gimplify: Emit clobbers for TARGET_EXPR_SLOT vars later [PR103984]

gimplify: Emit clobbers for TARGET_EXPR_SLOT vars later [PR103984]

As mentioned in the PR, we emit a bogus uninitialized warning but
easily could emit wrong-code for it or similar testcases too.
The bug is that we emit clobber for a TARGET_EXPR_SLOT too early:
          D.2499.e = B::qux (&h); [return slot optimization]
          D.2516 = 1;
          try
            {
              B::B (&D.2498, &h);
              try
                {
                  _2 = baz (&D.2498);
                  D.2499.f = _2;
                  D.2516 = 0;
                  try
                    {
                      try
                        {
                          bar (&D.2499);
                        }
                      finally
                        {
                          C::~C (&D.2499);
                        }
                    }
                  finally
                    {
                      D.2499 = {CLOBBER(eol)};
                    }
                }
              finally
                {
                  D.2498 = {CLOBBER(eol)};
                }
            }
          catch
            {
              if (D.2516 != 0) goto <D.2517>; else goto <D.2518>;
              <D.2517>:
              A::~A (&D.2499.e);
              goto <D.2519>;
              <D.2518>:
              <D.2519>:
            }
The CLOBBER for D.2499 is essentially only emitted on the non-exceptional
path, if B::B or baz throws, then there is no CLOBBER for it but there
is a conditional destructor A::~A (&D.2499.e).  Now, ehcleanup1
sink_clobbers optimization assumes that clobbers in the EH cases are
emitted after last use and so sinks the D.2499 = {CLOBBER(eol)}; later,
so we then have
  # _3 = PHI <1(3), 0(9)>
<L2>:
  D.2499 ={v} {CLOBBER(eol)};
  D.2498 ={v} {CLOBBER(eol)};
  if (_3 != 0)
    goto <bb 11>; [INV]
  else
    goto <bb 15>; [INV]

  <bb 11> :
  _35 = D.2499.a;
  if (&D.2499.b != _35)
where that _35 = D.2499.a comes from inline expansion of the A::~A dtor,
and that is a load from a clobbered memory.

Now, what the gimplifier sees in this case is a CLEANUP_POINT_EXPR with
somewhere inside of it a TARGET_EXPR for D.2499 (with the C::~C (&D.2499)
cleanup) which in its TARGET_EXPR_INITIAL has another TARGET_EXPR for
D.2516 bool flag which has CLEANUP_EH_ONLY which performs that conditional
A::~A (&D.2499.e) call.
The following patch ensures that CLOBBERs (and asan poisoning) are emitted
after even those gimple_push_cleanup pushed cleanups from within the
TARGET_EXPR_INITIAL gimplification (i.e. the last point where the slot could
be in theory used).  In my first version of the patch I've done it by just
moving the
      /* Add a clobber for the temporary going out of scope, like
         gimplify_bind_expr.  */
      if (gimplify_ctxp->in_cleanup_point_expr
          && needs_to_live_in_memory (temp))
        {
...
        }
block earlier in gimplify_target_expr, but that regressed a couple of tests
where temp is marked TREE_ADDRESSABLE only during (well, very early during
that) the gimplification of TARGET_EXPR_INITIAL, so we didn't emit e.g. on
pr80032.C or stack2.C tests any clobbers for the slots and thus stack slot
reuse wasn't performed.
So that we don't regress those tests, this patch gimplifies
TARGET_EXPR_INITIAL as before, but doesn't emit it directly into pre_p,
emits it into a temporary sequence.  Then emits the CLOBBER cleanup
into pre_p, then asan poisoning if needed, then appends the
TARGET_EXPR_INITIAL temporary sequence and finally adds TARGET_EXPR_CLEANUP
gimple_push_cleanup.  The earlier a GIMPLE_WCE appears in the sequence, the
outer try/finally or try/catch it is.
So, with this patch the part of the testcase in gimple dump cited above
looks instead like:
          try
            {
              D.2499.e = B::qux (&h); [return slot optimization]
              D.2516 = 1;
              try
                {
                  try
                    {
                      B::B (&D.2498, &h);
                      _2 = baz (&D.2498);
                      D.2499.f = _2;
                      D.2516 = 0;
                      try
                        {
                          bar (&D.2499);
                        }
                      finally
                        {
                          C::~C (&D.2499);
                        }
                    }
                  finally
                    {
                      D.2498 = {CLOBBER(eol)};
                    }
                }
              catch
                {
                  if (D.2516 != 0) goto <D.2517>; else goto <D.2518>;
                  <D.2517>:
                  A::~A (&D.2499.e);
                  goto <D.2519>;
                  <D.2518>:
                  <D.2519>:
                }
            }
          finally
            {
              D.2499 = {CLOBBER(eol)};
            }

2022-03-17  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/103984
	* gimplify.cc (gimplify_target_expr): Gimplify type sizes and
	TARGET_EXPR_INITIAL into a temporary sequence, then push clobbers
	and asan unpoisioning, then append the temporary sequence and
	finally the TARGET_EXPR_CLEANUP clobbers.

	* g++.dg/opt/pr103984.C: New test.