On 8/12/24 7:21 PM, Marek Polacek wrote:
On Fri, Aug 09, 2024 at 05:15:05PM -0400, Jason Merrill wrote:
On 8/9/24 4:21 PM, Marek Polacek wrote:
On Fri, Aug 09, 2024 at 12:58:34PM -0400, Jason Merrill wrote:
On 8/8/24 1:37 PM, Marek Polacek wrote:
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

-- >8 --
The problem in this PR is that we ended up with

     {.rows=(&<PLACEHOLDER_EXPR struct Widget>)->n,
      .outer_stride=(&<PLACEHOLDER_EXPR struct MatrixLayout>)->rows}

that is, two PLACEHOLDER_EXPRs for different types on the same level
in one { }.  That should not happen; we may, for instance, neglect to
replace a PLACEHOLDER_EXPR due to CONSTRUCTOR_PLACEHOLDER_BOUNDARY on
the constructor.

The same problem happened in PR100252, which I fixed by introducing
replace_placeholders_for_class_temp_r.  That didn't work here, though,
because r_p_for_c_t_r only works for non-eliding TARGET_EXPRs: replacing
a PLACEHOLDER_EXPR with a temporary that is going to be elided will
result in a crash in gimplify_var_or_parm_decl when it encounters such
a loose decl.

But leaving the PLACEHOLDER_EXPRs in is also bad because then we end
up with this PR.

TARGET_EXPRs for function arguments are elided in gimplify_arg.  The
argument will get a real temporary only in get_formal_tmp_var.  One
idea was to use the temporary that is going to be elided anyway, and
then replace_decl it with the real object once we get it.  But that
didn't work out: one problem is that we elide the TARGET_EXPR for an
argument before we create the real temporary for the argument, and
when we get it, the context that this was a TARGET_EXPR for an argument
has been lost.  We're also in the middle end territory now, even though
this is a C++-specific problem.

How complex!

I figured that since the to-be-elided temporary is going to stay around
until gimplification, the front end is free to use it.  Once we're done
with things like store_init_value, which replaces PLACEHOLDER_EXPRs with
the decl it is initializing, we can turn those to-be-elided temporaries
into PLACEHOLDER_EXPRs again, so that cp_gimplify_init_expr can replace
them with the real object once available.  The context is not lost so we
do not need an extra flag for these makeshift temporaries.

Clever, that makes a lot of sense.  But I wonder if we can avoid the problem
more simply than working around it?

I see that the get_formal_tmp_var happens directly from gimplify_arg, so it
strips the TARGET_EXPR to avoid a temporary...and then immediately turns
around and creates a new temporary.

Would it work to stop stripping the TARGET_EXPR in gimplify_arg and
therefore stop setting TARGET_EXPR_ELIDING_P in convert_for_arg_passing?

Well, it does fix the ICE.  But then a number of testcases fail :(.
For instance, pr23372.C.  .gimple diff w/ and w/o stripping the TARGET_EXPR:

@@ -1,6 +1,9 @@
   void g (struct A * a)
   {
-  f (MEM[(const struct A &)a]);
+  struct A D.2829;
+
+  D.2829 = MEM[(const struct A &)a];
+  f (D.2829);
   }

The extra copy is there even in .optimized with -O2.


It's always sad when we have to add complicated code just to work around
a corner case, but the above pessimization looks pretty important :(.

Ah, good point.  In that case, the stripping avoids the copy because the
TARGET_EXPR_INITIAL is already (adjustable into) a suitable lvalue.  The
current code already fails to avoid the redundant copy when _INITIAL is a
CONSTRUCTOR:

void g (struct A * a)
{
   struct A D.2805;

   D.2805 = {}; // boo
   f (D.2805);
}

I'm failing to find the PR about this issue.

I also haven't found it (that doesn't mean it doesn't exist :)).  I can file
one if you'd like...

Note that if we do fix that, we may be facing this problem again.

Please do. The way I would expect the bug to get fixed would be to assign the temporary the location of the argument slot, and then recognize that the copy is a no-op.

How about the change I mentioned only in the CONSTRUCTOR case?

That works!

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

-- >8 --
The problem in this PR is that we ended up with

   {.rows=(&<PLACEHOLDER_EXPR struct Widget>)->n,
    .outer_stride=(&<PLACEHOLDER_EXPR struct MatrixLayout>)->rows}

that is, two PLACEHOLDER_EXPRs for different types on the same level
in one { }.  That should not happen; we may, for instance, neglect to
replace a PLACEHOLDER_EXPR due to CONSTRUCTOR_PLACEHOLDER_BOUNDARY on
the constructor.

The same problem happened in PR100252, which I fixed by introducing
replace_placeholders_for_class_temp_r.  That didn't work here, though,
because r_p_for_c_t_r only works for non-eliding TARGET_EXPRs: replacing
a PLACEHOLDER_EXPR with a temporary that is going to be elided will
result in a crash in gimplify_var_or_parm_decl when it encounters such
a loose decl.

But leaving the PLACEHOLDER_EXPRs in is also bad because then we end
up with this PR.

TARGET_EXPRs for function arguments are elided in gimplify_arg.  The
argument will get a real temporary only in get_formal_tmp_var.  One
idea was to use the temporary that is going to be elided anyway, and
then replace_decl it with the real object once we get it.  But that
didn't work out: one problem is that we elide the TARGET_EXPR for an
argument before we create the real temporary for the argument, and
when we get it, the context that this was a TARGET_EXPR for an argument
has been lost.  We're also in the middle end territory now, even though
this is a C++-specific problem.

A solution is to simply stop eliding TARGET_EXPRs whose initializer is
a CONSTRUCTOR.  Such copies can't be (at the moment) elided anyway.  But
not eliding all TARGET_EXPRs would be a pessimization.

        PR c++/116015

gcc/cp/ChangeLog:

        * call.cc (convert_for_arg_passing): Don't set_target_expr_eliding
        when the TARGET_EXPR initializer is a CONSTRUCTOR.

gcc/ChangeLog:

        * gimplify.cc (gimplify_arg): Do not strip a TARGET_EXPR whose
        initializer is a CONSTRUCTOR.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp1y/nsdmi-aggr23.C: New test.
---
  gcc/cp/call.cc                            |  7 ++++--
  gcc/gimplify.cc                           | 14 +++++++++++-
  gcc/testsuite/g++.dg/cpp1y/nsdmi-aggr23.C | 26 +++++++++++++++++++++++
  3 files changed, 44 insertions(+), 3 deletions(-)
  create mode 100644 gcc/testsuite/g++.dg/cpp1y/nsdmi-aggr23.C

diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 94015db4e65..0fe679aae9f 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -9468,8 +9468,11 @@ convert_for_arg_passing (tree type, tree val, 
tsubst_flags_t complain)
    if (complain & tf_warning)
      warn_for_address_of_packed_member (type, val);
- /* gimplify_arg elides TARGET_EXPRs that initialize a function argument. */
-  if (SIMPLE_TARGET_EXPR_P (val))
+  /* gimplify_arg elides TARGET_EXPRs that initialize a function argument,
+     unless the initializer is a CONSTRUCTOR.  In that case, we fail to
+     elide the copy anyway.  See that function for more information.  */
+  if (SIMPLE_TARGET_EXPR_P (val)
+      && TREE_CODE (TARGET_EXPR_INITIAL (val)) != CONSTRUCTOR)
      set_target_expr_eliding (val);
return val;
diff --git a/gcc/gimplify.cc b/gcc/gimplify.cc
index 71cc6c38d80..37d0d3a59f5 100644
--- a/gcc/gimplify.cc
+++ b/gcc/gimplify.cc
@@ -3760,7 +3760,19 @@ gimplify_arg (tree *arg_p, gimple_seq *pre_p, location_t 
call_location,
        {
          tree init = TARGET_EXPR_INITIAL (*arg_p);
          if (init
-             && !VOID_TYPE_P (TREE_TYPE (init)))
+             && !VOID_TYPE_P (TREE_TYPE (init))
+             /* Currently, due to c++/116015, it is not desirable to
+                strip a TARGET_EXPR whose initializer is a {}.  The
+                problem is that if we do elide it, we also have to
+                replace all the occurrences of the slot temporary in the
+                initializer with the temporary created for the argument.
+                But we do not have that temporary yet so the replacement
+                would be quite awkward and it might be needed to resort
+                back to a PLACEHOLDER_EXPR.
+
+                See convert_for_arg_passing for the C++ code that marks
+                the TARGET_EXPR as eliding or not.  */

Please also mention that stripping the TARGET_EXPR wouldn't help anyway in this case, as gimplify_expr will immediately allocate a temporary to store the CONSTRUCTOR into. And reference the new PR discussed above.

OK with that change.

Jason

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