[Bug tree-optimization/95481] New: Failure to optimize infinite recursion for struct types

[Bug tree-optimization/95481] New: Failure to optimize infinite recursion for struct types

[gabravier at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95481

            Bug ID: 95481
           Summary: Failure to optimize infinite recursion for struct
                    types
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gabravier at gmail dot com
  Target Milestone: ---

struct A {};

struct A foo()
{
  return foo();
}

This can be optimized to an infinite loop. With -O3, GCC currently outputs an
explicit recursive call like this :

foo():
  sub rsp, 8
  call foo()
  xor eax, eax
  add rsp, 8
  ret

When it could just do this :

foo():
.L2:
  jmp .L2

[Bug middle-end/95481] Failure to optimize infinite recursion for struct types

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95481

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement

[Bug tree-optimization/95481] Failure to optimize infinite recursion for empty struct types

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95481

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|middle-end                  |tree-optimization
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2021-05-30
            Summary|Failure to optimize         |Failure to optimize
                   |infinite recursion for      |infinite recursion for
                   |struct types                |empty struct types

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The problem only exist for empty structs.  If we add a field, we get the better
code:
foo:
.LFB0:
        .cfi_startproc
        .p2align 3,,7
.L2:
        b       .L2


The reason why this happens is because we remove the assignment for empty
struct assignment during gimplification.  This messes up how tailr and tailc
works.

Note here is a testcase for tailc:
struct A {};
struct A goo(void);
struct A foo(void)
{
  return goo();
}

[Bug tree-optimization/95481] Failure to optimize infinite recursion for empty struct types

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95481

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |pinskia at gcc dot gnu.org
             Status|NEW                         |ASSIGNED

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Created attachment 50892
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50892&action=edit
Patch which I am testing

I need to change the comment before the if statement and add a few testcases. 
But this is the patch which I am testing which fixes the problem.

[Bug tree-optimization/95481] Failure to optimize infinite recursion for empty struct types

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95481

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |patch
                URL|                            |https://gcc.gnu.org/piperma
                   |                            |il/gcc-patches/2021-May/571
                   |                            |490.html

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Patch submitted
https://gcc.gnu.org/pipermail/gcc-patches/2021-May/571490.html

[Bug tree-optimization/95481] Failure to optimize infinite recursion for empty struct types

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95481

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Andrew Pinski <pinskia@gcc.gnu.org>:

https://gcc.gnu.org/g:ea418485c700494c3efdc282854c5f5a08702416

commit r12-1151-gea418485c700494c3efdc282854c5f5a08702416
Author: Andrew Pinski <apinski@marvell.com>
Date:   Mon May 31 00:17:22 2021 +0000

    Fix PR 95481: tail call fails with empty struct types

    The problem here is we don't have an assignment type any more
    for empty structs as they were removed during gimplifcation.
    This adds a special case where the assignment var does not exist
    and the return decl is empty typed.

    OK? Tested on aarch64-linux-gnu with no regressions.

    Thanks,
    Andrew Pinski

    changes since v1:
    v2: Use is_empty_type instead of zero-sized type.

    gcc/ChangeLog:
            PR tree-optimization/95481
            * tree-tailcall.c (find_tail_calls): Handle empty typed
            return decls.

    gcc/testsuite/ChangeLog:

            PR tree-optimization/95481
            * gcc.dg/tree-ssa/tailcall-10.c: New test.
            * gcc.dg/tree-ssa/tailcall-11.c: New test.
            * gcc.dg/tree-ssa/tailcall-12.c: New test.
            * gcc.dg/tree-ssa/tailcall-13.c: New test.
            * gcc.dg/tree-ssa/tailrecursion-8.c: New test.

[Bug tree-optimization/95481] Failure to optimize infinite recursion for empty struct types

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95481

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|ASSIGNED                    |RESOLVED
   Target Milestone|---                         |12.0

--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Fixed.