3 ms·

I think it is used for code layout: typically you want to make the most often taken branch as fallthrough as it can be slightly faster even when perfectly predi

I think it is used for code layout: typically you want to make the most often taken branch as fallthrough as it can be slightly faster even when perfectly predicted. I also tried (and failed) to use expect with probability to generate a cmov, but in retrospect it is obvious: the parameter is not the prediction probability, but the taken/not-taken probability, and even a branch that goes either way with 50% probability can still be predicted perfectly (for example a branch that switches every other iteration), so the parameter can't be used to decide between predication and prediction. edit: what we really need is an [[unpredictable]] attribute.

twoodfin 3y ago
Indeed: A branch taken 50% of the time can be readily predicted 100% of the time if it follows a pattern the CPU can infer. Why you have to be careful choosing realistic input data if you are microbenchmarking at this level for a real purpose.
pixelesque 3y ago
clang has one: https://clang.llvm.org/docs/LanguageExtensions.html#builtin-unpredictable https://clang.llvm.org/docs/LanguageExtensions.html#builtin-...
- ot 3y ago
  Which also does not affect cmov conversion. Every time this comes up I bring up this long-standing LLVM bug: https://bugs.llvm.org/show_bug.cgi?id=40027 https://bugs.llvm.org/show_bug.cgi?id=40027
  - dzaima 3y ago
    __builtin_unpredictable is gonna be fixed in LLVM/clang 17: https://github.com/llvm/llvm-project/commit/09515f2c20111628ce81ad5f40e12e5f6af5ed2f https://github.com/llvm/llvm-project/commit/09515f2c20111628... (also, bugs.llvm.org is old; the more up-to-date (but still open) issue is https://github.com/llvm/llvm-project/issues/39374 https://github.com/llvm/llvm-project/issues/39374)
    - ot 3y ago
      Amazing, finally!!!
foldr 3y ago
>typically you want to make the most often taken branch as fallthrough as it can be slightly faster even when perfectly predicted. Sure, but that application doesn't require a fine-grained probability estimate.

twoodfin 3y ago

Indeed: A branch taken 50% of the time can be readily predicted 100% of the time if it follows a pattern the CPU can infer. Why you have to be careful choosing realistic input data if you are microbenchmarking at this level for a real purpose.

pixelesque 3y ago

clang has one: https://clang.llvm.org/docs/LanguageExtensions.html#builtin-unpredictable https://clang.llvm.org/docs/LanguageExtensions.html#builtin-...

ot 3y ago
Which also does not affect cmov conversion. Every time this comes up I bring up this long-standing LLVM bug: https://bugs.llvm.org/show_bug.cgi?id=40027 https://bugs.llvm.org/show_bug.cgi?id=40027
- dzaima 3y ago
  __builtin_unpredictable is gonna be fixed in LLVM/clang 17: https://github.com/llvm/llvm-project/commit/09515f2c20111628ce81ad5f40e12e5f6af5ed2f https://github.com/llvm/llvm-project/commit/09515f2c20111628... (also, bugs.llvm.org is old; the more up-to-date (but still open) issue is https://github.com/llvm/llvm-project/issues/39374 https://github.com/llvm/llvm-project/issues/39374)
  - ot 3y ago
    Amazing, finally!!!

foldr 3y ago

>typically you want to make the most often taken branch as fallthrough as it can be slightly faster even when perfectly predicted. Sure, but that application doesn't require a fine-grained probability estimate.