Fix: Strtof Check Fails On Cray Frontier

Hey guys! Ever run into a snag when building code, especially on a supercomputer like the HPE Cray EX (Frontier)? I recently did, and it involved the CHECK_FUNCTION_EXISTS(strtof) test failing during a build process. This might sound like a bunch of technical jargon, but trust me, we'll break it down. Essentially, this failure led to some unexpected behavior in how the code handled floating-point number conversions. Let's dive into what went wrong and how we can fix it.

The Problem: Incorrect Test, Not Missing Function

First off, let's understand the core issue. The CHECK_FUNCTION_EXISTS(strtof) test, which is supposed to confirm if the strtof function (used for converting strings to floating-point numbers) is available, was failing. But here's the kicker: strtof is actually provided by the C standard library. The problem wasn't the absence of strtof; it was an issue with how the test was implemented. The test was trying to call the function in a way that didn't match how strtof is supposed to be used. This mismatch caused the compiler to throw errors, making it seem like strtof wasn't there.

To give you a bit more context, the error messages from the build logs were pretty clear about this. They showed conflicting types and incorrect argument counts when trying to use strtof. Because of the test's failure, the build system incorrectly assumed that strtof wasn't available. This led to a fallback implementation being compiled, which, in turn, caused symbol conflicts with the standard library's version of strtof. It's like having two of the same things trying to do the same job – chaos ensues!

Diving into the Details: The Error Messages

Let's get a bit more technical to understand the errors. The build process uses CMake, a popular build system, to configure the project. When the CHECK_FUNCTION_EXISTS macro runs, it tries to compile a small test program to see if strtof is available. Here's a snippet from the CMake log that highlights the problem:

/lustre/orion/nfu106/proj-shared/ylan/src/nekrs_stefan_v25_next010726_rocm7/build/CMakeFiles/CMakeScratch/TryCompile-IjXQtE/CheckFunctionExists.c:7:3: error: conflicting types for 'strtof'
    7 |   CHECK_FUNCTION_EXISTS(void);
      |   ^
<command line>:5:31: note: expanded from macro 'CHECK_FUNCTION_EXISTS' ...

This error indicates that there's a type conflict. The macro CHECK_FUNCTION_EXISTS is defined to strtof, but the test code is using it in a way that doesn't align with the actual function signature of strtof. The second error shows the test program calling strtof with the wrong number of arguments, which triggered another compile error:

/lustre/orion/nfu106/proj-shared/ylan/src/nekrs_stefan_v25_next010726_rocm7/build/CMakeFiles/CMakeScratch/TryCompile-IjXQtE/CheckFunctionExists.c:17:25: error: too few arguments to function call, expected 2, have 0
   17 |   CHECK_FUNCTION_EXISTS();
      |   ~~~~~~~~~~~~~~~~~~~~~ ^

This confirms the test program's incorrect invocation of strtof. The function was called without the required arguments, causing the compilation to fail. These errors resulted in HAVE_STRTOF being set to false, leading to the problems.

The Fallout: Symbol Conflicts and Duplicate Definitions

The most significant consequence of the failed strtof check was the compilation of a fallback implementation of strtof. This fallback code, intended to be used if strtof wasn't available in the standard library, created a duplicate definition of the function. This situation caused symbol conflicts during the linking phase, leading to errors and preventing the program from building correctly. It's like having two people with the same name trying to do the same job – the compiler doesn't know which one to choose.

This is particularly problematic on systems like the Cray Frontier, which use advanced compilers and build tools. These systems are highly optimized and sensitive to such conflicts. The duplicate definitions could lead to unpredictable behavior or even complete build failures, making it crucial to fix this issue.

The Solution: Fixing the Test

The fix for this problem involves correcting the CHECK_FUNCTION_EXISTS test. Instead of incorrectly calling strtof, the test needs to be updated to use the function correctly. This typically means ensuring the test code includes the necessary headers and calls the function with the correct arguments. Here’s a conceptual outline of what needs to be changed:

1. Include the necessary headers: Make sure the test includes <stdlib.h>, which declares strtof. This ensures that the compiler knows the function signature.
2. Call strtof correctly: The test should call strtof with the correct arguments. Generally, strtof takes a string and a pointer to a character as arguments. The pointer is used to store the location of the first character after the converted number.

By fixing the test, we ensure that the build system correctly identifies the availability of strtof. This prevents the compilation of the fallback implementation, resolving the symbol conflicts and allowing the program to build successfully. This approach is much more reliable and aligns with standard practices for checking the availability of library functions during the build process.

Code Example

Here’s a simplified example of how the corrected test might look. Remember, the exact implementation will depend on your build system (CMake, in this case), but this should give you the general idea:

#include <stdlib.h>

int main() {
    const char *str = "3.14159";
    char *endptr;
    float result = strtof(str, &endptr);
    if (endptr == str) {
        // Error: No conversion performed
        return 1;
    }
    return 0;
}

In this example, the code includes <stdlib.h>, declares a string to convert, calls strtof correctly with the string and a pointer, and checks if the conversion was successful. This is a basic example, but it shows the correct way to test if strtof is available and functioning as expected.

Impact and Importance

The impact of fixing this issue is significant, especially when building on HPC systems. It ensures that the code correctly utilizes the standard library's implementation of strtof, avoiding potential conflicts and ensuring compatibility. It improves build reliability and overall code portability. By addressing the root cause, developers can prevent similar issues from arising in the future and maintain a clean and efficient build process.

Conclusion

So, in summary, the CHECK_FUNCTION_EXISTS(strtof) failure on the Cray Frontier wasn't due to a missing function, but a flawed test. By fixing the test to correctly verify the availability of strtof, we avoided symbol conflicts and ensured the correct use of standard library functions. This simple fix can save a lot of headaches and ensures the smooth building of code on HPC environments. Hopefully, this helps you guys if you ever encounter a similar issue. Happy coding!

I hope this explanation was helpful and easy to understand. If you have any questions or want to dive deeper into any aspect of this, feel free to ask. Let's keep those builds running smoothly!