On Tue, Aug 13, 2024 at 03:12:01PM -0700, Jason Merrill wrote:
> 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.
https://gcc.gnu.org/PR116375
> > > 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.
Done.
> OK with that change.
Here is what I'm pushing. Thanks!
-- >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 | 17 ++++++++++++++-
gcc/testsuite/g++.dg/cpp1y/nsdmi-aggr23.C | 26 +++++++++++++++++++++++
3 files changed, 47 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..26a216e151d 100644
--- a/gcc/gimplify.cc
+++ b/gcc/gimplify.cc
@@ -3760,7 +3760,22 @@ 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. Note that stripping the
+ TARGET_EXPR wouldn't help anyway, as gimplify_expr would
+ just allocate a temporary to store the CONSTRUCTOR into.
+ (FIXME PR116375.)
+
+ 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: a247088adaf122116919235f4a40189506139495
--
2.46.0
