[Bug tree-optimization/98925] New: Extend modref to handle return slot optimization

[Bug tree-optimization/98925] New: Extend modref to handle return slot optimization

[slyfox at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98925

            Bug ID: 98925
           Summary: Extend modref to handle return slot optimization
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: hubicka at gcc dot gnu.org
          Reporter: slyfox at gcc dot gnu.org
                CC: rguenth at gcc dot gnu.org
  Target Milestone: ---

Moving out https://gcc.gnu.org/PR98499#c9 to a separate enhancement PR:

Right now analyze_ssa_name_flags() does not perform detailed modref analysis of
return slot optimization and falls back to conservative assumption:

    analyze_ssa_name_flags()

          /* Return slot optiomization would require bit of propagation;
             give up for now.  */
          if (gimple_call_return_slot_opt_p (call)
              && gimple_call_lhs (call) != NULL_TREE
              && TREE_ADDRESSABLE (TREE_TYPE (gimple_call_lhs (call))))
            {
              if (dump_file)
                fprintf (dump_file, "%*s  Unhandled return slot opt\n",
                         depth * 4, "");
              lattice[index].merge (0);
            }

I don't know what would be a good test for it.

If I don't overstimate modref and alias analysis   destructor should disappear
completely in the example below (from https://gcc.gnu.org/PR98499#c4):

struct string {
  char * _M_buf;
  // local store
  char _M_local_buf[16];

  __attribute__((noinline)) string() : _M_buf(_M_local_buf) {}

  ~string() {
    if (_M_buf != _M_local_buf)
      __builtin_trap();
  }

  string(const string &__str); // no copies
};

// main.cc

__attribute__((noinline)) static string dir_name() { return string(); }
class Importer {
  string base_path;

public:
  __attribute__((noinline)) Importer() : base_path (dir_name()) {}
};

int main() {
  Importer imp;
}

[Bug tree-optimization/98925] Extend modref to handle return slot optimization

[hubicka at ucw dot cz]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98925

--- Comment #1 from Jan Hubicka <hubicka at ucw dot cz> ---
> If I don't overstimate modref and alias analysis   destructor should disappear
> completely in the example below (from https://gcc.gnu.org/PR98499#c4):
> 
> struct string {
>   char * _M_buf;
>   // local store
>   char _M_local_buf[16];
> 
>   __attribute__((noinline)) string() : _M_buf(_M_local_buf) {}
> 
>   ~string() {
>     if (_M_buf != _M_local_buf)
>       __builtin_trap();
>   }
> 
>   string(const string &__str); // no copies
> };

To see that _M_buf == _M_local_buf at the destruction time would require
tracking the aggregate alue from the inlinined constructor to destructor
which is something fitting to ipa-prop.

Modref can helip in case _M_buf is initialized to NULL.  In that case
the address of return value would be non-escaping and we could optimize
the if condition to true (is we did incorrectly before).

I have WIP patch for this and will discuss with Martin Jambor the jump
functions for return values.

Thanks!
Honza

[Bug tree-optimization/98925] Extend ipa-prop to handle return functions for slot optimization

[hubicka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98925

Jan Hubicka <hubicka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|Extend modref to handle     |Extend ipa-prop to handle
                   |return slot optimization    |return functions for slot
                   |                            |optimization
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
                 CC|                            |mjambor at suse dot cz
           Severity|normal                      |enhancement
   Last reconfirmed|                            |2021-11-07

--- Comment #2 from Jan Hubicka <hubicka at gcc dot gnu.org> ---
Modref now handles return slots.
However as discussed in earlier comment we also need return function in
ipa-prop to propagate constant from callee to caller. So I am keeping the PR to
track this (I think it would be useful especially for offlined ctors)

[Bug ipa/98925] Extend ipa-prop to handle return functions for slot optimization

[hubicka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98925

--- Comment #3 from Jan Hubicka <hubicka at gcc dot gnu.org> ---
Return value range propagation was added in
r:53ba8d669550d3a1f809048428b97ca607f95cf5

however it works on scalar return values only for now. Extending it to
aggregates is a logical next step and should not be terribly hard.

The code also misses logic for IPA streaming so it works only in ealry and late
opts.