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.
> 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. */
+ && TREE_CODE (init) != CONSTRUCTOR)
*arg_p = init;
}
}
diff --git a/gcc/testsuite/g++.dg/cpp1y/nsdmi-aggr23.C
b/gcc/testsuite/g++.dg/cpp1y/nsdmi-aggr23.C
new file mode 100644
index 00000000000..2f5b8ca97bf
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/nsdmi-aggr23.C
@@ -0,0 +1,26 @@
+// PR c++/116015
+// { dg-do compile { target c++14 } }
+// { dg-additional-options "-Wno-c++20-extensions" }
+
+struct MatrixLayout {
+ int rows = 0;
+ int outer_stride = rows;
+};
+struct Matrix {
+ Matrix(MatrixLayout m) {}
+};
+struct Widget {
+ int n = 5;
+ Matrix A0{{}};
+ Matrix A1{{n}};
+ Matrix A1_{{.rows = n}};
+ Matrix A2{{n, n}};
+};
+
+int
+main ()
+{
+ Widget w{};
+ Widget w1;
+ Widget w2 = {};
+}
base-commit: 0451bc503da9c858e9f1ddfb8faec367c2e032c8
--
2.46.0
