[llvm-bugs] [Bug 52229] New: CNTTP materialisation exported even for unnamed-namespace content
via llvm-bugs
llvm-bugs at lists.llvm.org
Tue Oct 19 14:11:34 PDT 2021
https://bugs.llvm.org/show_bug.cgi?id=52229
Bug ID: 52229
Summary: CNTTP materialisation exported even for
unnamed-namespace content
Product: clang
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: C++2a
Assignee: unassignedclangbugs at nondot.org
Reporter: thiago at kde.org
CC: blitzrakete at gmail.com, erik.pilkington at gmail.com,
llvm-bugs at lists.llvm.org, richard-llvm at metafoo.co.uk
When a non-type class template parameter is materialised because its address is
taken, clang emits it as weak but always global. That content does not obey
either -fvisibility=hidden nor the fact that the type may be in an unnamed
namespace. The latter produces broken code.
$ cat test1.cpp
#include <stdio.h>
[[gnu::noinline]] void dump(const void *data, size_t len)
{
const char *ptr = reinterpret_cast<const char *>(data);
const char *end = ptr + len;
printf("%p(%zd) = ", data, len);
for ( ; ptr != end; ++ptr)
printf("%02x", *ptr);
puts("");
}
namespace { struct Foo { short i; }; }
template <Foo f> void test1()
{
dump(&f, sizeof(f));
printf("expected: %d\n", f.i);
}
void test2();
int main()
{
test1<Foo{}>();
test2();
}
$ cat test2.cpp
#include <stdio.h>
void dump(const void *data, size_t len);
namespace { struct Foo { int Foo::* i; }; }
template <Foo f> void test2()
{
dump(&f, sizeof(f));
printf("expected: %td\n", f.i);
}
void test2()
{
test2<Foo{}>();
}
$ g++ -O2 -std=c++20 test1.cpp test2.cpp
$ ./a.out
0x402022(2) = 0000
expected: 0
0x402038(8) = ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff
expected: -1
$ clang -O2 -std=c++20 test1.cpp test2.cpp
tjmaciei at tjmaciei-mobl5 /tmp $ ./a.out
0x402028(2) = 0000
expected: 0
0x402028(8) = 0000657870656374
expected: -1
The assembly for test2.cpp ends in:
.type _ZTAXtlN12_GLOBAL__N_13FooEEE, at object #
@_ZTAXtlN12_GLOBAL__N_13FooEEE
.section
.rodata._ZTAXtlN12_GLOBAL__N_13FooEEE,"aG", at progbits,_ZTAXtlN12_GLOBAL__N_13FooEEE,comdat
.weak _ZTAXtlN12_GLOBAL__N_13FooEEE
.p2align 3
_ZTAXtlN12_GLOBAL__N_13FooEEE:
.quad -1 # 0xffffffffffffffff
The _ZTA symbol is .weak but exported. Therefore, the <unnamed
namespace>::Foo{} expression in it matches the other Foo expression, thus
making the two symbols get merged by the linker, even though they don't have
the same size or contents.
$ clang --version
clang version 13.0.0
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
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