alt Hacker NewsI am rather thinking, if one is so much faster, and they are truly equal, why is the compiler too stupid to convert one into the other?
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The two code snippets do different things, apples and oranges... e.g. the array modification in the second example needs to move in front of the if for the two snippets to behave identically. I bet then the compiler output is the same with -O1 or higher.
PS: e.g. note how bla() (first code snippet) and blob() (fixed second code snippet) have identical output (both are turned into the same 'branchless' code via a conditional 'setl' instruction), but the blub() function (original second code snippet) differs because that function has different behaviour:
https://www.godbolt.org/z/h9Kfbn5bc
TL;DR: most 'branchless advice' that only tinkers with language features (like "x = a ? b : c" instead of an if) is useless because to the optimizer passes both are the same thing (a condition).
When there's a difference in the generated code then it's usually a bug and the before-after code are not actually equivalent (like in the code examples above).
It doesn't convert bogosort into heapsort either, despite the second being much faster than the first. I'm guessing that it's not that easy going from one to the other because the only thing they have in common is the output (and only after you have checked the last value), so if the transformation is not hard-coded into the compiler, the odds of it randomly discovering the optimization is close to zero