On Wed, 30 Jul 2025, Patrick Palka wrote:
On Wed, 30 Jul 2025, Patrick Palka wrote:
On Wed, 30 Jul 2025, Patrick Palka wrote:
Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look
OK for trunk?
-- >8 --
Here the result of A::make(0, 0, 1), (0, 1, 0) and (1, 0, 0) are
each
represented as a single-element CONSTRUCTOR with
CONSTRUCTOR_NO_CLEARING
cleared, and we end up mangling them all as A{1}, i.e. eliding
both
the
implicit initial and trailing zeros. Mangling them all the same
seems
clearly wrong since they're logically different values.
Just realized that A::make(1, 0, 1) is also mangled incorrectly --
as
A{1, 1} instead of A{1, 0, 1}. So we need to consider intermediate
implicit zeroes as well, not just initial zeroes...
Here's an updated patch that also considers intermediate implicit
zeros.
Bootstrap and regtest on x86_64-pc-linux-gnu in progress, does this
look
OK for
trunk if successful?
-- >8 --
Subject: [PATCH] c++: mangling cNTTP object w/ implicit non-trailing
zeros
[PR121231]
Here the result of A::make(0, 0, 1), (0, 1, 0) and (1, 0, 0) are each
represented as a single-element CONSTRUCTOR with
CONSTRUCTOR_NO_CLEARING
cleared, and we end up mangling them all as A{1}, i.e. eliding both
the
implicit initial and trailing zeros. Mangling them all the same seems
clearly wrong since they're logically different values.
It turns out we also omit intermediate zeros, e.g. A::make(1, 0, 1) is
mangled as A{1, 1} same as A::make(1, 1, 0).
It seems we can't omit both trailing and non-trailing implicit zeros
without introducing mangling ambiguities. This patch makes us include
non-trailing zeros in these manglings, while continuing to omit
trailing zeros, which matches e.g. Clang.
PR c++/121231
gcc/ChangeLog:
* common.opt: Document additional ABI version 21 change.
* doc/invoke.texi: Likewise.
gcc/cp/ChangeLog:
* mangle.cc (write_expression): Write out implicit
non-trailing
zeroes of a CONSTRUCTOR when the ABI version is at least 21.
gcc/testsuite/ChangeLog:
* g++.dg/abi/mangle82.C: New test.
---
gcc/common.opt | 2 +
gcc/cp/mangle.cc | 49 ++++++++++++++++
gcc/doc/invoke.texi | 5 +-
gcc/testsuite/g++.dg/abi/mangle82.C | 86
+++++++++++++++++++++++++++++
4 files changed, 140 insertions(+), 2 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/abi/mangle82.C
diff --git a/gcc/common.opt b/gcc/common.opt
index 70659fabebd5..bf38f60d194b 100644
--- a/gcc/common.opt
+++ b/gcc/common.opt
@@ -1067,6 +1067,8 @@ Driver Undocumented
;
; 21: Fix noexcept lambda capture pruning.
; Fix C++20 layout of base with all explicitly defaulted
constructors.
+; Fix mangling of class and array objects with implicitly
+; zero-initialized non-trailing subojects.
; Default in G++ 16.
;
; Additional positive integers will be assigned as new versions of
diff --git a/gcc/cp/mangle.cc b/gcc/cp/mangle.cc
index 13d5dedebd29..296e0f34552e 100644
--- a/gcc/cp/mangle.cc
+++ b/gcc/cp/mangle.cc
@@ -3745,9 +3745,55 @@ write_expression (tree expr)
|| !zero_init_expr_p (ce->value))
last_nonzero = i;
+ tree prev_field = NULL_TREE;
+ if (TREE_CODE (TREE_TYPE (expr)) == RECORD_TYPE)
+ prev_field = first_field (etype);
+ else if (TREE_CODE (TREE_TYPE (expr)) == ARRAY_TYPE)
+ prev_field = size_int (0);
On second thought I think the logic is a little clearer if prev_field is
NULL_TREE on the first iteration, and we adjust the i == 0 logic
accordingly. Like so (passes initial testisg, full testing in
progress):
-- >8 --
Here the result of A::make(0, 0, 1), (0, 1, 0) and (1, 0, 0) are each
represented as a single-element CONSTRUCTOR with CONSTRUCTOR_NO_CLEARING
cleared, and we end up mangling them all as A{1}, i.e. eliding both the
implicit initial and trailing zeros. Mangling them all the same seems
clearly wrong since they're logically different values.
It turns out we also omit intermediate zeros, e.g. A::make(1, 0, 1) is
mangled as A{1, 1}, the same as A::make(1, 1, 0).
It seems we can't omit both trailing and non-trailing implicit zeros
without introducing mangling ambiguities. This patch makes us include
non-trailing zeros in these manglings, while continuing to omit
trailing zeros, which matches e.g. Clang.
PR c++/121231
gcc/ChangeLog:
* common.opt: Document additional ABI version 21 change.
* doc/invoke.texi: Likewise.
gcc/cp/ChangeLog:
* mangle.cc (write_expression): Write out implicit non-trailing
zeroes of a CONSTRUCTOR when the ABI version is at least 21.
gcc/testsuite/ChangeLog:
* g++.dg/abi/mangle82.C: New test.
---
gcc/common.opt | 2 +
gcc/cp/mangle.cc | 50 +++++++++++++++++
gcc/doc/invoke.texi | 5 +-
gcc/testsuite/g++.dg/abi/mangle82.C | 86
+++++++++++++++++++++++++++++
4 files changed, 141 insertions(+), 2 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/abi/mangle82.C
diff --git a/gcc/common.opt b/gcc/common.opt
index 70659fabebd5..bf38f60d194b 100644
--- a/gcc/common.opt
+++ b/gcc/common.opt
@@ -1067,6 +1067,8 @@ Driver Undocumented
;
; 21: Fix noexcept lambda capture pruning.
; Fix C++20 layout of base with all explicitly defaulted
constructors.
+; Fix mangling of class and array objects with implicitly
+; zero-initialized non-trailing subojects.
; Default in G++ 16.
;
; Additional positive integers will be assigned as new versions of
diff --git a/gcc/cp/mangle.cc b/gcc/cp/mangle.cc
index 13d5dedebd29..23b21c0a5b7a 100644
--- a/gcc/cp/mangle.cc
+++ b/gcc/cp/mangle.cc
@@ -3745,11 +3745,58 @@ write_expression (tree expr)
|| !zero_init_expr_p (ce->value))
last_nonzero = i;
+ tree prev_field = NULL_TREE;
if (undigested || last_nonzero != UINT_MAX)
for (HOST_WIDE_INT i = 0; vec_safe_iterate (elts, i,
&ce);
++i)
{
if (i > last_nonzero)
break;
+ if (!undigested && !CONSTRUCTOR_NO_CLEARING (expr)
+ && (TREE_CODE (etype) == RECORD_TYPE
+ || TREE_CODE (etype) == ARRAY_TYPE))
+ {
+ /* Write out any implicit non-trailing zeros
+ (which we neglected to do before v21). */
+ if (TREE_CODE (etype) == RECORD_TYPE)
+ {
+ tree field;
+ if (i == 0)
+ field = first_field (etype);
+ else
+ field = DECL_CHAIN (prev_field);
+ for (; field; field = DECL_CHAIN (field))
+ {
+ field = next_subobject_field (field);
+ if (!field || field == ce->index)
+ break;
+ if (abi_check (21))
+ write_expression (build_zero_cst
+ (TREE_TYPE (field)));
