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JIT doesn't elide bounds checking when iterating over a span and an array of known length #110986
Description
Description
If you ensure that the length of an array and a span are the same, the JIT will still emit bounds checking on only the span when indexing.
This does not occur when iterating over a span and a span, or an array and an array.
public static int SumSpanArray(int[] arr, Span<int> span)
{
int counter = 0;
if(arr.Length != span.Length)
return 0;
//bounds check here
for(int i = 0; i < arr.Length; i++)
counter += arr[i] + span[i];
return counter;
}Sharplab example here
Regression?
I have only used Sharplab to test this, but it doesn't seem likely to be a regression.
Analysis
I don't know why, but curiously enough when checking the span length when iterating over a span + array, there is no bound check
public static int SumSpanArray2(int[] arr, Span<int> span)
{
int counter = 0;
if(arr.Length != span.Length)
return 0;
//no bound check
for(int i = 0; i < span.Length; i++)
counter += arr[i] + span[i];
return counter;
}
Activity
dotnet-policy-service commentedon Dec 29, 2024
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dotnet-policy-service commentedon Dec 29, 2024
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EgorBo commentedon Dec 29, 2024
It works if both are arrays. Presumably it should work here as well once #82946 is implemented.
tannergooding commentedon Dec 31, 2024
Notably as well it does work if you insert a
span1 = span1.Slice(span2.Length)after aif (span1.Length != span2.Length) { return; }check.So there is a relatively easy workaround, but ideally we'd still just handle correlating the array/span or span/span lengths directly.