[patch] Fix dangling references in thunks at -O0

[Eric Botcazou]

> 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.

> 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