[llvm-bugs] [Bug 117899] [C++20][Modules] clang++ not inlining tiny function exported in module

[LLVM Bugs via llvm-bugs]

Issue	117899
Summary	[C++20][Modules] clang++ not inlining tiny function exported in module
Labels	clang
Assignees
Reporter	aleck099

    **f1.cpp**:
```cpp
module;
extern "C" int puts(const char* s);
export module f1;

export void foo() {
 puts("Hello World");
}
```

**main.cpp**:
```cpp
import f1;

int main() {
    foo();
}
```

compile with:
```sh
clang++ -std=c++23 -fprebuilt-module-path=. -O3 --precompile -x c++-module f1.cpp
clang++ -std=c++23 -fprebuilt-module-path=. -O3 -c f1.pcm
clang++ -std=c++23 -fprebuilt-module-path=. -O3 -c main.cpp
clang++ -fuse-ld=lld *.o
```
And `objdump` the output, the function `foo()` is not inlined:
```
0000000000001710 <foo@f1()>:
    1710:       48 8d 3d 65 ee ff ff    lea    rdi,[rip+0xffffffffffffee65]        # 57c <_IO_stdin_used+0x4>
    1717:       e9 64 00 00 00          jmp    1780 <puts@plt>
    171c:       cc                      int3
    171d:       cc int3
    171e:       cc                      int3
 171f:       cc                      int3

0000000000001720 <initializer for module f1>:
    1720:       c3                      ret
    1721:       cc int3
    1722:       cc                      int3
 1723:       cc                      int3
    1724:       cc int3
    1725:       cc                      int3
    1726:       cc int3
    1727:       cc                      int3
 1728:       cc                      int3
    1729:       cc int3
    172a:       cc                      int3
    172b:       cc int3
    172c:       cc                      int3
 172d:       cc                      int3
    172e:       cc int3
    172f:       cc                      int3

0000000000001730 <main>:
    1730:       50                      push   rax
    1731: e8 da ff ff ff          call   1710 <foo@f1()>
    1736:       31 c0 xor    eax,eax
    1738:       59                      pop rcx
    1739:       c3                      ret
```

This is different behavior from what we did without modules

Before c++20 we write **main2.cpp**:
```cpp
extern "C" int puts(const char* s);

void foo() {
    puts("Hello World");
}

int main() {
    foo();
}
```
Compile with `clang++ -O3 main2.cpp`, and we get:
```
0000000000001140 <foo()>:
 1140:       48 8d 3d bd 0e 00 00    lea    rdi,[rip+0xebd]        # 2004 <_IO_stdin_used+0x4>
    1147:       e9 e4 fe ff ff          jmp    1030 <puts@plt>
    114c:       0f 1f 40 00             nop    DWORD PTR [rax+0x0]

0000000000001150 <main>:
    1150:       50 push   rax
    1151:       48 8d 3d ac 0e 00 00    lea    rdi,[rip+0xeac] # 2004 <_IO_stdin_used+0x4>
    1158:       e8 d3 fe ff ff call   1030 <puts@plt>
    115d:       31 c0                   xor eax,eax
    115f:       59                      pop    rcx
    1160: c3                      ret
```

There is no call to `foo()` in `main()`

# Version Info
```
clang version 20.0.0git (https://github.com/llvm/llvm-project.git 74392bde2e603c8ca3f0956d4c55ba181fe9989b)
Target: x86_64-unknown-linux-gnu
Thread model: posix
```

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