Changes from released compilers

ACLE specification changes

• The set of supported features has changed as indicated by the ACLE Q3 change log and the ACLE Q4 change log .
• The runtime detection of features has changed. Dependent-on features get detected as indicated by the dependencies table .
• The most appropriate version of a function is determined as indicated by the new selection rules . Previously, the most specific version (the one with most features) was favored over any other version.
• A new predefined macro __FUNCTION_MULTI_VERSIONING_SUPPORT_LEVEL has been added to indicate which ACLE version is implemented by the compiler.

Semantic changes between LLVM 19 and LLVM 20

With LLVM 19 at least one more version other than the default is needed to trigger function multiversioning.

With LLVM 20 a header file declaration:

    

        
        
__attribute__((target_version("default"))) void f(void);

    

guarantees that there will be a mangled version f.default. Conversely, LLVM 19 would generate an unmangled symbol here since function multiversioning does not trigger when compiling this code in the absence of other versions.

Static resolution in LLVM 20

LLVM can optimize calls to versioned functions when they can be statically resolved. For example:

    

        
        
__attribute__((target_version("mops"))) int f(void);
__attribute__((target_version("sve2"))) int f(void);
__attribute__((target_version("sve"))) int f(void);
__attribute__((target_version("default"))) int f(void) { return 0; }

__attribute__((target_version("mops+sve2"))) int caller(void) {
  return f(); // f._Mmops is called directly
}
__attribute__((target_version("mops"))) int caller(void) {
  return f(); // f._Mmops is called directly
}
__attribute__((target_version("sve"))) int caller(void) {
  return f(); // cannot be optimized since SVE2 may be available on target
}
__attribute__((target_version("default"))) int caller(void) {
  return f(); // f.default is called directly
}