Re: [PATCH] Fix PR71815 (SLSR misses PHI opportunities)

[Bill Schmidt <wschmidt@linux.vnet.ibm.com>]

On Jun 20, 2017, at 6:23 AM, Richard Biener <richard.guenther@gmail.com> wrote:
> 
> On Fri, Jun 16, 2017 at 6:10 PM, Bill Schmidt
> <wschmidt@linux.vnet.ibm.com> wrote:
>> Hi,
>> 
>> PR71815 identifies a situation where SLSR misses opportunities for
>> PHI candidates when code hoisting is enabled (which is now on by
>> default).  The basic problem is that SLSR currently uses an overly
>> simple test for profitability of the transformation.  The algorithm
>> currently requires that the PHI basis (through which the non-local
>> SLSR candidate is propagated) has only one use, which is the
>> candidate statement.  The true requirement for profitability is
>> that, if the candidate statement will be dead after transformation,
>> then so will the PHI candidate.
>> 
>> This patch fixes the problem by looking at the transitive reachability
>> of the PHI definitions.  If all paths terminate in the candidate
>> statement, then we know the PHI basis will go dead and we will not
>> make the code worse with the planned replacement.  To avoid compile
>> time issues, path search is arbitrarily terminated at depth 10.  The
>> new test is used throughout the cost calculation, so appears multiple
>> times in the code.
>> 
>> Also, I've added a check to avoid replacing multiply candidates with
>> a stride of 1.  Such a candidate is really a copy or cast statement,
>> and if we replace it, we will just generate a different copy or cast
>> statement.  I noticed this with one of the test cases from the PR
>> while debugging the problem.
>> 
>> I've updated the two test cases that were previously enabled only
>> with -fno-code-hoisting, removing that restriction.
>> 
>> Bootstrapped and tested on powerpc64le-unknown-linux-gnu with no
>> regressions.  I've also tested this with SPEC cpu2006 and the
>> patch is performance neutral on a POWER8 box (as expected).  Is
>> this ok for trunk?
>> 
>> Thanks,
>> Bill
>> 
>> 
>> [gcc]
>> 
>> 2016-06-16  Bill Schmidt  <wschmidt@linux.vnet.ibm.com>
>> 
>>        * gimple-ssa-strength-reduction.c (uses_consumed_by_stmt): New
>>        function.
>>        (find_basis_for_candidate): Call uses_consumed_by_stmt rather than
>>        has_single_use.
>>        (slsr_process_phi): Likewise.
>>        (replace_uncond_cands_and_profitable_phis): Don't replace a
>>        multiply candidate with a stride of 1 (copy or cast).
>>        (phi_incr_cost): Call uses_consumed_by_stmt rather than
>>        has_single_use.
>>        (lowest_cost_path): Likewise.
>>        (total_savings): Likewise.
>> 
>> [gcc/testsuite]
>> 
>> 2016-06-16  Bill Schmidt  <wschmidt@linux.vnet.ibm.com>
>> 
>>        * gcc.dg/tree-ssa/slsr-35.c: Remove -fno-code-hoisting workaround.
>>        * gcc.dg/tree-ssa/slsr-36.c: Likewise.
>> 
>> 
>> Index: gcc/gimple-ssa-strength-reduction.c
>> ===================================================================
>> --- gcc/gimple-ssa-strength-reduction.c (revision 239241)
>> +++ gcc/gimple-ssa-strength-reduction.c (working copy)
>> @@ -475,6 +475,48 @@ find_phi_def (tree base)
>>   return c->cand_num;
>> }
>> 
>> +/* Determine whether all uses of NAME are directly or indirectly
>> +   used by STMT.  That is, we want to know whether if STMT goes
>> +   dead, the definition of NAME also goes dead.  */
>> +static bool
>> +uses_consumed_by_stmt (tree name, gimple *stmt, unsigned recurse)
> 
> use a default arg 'unsigned recurse = 0' to hide this implementation
> detail at users.

Good idea, thanks.

> 
>> +{
>> +  gimple *use_stmt;
>> +  imm_use_iterator iter;
>> +  bool retval = true;
>> +
>> +  FOR_EACH_IMM_USE_STMT (use_stmt, iter, name)
>> +    {
>> +      if (use_stmt == stmt || is_gimple_debug (use_stmt))
>> +       continue;
>> +
>> +      if (!is_gimple_assign (use_stmt))
>> +       {
>> +         retval = false;
>> +         BREAK_FROM_IMM_USE_STMT (iter);
>> +       }
>> +
>> +      /* Limit recursion.  */
>> +      if (recurse >= 10)
>> +       {
>> +         retval = false;
>> +         BREAK_FROM_IMM_USE_STMT (iter);
>> +       }
> 
> Put this limit right before the recursion.
> 
>> +      tree next_name = gimple_get_lhs (use_stmt);
>> +      if (!next_name || !is_gimple_reg (next_name))
>> +       {
>> +         retval = false;
>> +         BREAK_FROM_IMM_USE_STMT (iter);
>> +       }
>> +
>> +      if (uses_consumed_by_stmt (next_name, stmt, recurse + 1))
>> +       continue;
> 
> So this doesn't change dependent on the result which means you likely meant
> 
>          if (! uses....)
>           {
>             retval = false;
>             BREAK...
>           }
> 
> which possibly also invalidates your testing?

Grumble.  Can't believe I did that.  Yep, will respin.

> 
> The whole thing is probably easier to optimize if you merge the ifs
> that break into one.

Will do!  Thanks, Richard!

Bill

> 
> Richard.
> 
>> +    }
>> +
>> +  return retval;
>> +}
>> +
>> /* Helper routine for find_basis_for_candidate.  May be called twice:
>>    once for the candidate's base expr, and optionally again either for
>>    the candidate's phi definition or for a CAND_REF's alternative base
>> @@ -550,7 +592,8 @@ find_basis_for_candidate (slsr_cand_t c)
>> 
>>          /* If we found a hidden basis, estimate additional dead-code
>>             savings if the phi and its feeding statements can be removed.  */
>> -         if (basis && has_single_use (gimple_phi_result (phi_cand->cand_stmt)))
>> +         tree feeding_var = gimple_phi_result (phi_cand->cand_stmt);
>> +         if (basis && uses_consumed_by_stmt (feeding_var, c->cand_stmt, 0))
>>            c->dead_savings += phi_cand->dead_savings;
>>        }
>>     }
>> @@ -777,7 +820,7 @@ slsr_process_phi (gphi *phi, bool speed)
>> 
>>          /* Gather potential dead code savings if the phi statement
>>             can be removed later on.  */
>> -         if (has_single_use (arg))
>> +         if (uses_consumed_by_stmt (arg, phi, 0))
>>            {
>>              if (gimple_code (arg_stmt) == GIMPLE_PHI)
>>                savings += arg_cand->dead_savings;
>> @@ -2384,7 +2427,9 @@ replace_uncond_cands_and_profitable_phis (slsr_can
>> {
>>   if (phi_dependent_cand_p (c))
>>     {
>> -      if (c->kind == CAND_MULT)
>> +      /* A multiply candidate with a stride of 1 is just an artifice
>> +        of a copy or cast; there is no value in replacing it.  */
>> +      if (c->kind == CAND_MULT && wi::to_widest (c->stride) != 1)
>>        {
>>          /* A candidate dependent upon a phi will replace a multiply by
>>             a constant with an add, and will insert at most one add for
>> @@ -2630,8 +2675,9 @@ phi_incr_cost (slsr_cand_t c, const widest_int &in
>>          if (gimple_code (arg_def) == GIMPLE_PHI)
>>            {
>>              int feeding_savings = 0;
>> +             tree feeding_var = gimple_phi_result (arg_def);
>>              cost += phi_incr_cost (c, incr, arg_def, &feeding_savings);
>> -             if (has_single_use (gimple_phi_result (arg_def)))
>> +             if (uses_consumed_by_stmt (feeding_var, phi, 0))
>>                *savings += feeding_savings;
>>            }
>>          else
>> @@ -2644,7 +2690,7 @@ phi_incr_cost (slsr_cand_t c, const widest_int &in
>>                  tree basis_lhs = gimple_assign_lhs (basis->cand_stmt);
>>                  tree lhs = gimple_assign_lhs (arg_cand->cand_stmt);
>>                  cost += add_cost (true, TYPE_MODE (TREE_TYPE (basis_lhs)));
>> -                 if (has_single_use (lhs))
>> +                 if (uses_consumed_by_stmt (lhs, phi, 0))
>>                    *savings += stmt_cost (arg_cand->cand_stmt, true);
>>                }
>>            }
>> @@ -2721,7 +2767,7 @@ lowest_cost_path (int cost_in, int repl_savings, s
>>       gimple *phi = lookup_cand (c->def_phi)->cand_stmt;
>>       local_cost += phi_incr_cost (c, incr, phi, &savings);
>> 
>> -      if (has_single_use (gimple_phi_result (phi)))
>> +      if (uses_consumed_by_stmt (gimple_phi_result (phi), c->cand_stmt, 0))
>>        local_cost -= savings;
>>     }
>> 
>> @@ -2765,7 +2811,7 @@ total_savings (int repl_savings, slsr_cand_t c, co
>>       gimple *phi = lookup_cand (c->def_phi)->cand_stmt;
>>       savings -= phi_incr_cost (c, incr, phi, &phi_savings);
>> 
>> -      if (has_single_use (gimple_phi_result (phi)))
>> +      if (uses_consumed_by_stmt (gimple_phi_result (phi), c->cand_stmt, 0))
>>        savings += phi_savings;
>>     }
>> 
>> Index: gcc/testsuite/gcc.dg/tree-ssa/slsr-35.c
>> ===================================================================
>> --- gcc/testsuite/gcc.dg/tree-ssa/slsr-35.c     (revision 239241)
>> +++ gcc/testsuite/gcc.dg/tree-ssa/slsr-35.c     (working copy)
>> @@ -3,7 +3,7 @@
>>    phi has an argument which is a parameter.  */
>> 
>> /* { dg-do compile } */
>> -/* { dg-options "-O3 -fno-code-hoisting -fdump-tree-optimized" } */
>> +/* { dg-options "-O3 -fdump-tree-optimized" } */
>> 
>> int
>> f (int c, int i)
>> Index: gcc/testsuite/gcc.dg/tree-ssa/slsr-36.c
>> ===================================================================
>> --- gcc/testsuite/gcc.dg/tree-ssa/slsr-36.c     (revision 239241)
>> +++ gcc/testsuite/gcc.dg/tree-ssa/slsr-36.c     (working copy)
>> @@ -3,7 +3,7 @@
>>    phi has an argument which is a parameter.  */
>> 
>> /* { dg-do compile } */
>> -/* { dg-options "-O3 -fno-code-hoisting -fdump-tree-optimized" } */
>> +/* { dg-options "-O3 -fdump-tree-optimized" } */
>> 
>> int
>> f (int s, int c, int i)