[Clang] C++ Modules: Setting section attribute on variable in module causes miscompilation

[Gurah]

Setting the output section of a variable by attribute in a module interface causes miscompilation.
Code works fine if not using c++ modules or if removing section attribute.

Example
test.ccm:

export module test;

int foo  __attribute__((section(".data.foo"))) = 2;

export int get_foo()
{
	return foo;
}

export void set_foo(int val)
{
	foo = val;
}

main.cc:

import test;

int main()
{
	set_foo(5);
	return get_foo();
}

Compile and run:
clang++ -std=c++23 -O1 -c test.ccm -fmodule-output=test.pcm
clang++ -std=c++23 -O1 -c main.cc -fmodule-file=test=test.pcm
clang++ test.o main.o -o test
./test
echo $?

Expected result:
5

Actual result:
2

Clang version
clang++ --version
clang version 19.1.7
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

[llvmbot]

@llvm/issue-subscribers-clang-modules

Author: None (Gurah)

Setting the output section of a variable by attribute in a module interface causes miscompilation. Code works fine if not using c++ modules or if removing section attribute.

Example
test.ccm:

export module test;

int foo  __attribute__((section(".data.foo"))) = 2;

export int get_foo()
{
	return foo;
}

export void set_foo(int val)
{
	foo = val;
}

main.cc:

import test;

int main()
{
	set_foo(5);
	return get_foo();
}

Compile and run:
clang++ -std=c++23 -O1 -c test.ccm -fmodule-output=test.pcm
clang++ -std=c++23 -O1 -c main.cc -fmodule-file=test=test.pcm
clang++ test.o main.o -o test
./test
echo $?

Expected result:
5

Actual result:
2

Clang version
clang++ --version
clang version 19.1.7
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

[zwuis]

Could you please try 20 or main branch?

[Gurah]

Hi

Just finished compiling main (cca454b)
Still seeing the same issue.

The generated code doesn't even touch foo:

Disassembly of section .text:

0000000000000000 <get_foo@test()>:
   0:	b8 02 00 00 00       	mov    $0x2,%eax
   5:	c3                   	ret
   6:	66 2e 0f 1f 84 00 00 	cs nopw 0x0(%rax,%rax,1)
   d:	00 00 00 

0000000000000010 <set_foo@test(int)>:
  10:	c3                   	ret

Interestingly, Compiler explorer is generating correct code with both clang 19.1.0 and 20.1.0.
(https://godbolt.org/z/vEf1xWcr6) but I don't know if it really cares about the section attribute.