Re: question about DSE

[Andrew Haley <aph@redhat.com>]

Alex Turjan wrote:
> Dear all,
> Im writing to you regarding the dead store elimination (dse) which runs after register allocation. Apparently dse removes wrongly the following store (present in bb2):
> 
> (insn 374 47 52 2 test.c:107 (set (mem/c:SI (plus:PSI (reg/f:PSI 55 ptr15)
>                 (const_int 96 [0x60])) [19 fac_iter+0 S4 A32])
>         (reg/v:SI 16 r16 [orig:161 step109 ] [161])) 48  {si_indexed_store_incl_ra} (nil))
> 
> despite being consumed (in bb3) by the following 2 loads:
> (insn 380 71 64 3 test.c:112 (set (reg:HI 1 r1)
>         (mem:HI (plus:PSI (reg/f:PSI 55 ptr15)
>                 (const_int 96 [0x60])) [0 S2 A16])) 12 {load} (nil))
> 
> (insn 382 346 65 3 test.c:112 (set (reg:HI 5 r5)
>         (mem:HI (plus:PSI (reg/f:PSI 55 ptr15)
>                 (const_int 98 [0x62])) [0 S2 A16])) 12 {load} (nil))
> 
> 
> Can anyone point what may be the problem?
> 
> As you can see the store is SI while the loads are HI. While looking to the comments from dse.c I get to the following remark:
> 
> " There are three cases where dse falls short:
>      a) Reload sometimes creates the slot for one mode of access, and
>      then inserts loads and/or stores for a smaller mode. "
> 
> Does it mean that such cases are not treated properly by dse?
> 
> I observed that if I run with the flag -fno-strict-aliasing the wrongly removed store is no longer removed and the code is runs correctly. 
> Im wondering does the dse after register allocation make use of type based alias analysis? 

Here's part of the comment in alias.c:

/* The alias sets assigned to MEMs assist the back-end in determining
   which MEMs can alias which other MEMs.  In general, two MEMs in
   different alias sets cannot alias each other ...

There's a lot more information in the comments there.

Andrew.