Duplicated loop body

[iSazonov]

Follow pattern results in a duplicated loop body:

        for (int i = 0; i < arr.Length; i++)
        {
            for (int j = i; j < arr.Length; j++)
            {
                var b = arr[j];
                Console.WriteLine(b);
            }
        }

See https://sharplab.io/#v2:EYLgtghglgdgNAFxBAzmAPgAQEwEYCwAUJgMwAEOZAwmQN5FmMXmYAsZAsgBSwJkwBKOgyaiAbhABOZKdIC8/AKYB3MrwDaMALoBuEaMYAzAPbSeMPlDIKADDrVkAPDMmSAdABlFMAOYIAFvZQANTBAvoG9IQGMWQmZrxkAFbWavYpzrKe3n6ByaHh0bGiUcXFAPTlEtIQqbLqULoRZUyVmLgAnFwQAnpFLeJSZMB1rupJTf0DjO1dwL3NsQC+i4wrU+tLQA

category:cq
theme:loop-opt
skill-level:expert
cost:medium
impact:small

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[EgorBo]

The nested loop doesn't know that i is never negative (assertprop?) so it can't elide bound checks and is cloned by LoopCloning so at least the cloned loop will be able to iterate without them.

[iSazonov]

The nested loop doesn't know

I guess you mean "CodeGen doesn't know". From this I conclude this comes from the fact CodeGen generates the code based only on static analysis. But at runtime we can check "j = i is less arr.Length" before the cycle.

[AndyAyersMS]

cc @BruceForstall

[BruceForstall]

#8558 Covers this somewhat: loop cloning kicks in for the inner loop, eliminates the "fast path" bounds check. But later on, assertion prop gets rid of the array null check it inserted, and range prop gets rid of the "slow path" bounds check, so the loops are the same. Assertion prop doesn't get rid of the extra "i < 0" check that gets inserted (which it should). If that was eliminated, we're left with #5820.