Bug Fix Report: AMX Instruction Compatibility with GCC
Issue Summary
When compiling a C++ project using GCC 13 with CUDA enabled, the project encountered errors related to AMX Tile Matrix instructions (ldtilecfg and sttilecfg). These errors stemmed from conflicts between the __builtin_ia32_ldtilecfg and __builtin_ia32_sttilecfg intrinsics and GCC’s internal handling of these instructions on specific hardware configurations.
Root Cause
The root cause of the issue was the use of GCC’s built-in functions for AMX instructions, which conflicted with the underlying hardware’s support for these intrinsics. Specifically, the __builtin_ia32_ldtilecfg and __builtin_ia32_sttilecfg functions failed due to improper handling of the constexpr keyword, which GCC enforces on its built-in cmath functions. As a result, this caused redefinition and incompatibility errors during compilation.
Solution Implemented
To resolve the issue, we replaced the GCC built-in intrinsics with equivalent inline assembly instructions. This bypasses GCC’s handling and directly invokes the AMX instructions:
Modifications Made
- File Modified:
/usr/lib/gcc/x86_64-linux-gnu/13/include/amxtileintrin.h - Original Code:
extern __inline void
__attribute__((__gnu_inline__, __always_inline__, __artificial__))
_tile_loadconfig (const void *__config)
{
__builtin_ia32_ldtilecfg (__config);
}
extern __inline void
__attribute__((__gnu_inline__, __always_inline__, __artificial__))
_tile_storeconfig (void *__config)
{
__builtin_ia32_sttilecfg (__config);
}
Modified Code
extern __inline void
__attribute__((__gnu_inline__, __always_inline__, __artificial__))
_tile_loadconfig (const void *__config)
{
__asm__ volatile ("ldtilecfg\t%X0" :: "m" (*((const void **)__config)));
}
extern __inline void
__attribute__((__gnu_inline__, __always_inline__, __artificial__))
_tile_storeconfig (void *__config)
{
__asm__ volatile ("sttilecfg\t%X0" : "=m" (*((void **)__config)));
}
Explanation of the Changes
The updated code uses inline assembly to invoke the AMX instructions directly, allowing more precise control over the instruction handling without relying on GCC’s intrinsics. This change enables the code to compile without triggering constexpr conflicts and ensures compatibility with the AMX Tile Matrix instructions.
Impact Analysis
- System Compatibility: No changes to system-wide configuration. These changes apply only to the specific project that includes this header.
- Future Maintenance: If GCC is updated, the modified header may be overwritten. To avoid this, it’s recommended to encapsulate these changes in a custom project-specific header.
- Testing: The modified code was tested on an Intel processor supporting AMX, and the instructions were verified to execute correctly without further errors.
Recommendation
It is advised to encapsulate these changes within the project or maintain a patch file to reapply modifications as necessary after compiler updates. Avoid modifying system headers directly to maintain compatibility with future GCC versions and ensure that AMX functionality is only utilized where supported.