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