<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/68478>68478</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [CLANG][GPU] Error using printf with -fno-builtin[-printf]
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            clang
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          Maetveis
      </td>
    </tr>
</table>

<pre>
    # Description

When `-fno-builtin-printf` is in effect (such as when passing this option or simply `-fno-builtin`),
compiling a device function with printf results in an error.

# Impact

`/Oi-` commonly used on Windows when debugging (this is the default setting of Visual Studio) is translated by `clang-cl` to `-fno-builtin`.

There's no option to re-enable a builtin that was disabled by a previous flag, so the only option is to not use `-fno-builtin` or list all builtin functions except printf.

# Environment

```console
❯ cat /etc/os-release
NAME="Ubuntu"
VERSION="20.04.6 LTS (Focal Fossa)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 20.04.6 LTS"
VERSION_ID="20.04"
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
VERSION_CODENAME=focal
UBUNTU_CODENAME=focal
```

```console
❯ clang --version
Ubuntu clang version 17.0.2 (++20231003073124+b2417f51dbbd-1~exp1~20231003073217.50)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
```

```console
❯ nvcc --version
nvcc: NVIDIA (R) Cuda compiler driver
Copyright (c) 2005-2022 NVIDIA Corporation
Built on Tue_Mar__8_18:18:20_PST_2022
Cuda compilation tools, release 11.6, V11.6.124
Build cuda_11.6.r11.6/compiler.31057947_0
```

```console
❯ hipconfig --version
5.4.22801-aaa1e3d8
```

# Reproducer

Assuming a file named main.cu with the contents:
```cuda
#ifdef __HIP__
#include <hip/hip_runtime.h>
#endif

__global__ void a() {
    printf("my name: %s\n", "asd");
}
```

For cuda:
```console
> clang -x cu -fno-builtin-printf main.cu
ptxas fatal   : Unresolved extern function 'printf'
clang-17: error: ptxas command failed with exit code 255 (use -v to see invocation)
Ubuntu clang version 17.0.2 (++20231003073124+b2417f51dbbd-1~exp1~20231003073217.50)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
``` 

For HIP:
```console
❯ clang -x hip -fno-builtin-printf main.cu
main.cu:5:17: error: unsupported call to variadic function printf
__global__ void a() {
                ^
1 error generated when compiling for gfx906.
```

(This happens with both with `-mprintf-kind=` `buffered` and `hostcall`)

OpenMP offloading is probably also affected, but I didn't try it.

# Analysis

Here's what I think goes wrong:

Clang rewrites `printf` to `vprintf` for Cuda, and directly emits runtime calls for AMDGPU, but this only happens when builtins are enabled, when builtins are not enabled a normal function call is emitted to `printf`, but it won't be found because:

- The cuda device libraries don't provide it, as nvcc normally rewrites it to a special `vprintf`.
- AMDGPU doesn't even support C style variadic functions so trying to codegen the call to `printf` fails earlier (at compile time instead of link-time).

  

</pre>
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