Re: [PATCH] Support threading of just the exit edge

[Andrew MacLeod <amacleod@redhat.com>]

On 8/16/22 05:18, Richard Biener wrote:
> On Mon, 15 Aug 2022, Aldy Hernandez wrote:
>
>> On Mon, Aug 15, 2022 at 9:24 PM Andrew MacLeod <amacleod@redhat.com> wrote:
>>> heh. or just
>>>
>>>
>>> +      int_range<2> r;
>>> +      if (!fold_range (r, const_cast <gcond *> (cond_stmt))
>>> +      || !r.singleton_p (&val))
>>>
>>>
>>> if you do not provide a range_query to any of the fold_using_range code,
>>> it defaults to:
>>>
>>> fur_source::fur_source (range_query *q)
>>> {
>>>     if (q)
>>>       m_query = q;
>>>     else if (cfun)
>>>       m_query = get_range_query (cfun);
>>>     else
>>>       m_query = get_global_range_query ();
>>>     m_gori = NULL;
>>> }
>>>
>> Sweet.  Even better!
> So when I do the following incremental change ontop of the posted
> patch then I see that the path query is able to simplify more
> "single BB paths" than the global range folding.
>
> diff --git a/gcc/tree-ssa-threadbackward.cc
> b/gcc/tree-ssa-threadbackward.cc
> index 669098e4ec3..777e778037f 100644
> --- a/gcc/tree-ssa-threadbackward.cc
> +++ b/gcc/tree-ssa-threadbackward.cc
> @@ -314,6 +314,12 @@ back_threader::find_taken_edge_cond (const
> vec<basic_block> &path,
>   {
>     int_range_max r;
>   
> +  int_range<2> rf;
> +  if (path.length () == 1)
> +    {
> +      fold_range (rf, cond);
> +    }
> +
>     m_solver->compute_ranges (path, m_imports);
>     m_solver->range_of_stmt (r, cond);
>   
> @@ -325,6 +331,8 @@ back_threader::find_taken_edge_cond (const
> vec<basic_block> &path,
>   
>     if (r == true_range || r == false_range)
>       {
> +      if (path.length () == 1)
> +       gcc_assert  (r == rf);
>         edge e_true, e_false;
>         basic_block bb = gimple_bb (cond);
>         extract_true_false_edges_from_block (bb, &e_true, &e_false);
>
> Even doing the following (not sure what's the difference and in
> particular expense over the path range query) results in missed
> simplifications (checking my set of cc1files).
>
> diff --git a/gcc/tree-ssa-threadbackward.cc
> b/gcc/tree-ssa-threadbackward.cc
> index 669098e4ec3..1d43a179d08 100644
> --- a/gcc/tree-ssa-threadbackward.cc
> +++ b/gcc/tree-ssa-threadbackward.cc
> @@ -99,6 +99,7 @@ private:
>   
>     back_threader_registry m_registry;
>     back_threader_profitability m_profit;
> +  gimple_ranger *m_ranger;
>     path_range_query *m_solver;
>   
>     // Current path being analyzed.
> @@ -146,12 +147,14 @@ back_threader::back_threader (function *fun,
> unsigned flags, bool first)
>     // The path solver needs EDGE_DFS_BACK in resolving mode.
>     if (flags & BT_RESOLVE)
>       mark_dfs_back_edges ();
> -  m_solver = new path_range_query (flags & BT_RESOLVE);
> +  m_ranger = new gimple_ranger;
> +  m_solver = new path_range_query (flags & BT_RESOLVE, m_ranger);
>   }
>   
>   back_threader::~back_threader ()
>   {
>     delete m_solver;
> +  delete m_ranger;
>   
>     loop_optimizer_finalize ();
>   }
> @@ -314,6 +317,12 @@ back_threader::find_taken_edge_cond (const
> vec<basic_block> &path,
>   {
>     int_range_max r;
>   
> +  int_range<2> rf;
> +  if (path.length () == 1)
> +    {
> +      fold_range (rf, cond, m_ranger);
> +    }
> +
>     m_solver->compute_ranges (path, m_imports);
>     m_solver->range_of_stmt (r, cond);
>   
> @@ -325,6 +334,8 @@ back_threader::find_taken_edge_cond (const
> vec<basic_block> &path,
>   
>     if (r == true_range || r == false_range)
>       {
> +      if (path.length () == 1)
> +       gcc_assert  (r == rf);
>         edge e_true, e_false;
>         basic_block bb = gimple_bb (cond);
>         extract_true_false_edges_from_block (bb, &e_true, &e_false);
>
> one example is
>
> <bb 176> [local count: 14414059]:
> _128 = node_177(D)->typed.type;
> pretmp_413 = MEM[(const union tree_node *)_128].base.code;
> _431 = pretmp_413 + 65519;
> if (_128 == 0B)
>    goto <bb 199>; [18.09%]
> else
>    goto <bb 177>; [81.91%]
>
> where m_imports for the path is just _128 and the range computed is
> false while the ranger query returns VARYING.  But
> path_range_query::range_defined_in_block does
>
>    if (bb && POINTER_TYPE_P (TREE_TYPE (name)))
>      m_ranger->m_cache.m_exit.maybe_adjust_range (r, name, bb);
This is the coarse grained "side effect applies somewhere in the block" 
mechanism.  There is no understanding of where in the block it happens.
>
> which adjusts the range to ~[0, 0], probably because of the
> dereference in the following stmt.
>
> Why does fold_range not do this when folding the exit test?  Is there
> a way to make it do so?  It looks like the only routine using this
> in gimple-range.cc is range_on_edge and there it's used for e->src
> after calling range_on_exit for e->src (why's it not done in
> range_on_exit?).  A testcase for this is

Fold_range doesnt do this because it is simply another statement.  It 
makes no attempt to understand the context in which you are folding 
something. you could be folding that stmt from a different location (ie 
recomputing)   If your context is that you are looking for the range 
after the last statement has been executed, then one needs to check to 
see if there are any side effects.

ranger uses it for range_on_edge (), because  it knows all the 
statements in the block have been executed, and its safe to apply 
anything seen in the block.  It does it right after range_on_exit() is 
called internally.

Once upon a time, it was integrated with range-on-exit, but it turned 
out there were certain times that it was causing problems. There have 
been some cleanups since then, it probably safe now to return that call 
to range_on_exit.. but doesnt buy us a whole lot by itself.. except of 
course I have now OKd using range_on-entry/exit generally :-)

the cache also uses it when walking blocks to pick up inferred values 
during an on-entry cache fill.


> int foo (int **p, int i)
> {
>    int *q = *p;
>    int res = *q + i;
>    if (q)
>      return res;
>    return -1;
> }
>
> which we "thread" with the path and with the above ICEs because
> fold_range doesn't get that if (q) is always true.  Without the

Its a known limitation that, unless you are doing a walk, on-demand 
requests will "miss" some inferred ranges, because they are only 
maintained at the basic block level.  (ie, we will know that q is 
non-null in BB2,  but we don't know where, so we can make no assumptions 
at the exit condition about q in this case. the path_query is invoked in 
on-demand mode because it wont walk the entire IL,. so the first time 
you ask for the range of an ssa-name, it will quickly zip over the 
immediate use list and "fill" the on-exit structure for any blocks which 
a non-null reference is seen.  This allows the threader to pick up 
non-null from blocks outside the path that havent been examined.

VRP does a walk, and while during the walk, adjusts ranges on the fly 
for the current block via the gimple_ranger::register_inferred_ranges () 
routine.  which is really just a wrapper around 
ranger_cache::apply_inferred_ranges ()  (in gimple-range-cache.cc)

This is called after every statement and is where we take care of 
bookkeeping for adjusting values, and adding them to the blocks list.

if the path query is walking those statement, it could also "adjust" the 
range of q on the fly... but it has to have walked those statements.  
from that routine, the relevant bits use the gimple-infer class to find 
any inferred ranges from the statement, and would look something like:

   gimple_infer_range infer(s);
   for (unsigned x = 0; x < infer.num (); x++)
     {
       tree name = infer.name (x);
       if (!interesting_p (name))
          continue;
       get_current_path_range (r, name);
       if (r.intersect (infer.range (x)))
         set_current_path_range (name, r);
     }

That would adjust the value of q to be non-zero after   "int res = *q + i;"

but you need to have walked the statement before you get to the 
condition.   as long as they are all in your list of interesting 
statement to look at, then you'd be golden.

I dont know if, when, or what direction things are examined.

Andrew