On 1/18/2025 12:24 AM, H. Peter Anvin wrote:
In short, seems that __builtin_expect not work with switch(), at
least for
gcc version 8.5.0 20210514(RHEL).
For forward-facing optimizations, please don't use an ancient version
of gcc as the benchmark.
Even there is a latest Gcc built-in feature could work for this case, it
is highly unlikely that Linus would adopt such trick into upstream
kernel (only works for specific ver compiler). the same resultto those
downstream vendors/LTS kernels. thus, making an optimization with latest
only Gcc would construct an impractical benchmark-only performance
barrier. As to the __builtin_expect(), my understanding, it was designed
to only work for if(bool value) {
}
else if(bool value) {
} The value of the condition expression returned by __builtin_expect()
is a bool const. while switch(variable) expects a variable. so it is
normal for Gcc that it doesn't work with it.
If I got something wrong, please let me know.
Thanks,
Ethan
-hpa
