Re: Maybe we should get rid of ifuncs

[Peter Bergner <bergner@linux.ibm.com>]

On 4/24/24 9:43 AM, Zack Weinberg wrote:
> I'm very curious what the plan for function multiversioning in GCC
> and LLVM is, and how close to declarative it gets.

GCC (at least on powerpc) already supports it via the target_clones
attribute.  See gcc/testsuite/gcc.target/powerpc/clone*.c for examples.
Basically, it looks like (from clone3.c):

__attribute__((target_clones("cpu=power10,cpu=power9,default")))
long mod_func (long a, long b)
{
  return (a % b) + s;
}

long mod_func_or (long a, long b, long c)
{
  return mod_func (a, b) | c;
}


Mike knows how this works better than I, but GCC automatically emits an
ifunc resolver for the different clones and looks to use the HWCAP*
architecture mask associated with the cpu we're compiling for.
The "default" function being called in the case our ifunc resolver
doesn't match any of the HWCAP* masks from the cpus we're compiling
for.

Mike, it seems like this is more of a "cpu" clone and not a true HWCAP
test, so this specific thing doesn't (at least currently) work for
something like __attribute__((target_clones("vsx,mma,default"))) ?
Or did I misread the code?


I'll note I'm pretty sure we (IBM/powerpc) have added ifunc usage to
OpenBLAS and some other libraries outside of glibc.


Peter