From: Joseph Myers <joseph@codesourcery.com>
To: <gcc-patches@gcc.gnu.org>
Subject: stor-layout: Set TYPE_TYPELESS_STORAGE consistently for type variants
Date: Mon, 20 Mar 2023 19:46:32 +0000 [thread overview]
Message-ID: <9cbb57b3-3e66-3618-217f-bf2689cc8825@codesourcery.com> (raw)
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I've observed an LTO wrong-code bug with a large testcase in GCC 12,
that results from TYPE_TYPELESS_STORAGE not being set consistently on
type variants.
Specifically, in the LTO stage of compilation, there is an aggregate
type passed to get_alias_set, whose TYPE_MAIN_VARIANT does not have
TYPE_TYPELESS_STORAGE set. However, the TYPE_CANONICAL of that main
variant *does* have have TYPE_TYPELESS_STORAGE set; note that the use
of TYPE_CANONICAL in get_alias_set comes after the check of
TYPE_TYPELESS_STORAGE. The effect is that when (one-argument)
record_component_aliases is called, the recursive call to
get_alias_set gives alias set 0, and the aggregate type ends up not
being considered to alias its members, with wrong-code consequences.
I haven't managed to produce a self-contained executable testcase to
demonstrate this, but it clearly seems appropriate for
TYPE_TYPELESS_STORAGE to be consistent on type variants, so this patch
makes it so, which appears to be sufficient to resolve the bug. I've
attached a reduced test that does at least demonstrate main-variant
versions of a type (SB in this test) being written out to LTO IR both
with and without TYPE_TYPELESS_STORAGE, although not the subsequent
consequences of a type without TYPE_TYPELESS_STORAGE with a
TYPE_CANONICAL (as constructed after LTO type merging) with
TYPE_TYPELESS_STORAGE and following wrong-code.
Bootstrapped with no regressions for x86_64-pc-linux-gnu. OK to commit?
* stor-layout.cc (finalize_type_size): Copy TYPE_TYPELESS_STORAGE
to variants.
diff --git a/gcc/stor-layout.cc b/gcc/stor-layout.cc
index 45bf2d18639..023de8c37db 100644
--- a/gcc/stor-layout.cc
+++ b/gcc/stor-layout.cc
@@ -1996,6 +1996,7 @@ finalize_type_size (tree type)
unsigned int user_align = TYPE_USER_ALIGN (type);
machine_mode mode = TYPE_MODE (type);
bool empty_p = TYPE_EMPTY_P (type);
+ bool typeless = AGGREGATE_TYPE_P (type) && TYPE_TYPELESS_STORAGE (type);
/* Copy it into all variants. */
for (variant = TYPE_MAIN_VARIANT (type);
@@ -2020,6 +2021,8 @@ finalize_type_size (tree type)
TYPE_PRECISION (variant) = precision;
SET_TYPE_MODE (variant, mode);
TYPE_EMPTY_P (variant) = empty_p;
+ if (AGGREGATE_TYPE_P (variant))
+ TYPE_TYPELESS_STORAGE (variant) = typeless;
}
}
}
--
Joseph S. Myers
joseph@codesourcery.com
[-- Warning: decoded text below may be mangled, UTF-8 assumed --]
[-- Attachment #2: Type: text/x-c++src; name="typeless.cc", Size: 369 bytes --]
struct S {
int a;
char b[8];
};
template <int> class SB {
public:
operator bool() { return true; };
S x;
};
class T : public SB<0> {};
template <typename TT> class m1 {
public:
m1(TT) {}
void m2() {};
};
class U {
public:
U(int, T c) {
auto v = m1([] {});
if (c)
v.m2();
}
};
void f() {
T c = {};
U(0, c);
}
next reply other threads:[~2023-03-20 19:46 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-20 19:46 Joseph Myers [this message]
2023-03-21 7:29 ` Richard Biener
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