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Comment by Symmetry | Hacker News Reader

I'd hope that at least the branches would be turned into cmovs.

Depends on how predictable the branches are. LLVM used to use CMOVs whenever possible but I believe this was changed a few years back to generally improve performance.

See a related bug in GCC: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56309

"conditional moves instead of compare and branch result in almost 2x slower code"

Yeah, easy to predict branches are really cheap. Very non-deterministic, hard-to-predict ones on the other hand are still surprisingly expensive. In such a case, conditional moves might make sense, or sometimes you can also use the result of a comparison in arithmetic operations, something like:

  idx = 2*idx + (key > table[idx])

(to move a key down an implicitly-stored binary tree)

I remember Linus making much the same point: http://yarchive.net/comp/linux/cmov.html

It generally lines up with what I've observed. Surprisingly, I've found that even arithmetic comparisons like you mentioned ran a bit faster with branches instead of conditional moves. The one case where I have seen comparisons benefit is with SSE code where the comparison results go straight to a register instead of a flag.

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