My question is if compilers use "new" x86 instructions, as then the program won't work at all on old systems. For example, if Intel decided today that CPUs need a new "fast" hashing opcode (I don't know if they actually do), a compiler can't compiles to it, as programs won't work on older computers. Is it like the API cruft in Android, where "new" Lollipop APIs are introduced for 10 years from now, when no one uses a…
There are some methods to get around this. For example, there's an ELF extension called STT_GNU_IFUNC. It allows a symbol to be resolved at load time using a custom resolver function. This avoids the problem of figuring out which code-path to use on every invocation.
For example, you could have a function
void hash(char *out, const char *in);
void fast_hash(char *out, const char *in);
void slow_hash(char *out, const char *in);
void (*resolve_hasher(void))(char *, const char *)
{
if (cpuSupportsFancyInstructions()) {
return &fast_hash;
} else {
return &slow_hash;
}
}