Re: [PATCH] Add testcase for C++ placement new and the important difference between dtor clobbers and end of scope clobbers

[Jakub Jelinek]

On Mon, Jan 28, 2019 at 01:18:43PM +0100, Richard Biener wrote:
> Eww :/  That means they are not semantically the same which IMHO is
> bad.  If you make buf1 volatile does it trip over
> maybe_canonicalize_mem_ref_addr and miscompile?  Or are we somehow

If buf1 is volatile, then no clobbers are added for it for when it goes out
of scope:
      if (VAR_P (t)
          && !is_global_var (t)
          && DECL_CONTEXT (t) == current_function_decl)
        {
          if (!DECL_HARD_REGISTER (t)
              && !TREE_THIS_VOLATILE (t)
              && !DECL_HAS_VALUE_EXPR_P (t)
              /* Only care for variables that have to be in memory.  Others
                 will be rewritten into SSA names, hence moved to the
                 top-level.  */
              && !is_gimple_reg (t)
              && flag_stack_reuse != SR_NONE)
            {
              tree clobber = build_clobber (TREE_TYPE (t));
The clobbers from ctors and dtors are still there, but nothing folds them to
non-MEM_REF anyway:
  volatile char buf2[128];
  volatile char buf1[128];

  <bb 2> [local count: 1073741824]:
  foo (&buf1);
  MEM[(struct  &)&buf1] ={v} {CLOBBER};
  memset (&MEM[(void *)&buf1], 32, 128);
  bar (&buf1);
  MEM[(struct  &)&buf1] ={v} {CLOBBER};
  baz (&buf2);
  return;
in *.optimized.

        Jakub