Comment by torstenvl

I haven't run any benchmarks, but jump-if-equal and set-if-equal would seem to have the same level of predictability.

My naive, untested intuition is that there's only one meaningful difference: the former has to dump the entire pipeline on a miss, and the latter only has to nop a single instruction on a miss.

But maybe I'm missing something. I'll re-read his rant.

EDIT:

Linus rants a lot, but makes one concrete claim:

    You can always replace it by
    
      j<negated condition> forward
      mov ..., %reg
     forward:
    
    and assuming the branch is AT ALL predictable (and 95+% of all branches
    are), *the branch-over will actually be a LOT better for a CPU.*

So, I decided to test that.

    [18:50:14 user@boxer ~/src/looptest] $ diff -u loop2.s loop4.s
    --- loop2.s 2023-07-06 18:40:11.000000000 -0400
    +++ loop4.s 2023-07-06 18:46:58.000000000 -0400
    @@ -17,11 +17,15 @@
      incq %rdi
      xorl %edx, %edx
      cmpb $115, %cl
    - sete %dl
    + jne _run_switches_jmptgt1
    + mov $1,   %dl
    +_run_switches_jmptgt1:  
      addl %edx, %eax
      xorl %edx, %edx
      cmpb $112, %cl
    - sete %dl
    + jne _run_switches_jmptgt2
    + mov $1,   %dl
    +_run_switches_jmptgt2:  
      subl %edx, %eax
      testb %cl, %cl
      jne LBB0_1
    [18:50:29 user@boxer ~/src/looptest] $ gcc -O3 bench.c loop2.s -o l2
    [18:50:57 user@boxer ~/src/looptest] $ gcc -O3 bench.c loop4.s -o l4
    [18:51:02 user@boxer ~/src/looptest] $ time ./l2 1000 1
    449000
    ./l2 1000 1  0.69s user 0.00s system 99% cpu 0.697 total
    [18:51:09 user@boxer ~/src/looptest] $ time ./l4 1000 1
    449000
    ./l4 1000 1  4.53s user 0.01s system 99% cpu 4.542 total

I feel pretty confident that Linus has made a poor prediction about poor prediction here. Jumps are indeed slower.

To be fair to Linus, since Clang and I are using sete here, not cmov, I also tested cmov, and the difference was insignificant:

    [19:53:12 user@boxer ~/src/looptest] $ time ./l2 1000 1            
    449000
    ./l2 1000 1  0.69s user 0.00s system 99% cpu 0.700 total
    [19:53:15 user@boxer ~/src/looptest] $ time ./l5 1000 1            
    449000
    ./l5 1000 1  0.68s user 0.00s system 99% cpu 0.683 total

Jumps are slower.