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Usually not by default, but GCC and other compilers have intrinsics like __builtin_expect [1] which may generate branch hints. [1]: https://gcc.gnu.org/onlined

Usually not by default, but GCC and other compilers have intrinsics like __builtin_expect [1] which may generate branch hints. [1]: https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html#index-_005f_005fbuiltin_005fexpect https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html#index...

unwind 2y ago
Quite popular in the Linux kernel code, as far as I've seen over the years. See [1] for some discussion over on SO. [1]: https://stackoverflow.com/questions/109710 https://stackoverflow.com/questions/109710
menaerus 2y ago
In practice this doesn't result with a special-kinded instruction to hint the CPU branch-predictor but only affects the compiler to generate a codegen that is optimized for better CPU instruction-cache utilization. E.g. it will try to move the less likely code out from the hot code path and as such the likely code is going to be more dense and co-located next to each other.
- vlovich123 2y ago
  It can also change the instruction selection. For example, __builtin_unpredictable which is a close cousin of expect will indicate there's no prediction possible on a branch. This causes the compiler to select branchless instructions & is particularly useful in things like binary search where the CPU attempting to do prediction is worse than using a branchless version of the code.
Salgat 2y ago
I think in general we're overdue for more compiler directives, and not just branch hints. As heterogenous architectures become more standard, we really need a way to tag what functions are best to run with what types of instructions. For example, tag a function that will utilize SIMD so that if a thread is running this function, the operating system knows to try to have it run on the cores that support SIMD. As of now, with P and E Cores for Intel, Intel is straight up disabling instructions to ensure uniform instruction support.

unwind 2y ago

Quite popular in the Linux kernel code, as far as I've seen over the years. See [1] for some discussion over on SO. [1]: https://stackoverflow.com/questions/109710 https://stackoverflow.com/questions/109710

menaerus 2y ago

In practice this doesn't result with a special-kinded instruction to hint the CPU branch-predictor but only affects the compiler to generate a codegen that is optimized for better CPU instruction-cache utilization. E.g. it will try to move the less likely code out from the hot code path and as such the likely code is going to be more dense and co-located next to each other.

vlovich123 2y ago
It can also change the instruction selection. For example, __builtin_unpredictable which is a close cousin of expect will indicate there's no prediction possible on a branch. This causes the compiler to select branchless instructions & is particularly useful in things like binary search where the CPU attempting to do prediction is worse than using a branchless version of the code.

Salgat 2y ago

I think in general we're overdue for more compiler directives, and not just branch hints. As heterogenous architectures become more standard, we really need a way to tag what functions are best to run with what types of instructions. For example, tag a function that will utilize SIMD so that if a thread is running this function, the operating system knows to try to have it run on the cores that support SIMD. As of now, with P and E Cores for Intel, Intel is straight up disabling instructions to ensure uniform instruction support.