blog-code

> I assume that their test input (which isn't described in the post, and is also not in their GitHub repo) consists of random strings consisting almost entirely of s and p characters. test code is here: https://github.com/414owen/blog-code/blob/master/02-the-same... it randomly selects between 's' or 'p'. The characters can't be anything other than 's', 'p', or the terminating null. Knowing that particular fact about our input gives us this ...clever... optimization: int run_switches(const char* s) { int result = 0; while (*s) result += (1 | *s++) - 'r'; return result; } which compiles to: run_switches: movzx eax, BYTE PTR [rdi] xor edx, edx test al, al je .L1 .L3: or eax, 1 inc rdi

> because without FDO (or PGO) the compiler has no idea how likely each branch is to be taken So, the maximum amount of times you can hit '\0' is once in the string, because then the function returns, but you can hit the other characters many times, which seems to be information a compiler has access to without PGO. PGO does help, of course, and on my machine gives me 2.80s, which is better than the code at the end of the `Rearranging blocks` section :) > I assume that their test input (which isn't described in the post, and is also not in their GitHub repo) It's described under `Benchmarking setup`, and is in the repository here: https://github.com/414owen/blog-code/blob/master/01-six-time... Side note: There's a part two to this post (linked at the bottom) where I make the C code as fast as I possibly can, and it beats all the assembly