.
Seems quite reasonable.   The only question I would have would be 
whether or not you considered including the aliasing info into the 
hashing used by cselib.  You'd probably still need the bulk of this 
patch as well since we could presumably still get a hash conflict with 
two stores of the same value to the same location, but with different 
alias sets (it's just much less likely), so perhaps it doesn't really 
buy us anything.

Ideally this would include a testcase.  You might be able to turn that 
non-executawble reduced case into something useful by scanning the 
post-reload dumps.

Jeff