[Issue 19310] New: VRP for array casts

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19310

          Issue ID: 19310
           Summary: VRP for array casts
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Keywords: performance
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: dfj1esp02 at sneakemail.com

long[] f(byte[] a)
{
    return cast(long[])a[0..a.length/8*8];
}

Here the compiler can see that the array length is multiple of 8 and simply
divide the length by 8 for the cast.

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