Re: Potential incorrect simplification

[Linus Torvalds <torvalds@linux-foundation.org>]

On Tue, Mar 28, 2017 at 1:27 PM, Luc Van Oostenryck
<luc.vanoostenryck@gmail.com> wrote:
> On Tue, Mar 28, 2017 at 11:02:32AM -0700, Linus Torvalds wrote:
>>
>> The single-store shortcut is actually pretty important, because it is
>> what turns a *lot* of local variables from stack memory things into
>> pseudos. That's partly because the sparse optimizations aren't all
>> that smart.
>
> Everytime I look at this part of the code, I think the same:
>   why turn all vars into stack memory to have to turn them later
>   into pseudos?

Oh, you absolutely *cannot* turn them into pseudo's directly.

A pseudo is a "register", and in SSA format. Local variables are not
registers, and do not honor SSA.

So local variables are very much *not* pseudos at any time. Local
variables can have their address taken, local variables can be
assigned multiple times, local variables can have complex types, none
of which is pseudo-like behavior.

Now, some *very* limited cases of local variables end up being
trivially very similar to pseudos, and those simple cases get turned
into pseudos. But that only happens for the simple cases - when their
address is not taken or used, and when they only have a single
assignment to them.

But local variables do not start out as pseudos, exactly because the
pseudo case is a very very very limited case of the full local
variable case.

                        Linus