Following up to my own email, I think I found the missing magic. I
needed to set global_regs[16] to 1. Once global_regs was set for the
register, the assignment stopped getting optimized out.
Steve Ellcey
sell...@imgtec.com
On Wed, 2015-04-22 at 12:27 -0700, Steve Ellcey wrote:
> On Wed, 2015-04-22 at 12:28 +0200, Steven Bosscher wrote:
>
> > This is wrong for sure. You can't have DECL_RTL in GIMPLE.
> >
> > You will want to set has_local_explicit_reg_vars, DECL_HARD_REGISTER,
> > and DECL_ASSEMBLER_NAME, and leave it to the middle end to take care
> > of everything else.
> >
> > Ciao!
> > Steven
>
> Thanks for the advice, I switched to DECL_HARD_REGISTER and
> DECL_ASSEMBLER_NAME but I am still having the same problem, which is
> that the assignment to this global (register) variable is getting
> optimized away.
>
> If I have:
>
> ptr_type = build_pointer_type (char_type_node);
> id = get_identifier ("X");
> ptr_var = build_decl (UNKNOWN_LOCATION, VAR_DECL, id, ptr_type);
> TREE_PUBLIC (ptr_var) = 1;
> DECL_EXTERNAL (ptr_var) = 1;
> varpool_node::finalize_decl (ptr_var);
>
> The the assignment to this global variable is not removed by the
> optimizer, which makes sense because someone outside the function could
> access the value of the global variable.
>
> But if I change it to:
>
> ptr_type = build_pointer_type (char_type_node);
> id = get_identifier ("*$16");
> ptr_var = build_decl (UNKNOWN_LOCATION, VAR_DECL, id, ptr_type);
> TREE_PUBLIC (ptr_var) = 1;
> DECL_EXTERNAL (ptr_var) = 1;
> DECL_REGISTER (ptr_var) = 1;
> DECL_HARD_REGISTER (ptr_var) = 1;
> SET_DECL_ASSEMBLER_NAME (ptr_var, id);
> varpool_node::finalize_decl (ptr_var);
>
> Then the assignment to this variable is optimized away by the cse1
> optimization phase.
>
> Steve Ellcey
> sell...@imgtec.com
