A couple GIMPLE questions

[Gary Oblock]

>That's not a question? Are you asking why PHIs exist at all?
>They are the standard way to represent merging in SSA
>representations. You can iterate on the PHIs of a basic block, etc.

Marc,

I first worked with the SSA form twenty years ago so yes I am
aware of what a phi is... I've just never seen a compiler eliminate
an assignment of a variable to a constant and jam the constant into
the phi where the SSA variable should be. What a phi is all about
is representing data flow and a constant in the phi doesn't seem
to be related to that. I can deal with this but it seems that having to
crawl the phis looking for constants seems baroque. I would hope
there is a control that can suppress this or a transformation
that I can invoke to reverse it...

Thanks,

Gary
________________________________
From: Marc Glisse <marc.glisse@inria.fr>
Sent: Saturday, September 5, 2020 11:29 PM
To: Gary Oblock <gary@amperecomputing.com>
Cc: gcc@gcc.gnu.org <gcc@gcc.gnu.org>
Subject: Re: A couple GIMPLE questions

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On Sat, 5 Sep 2020, Gary Oblock via Gcc wrote:

> First off one of the questions just me being curious but
> second is quite serious. Note, this is GIMPLE coming
> into my optimization and not something I've modified.
>
> Here's the C code:
>
> type_t *
> do_comp( type_t *data, size_t len)
> {
>  type_t *res;
>  type_t *x = min_of_x( data, len);
>  type_t *y = max_of_y( data, len);
>
>  res = y;
>  if ( x < y ) res = 0;
>  return res;
> }
>
> And here's the resulting GIMPLE:
>
> ;; Function do_comp.constprop (do_comp.constprop.0, funcdef_no=5, decl_uid=4392, cgraph_uid=3, symbol_order=68) (executed once)
>
> do_comp.constprop (struct type_t * data)
> {
>  struct type_t * res;
>  struct type_t * x;
>  struct type_t * y;
>  size_t len;
>
>  <bb 5> [local count: 1073741824]:
>
>  <bb 2> [local count: 1073741824]:
>  x_2 = min_of_x (data_1(D), 10000);
>  y_3 = max_of_y (data_1(D), 10000);
>  if (x_2 < y_3)
>    goto <bb 3>; [29.00%]
>  else
>    goto <bb 4>; [71.00%]
>
>  <bb 3> [local count: 311385128]:
>
>  <bb 4> [local count: 1073741824]:
>  # res_4 = PHI <y_3(2), 0B(3)>
>  return res_4;
>
> }
>
> The silly question first. In the "if" stmt how does GCC
> get those probabilities? Which it shows as 29.00% and
> 71.00%. I believe they should both be 50.00%.

See the profile_estimate pass dump. One branch makes the function return
NULL, which makes gcc guess that it may be a bit less likely than the
other. Those are heuristics, which are tuned to help on average, but of
course they are sometimes wrong.

> The serious question is what is going on with this phi?
>    res_4 = PHI <y_3(2), 0B(3)>
>
> This makes zero sense practicality wise to me and how is
> it supposed to be recognized and used? Note, I really do
> need to transform the "0B" into something else for my
> structure reorganization optimization.

That's not a question? Are you asking why PHIs exist at all? They are the
standard way to represent merging in SSA representations. You can iterate
on the PHIs of a basic block, etc.

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Could you please get rid of this when posting on public mailing lists?

--
Marc Glisse