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> However, I’ve found that some compilers, e.g. GCC on x86-64 will refuse to make this variant branchless. I hate how fickle compilers can be sometimes, and I w

> However, I’ve found that some compilers, e.g. GCC on x86-64 will refuse to make this variant branchless. I hate how fickle compilers can be sometimes, and I wish compilers exposed not just the likely/unlikely attributes, but also an attribute that allows you to mark something as unpredictable to nudge the compiler towards using branchless techniques like CMOV’s. Clang has `__builtin_unpredictable`[^1]. Both of the have `__builtin_expect_with_probability` [^2], which could help. But these builtins are fickle/make no guarantees as far as I can tell. [^1] https://clang.llvm.org/docs/LanguageExtensions.html#builtin-unpredictable https://clang.llvm.org/docs/LanguageExtensions.html#builtin-... [^2] https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html#index-_005f_005fbuiltin_005fexpect_005fwith_005fprobability https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html#index...

orlp 3y ago
I wasn't aware of __builtin_unpredictable until yesterday. I was aware of __builtin_expect_with_probability. Unfortunately I've found both of them to be essentially useless with today's compilers.
- xoranth 3y ago
  Mmm, it looks like LLVM even has a pass to actively convert generated cmovs into branches if one of the operands touches memory [^1], and that only supported way to force things to be brancheless is inline assembly [^2]. [^1] https://reviews.llvm.org/D36858 https://reviews.llvm.org/D36858 [^2] See: - https://groups.google.com/g/llvm-dev/c/qwogIiwHCAc/m/lSP1jh2WBAAJ https://groups.google.com/g/llvm-dev/c/qwogIiwHCAc/m/lSP1jh2... - https://groups.google.com/g/llvm-dev/c/qwogIiwHCAc/m/EAt643eYBAAJ https://groups.google.com/g/llvm-dev/c/qwogIiwHCAc/m/EAt643e...