On Tue, Aug 15, 2017 at 10:43 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, Aug 15, 2017 at 1:14 PM, Luc Van Oostenryck
> <luc.vanoostenryck@xxxxxxxxx> wrote:
>>
>> For information, the current SSA construction is very broken,
>> both in simplify_one_symbol() (the main SSA construction)
>> and simplify_loads() (which also creates phi-nodes). They both
>> are 'load-driven' which is OK but they both put the phi-nodes
>> where the load is while phi-nodes need to placed where branches
>> meet. The loads are often placed in a BB following where the
>> join-point. Often this *seems* to be OK but in fact is very wrong
>> and create all sort of problems a bit everywhere
>
> I do think you're too hung up about the placement of the phi nodes.
>
> It may be an issue for simple models that just walk the linearized
> code directly and try to turn it into code. But the original intent of
> the existing phi node code was to just be as sources of the pseudo's -
> the location should be largely immaterial.
Yes, what I thought for a long while,
But when things like try_ti_simplify_bb() come in play,
with pack_basic_blocks() on top the things become
*very* messy I often you can't make sense of it anymore.
This has also consequences on liveness (but it can't explain
the root of the problem there, i's month's I realized it was useless.
> IOW, you could think of the phi nodes as being "outside" the
> instruction flow entirely, and just being the definition of the pseudo
> they define. They have to be placed somewhere, but the "somewhere" is
> somewhat arbitrary.
It's true to a certain point, after it, you need to have them placed at
the confluence point. I can't explain better for now. I'll have an
example tomorrow.
> That said, I like your new ssa construction -independently- of that
> issue - the old SSA constructor really was very ad-hoc. Look through
> the git history if you don't believe me ;)
Oh I believe ve *and* I have view the logs.
Not directly related to that is that I really think we need to associate
a ph-node's source to it's parent BB.
So if you have something:
| | |
A B C
\ | /
*rx: phi, *phsra, %phisrb, %phisrcx ,
We really want something like:
| | |
A B C
\ | /
*rx: phi, A:*phsra, B:%phisrb, C:%phisrcx
It's essential for a number of things (I suspect a number
of SSA algorithm, for liveliness , writing a trivial simulator, ....)
-- Luc
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