[Bug rtl-optimization/82683] Combine: GCC generates bad code with -tune=thunderx2t99

["wilco at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82683

--- Comment #4 from Wilco <wilco at gcc dot gnu.org> ---
(In reply to Segher Boessenkool from comment #3)
> Ah.  So we start with
> 
> insn_cost 4 for    18: r91:DI=r83:DI<<0x2
>       REG_DEAD r83:DI
> insn_cost 4 for    19: r78:DI=r76:DI+r91:DI
>       REG_DEAD r91:DI
>       REG_DEAD r76:DI
> insn_cost 20 for    20: r82:SI=[r78:DI]
>       REG_DEAD r78:DI
> insn_cost 4 for    21: r83:DI=sign_extend(r82:SI)
> 
> and first 18,19 are combined, and then 19,20,21.  Both 19 and 21 set
> r83, it is dead after 19, but that note is put on 21.

Yes, it doesn't show intermediate stages but I guess before merging 18+19+20
with 21 we have:

r82:SI=MEM([r83:DI*0x4+r76:DI]) REG_DEAD r83
r83:DI=sign_extend r82:SI

This is correct. However when it merges these, it swaps r82 for r83 on the
load, and now the REG_DEAD in the load needs to be removed as it is no longer
valid:

r83:DI=sign_extend([r83:DI*0x4+r76:DI]) ***REG_DEAD r83***