[Bug debug/86638] Og guality failures without -ftree-sra

[rsandifo at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86638

--- Comment #4 from rsandifo at gcc dot gnu.org <rsandifo at gcc dot gnu.org> 
---
Author: rsandifo
Date: Mon Jul 29 08:52:56 2019
New Revision: 273872

URL: https://gcc.gnu.org/viewcvs?rev=273872&root=gcc&view=rev
Log:
Prevent tree-ssa-dce.c from deleting stores at -Og

DCE tries to delete dead stores to local data and also tries to insert
debug binds for simple cases:

  /* If this is a store into a variable that is being optimized away,
     add a debug bind stmt if possible.  */
  if (MAY_HAVE_DEBUG_BIND_STMTS
      && gimple_assign_single_p (stmt)
      && is_gimple_val (gimple_assign_rhs1 (stmt)))
    {
      tree lhs = gimple_assign_lhs (stmt);
      if ((VAR_P (lhs) || TREE_CODE (lhs) == PARM_DECL)
          && !DECL_IGNORED_P (lhs)
          && is_gimple_reg_type (TREE_TYPE (lhs))
          && !is_global_var (lhs)
          && !DECL_HAS_VALUE_EXPR_P (lhs))
        {
          tree rhs = gimple_assign_rhs1 (stmt);
          gdebug *note
            = gimple_build_debug_bind (lhs, unshare_expr (rhs), stmt);
          gsi_insert_after (i, note, GSI_SAME_STMT);
        }
    }

But this doesn't help for things like "print *ptr" when ptr points
to the local variable (tests Og-dce-1.c and Og-dce-2.c).  It can
also introduce wrong debug info for earlier references (second test
in Og-dce-3.c) or make earlier references unavailable (first test
in Og-dce-3.c).

So for -Og I think it'd be better not to delete any stmts with
vdefs for now.  This also means that we can avoid the potentially
expensive vop walks (which already have a cut-off, but still).

The patch also fixes the Og failures in gcc.dg/guality/pr54970.c
(PR 86638).

2019-07-29  Richard Sandiford  <richard.sandif...@arm.com>

gcc/
        PR debug/86638
        * tree-ssa-dce.c (keep_all_vdefs_p): New function.
        (mark_stmt_if_obviously_necessary): Mark all stmts with vdefs as
        necessary if keep_all_vdefs_p is true.
        (mark_aliased_reaching_defs_necessary): Add a gcc_checking_assert
        that keep_all_vdefs_p is false.
        (mark_all_reaching_defs_necessary): Likewise.
        (propagate_necessity): Skip the vuse scan if keep_all_vdefs_p is true.

gcc/testsuite/
        * c-c++-common/guality/Og-dce-1.c: New test.
        * c-c++-common/guality/Og-dce-2.c: Likewise.
        * c-c++-common/guality/Og-dce-3.c: Likewise.

Added:
    trunk/gcc/testsuite/c-c++-common/guality/Og-dce-1.c
    trunk/gcc/testsuite/c-c++-common/guality/Og-dce-2.c
    trunk/gcc/testsuite/c-c++-common/guality/Og-dce-3.c
Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/testsuite/ChangeLog
    trunk/gcc/tree-ssa-dce.c