On Mon, Sep 14, 2020 at 12:56 PM Eric Botcazou <botca...@adacore.com> wrote:
>
> > No, so you're right - validity analysis is split. Still the cause you
> > reference comes from RTL expansion which means RTL expansion should
> > possibly do the disqualification here. The question is what makes the case
> > you run into at -O0 impossible to trigger with -O1+?
>
> The key test in use_register_for_decl is:
>
> if (optimize)
> return true;
>
> I think that all the preceding tests that return false cannot trigger in this
> context - a parameter passed by invisible reference - so, by returning true
> here, you're pretty much covered I think.
>
> > Maybe we can also avoid this spilling at -O0?
>
> The code wants to retrieve the parameter at all optimization levels. The
> question is: register or stack slot? It uses use_register_for_decl to decide
> as in other circumstances, but so you want to always force a register?
>
> Note that, since cgraph_node::expand_thunk has expanded the thunk in GIMPLE,
> the function is no longer a thunk for the middle-end (cfun->is_thunk false).
>
> That being said, I can add another bit to cfun, e.g. is_gimple_thunk, and
> force the use of a register above only in case the bit is true.
/* We don't set DECL_IGNORED_P for the function_result_decl. */
if (optimize)
return true;
/* We don't set DECL_REGISTER for the function_result_decl. */
return false;
or maybe set (and check) DECL_REGISTER? But the above certainly
looks like we can pick as we wish.
> > Yeah, but the asm thunk tail-calls also at -O0. I guess tail-calling is
> > forced here because gimple analysis sometimes would not mark the call.
>
> The low-level assembly thunks simply support the most simple thunks, see the
> condition in expand_thunk. Moreover targets can opt out as they wish.
>
> --
> Eric Botcazou
>
>
