Open
Description
The following method currently emits bound checks:
static byte Foo(uint index) // Extracted from CountDigits()
{
ReadOnlySpan<byte> table = new byte[]
{
0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, // 4294967296
0xF6, 0xFF, 0xFF, 0xFF, 0x01, 0x00, 0x00, 0x00, // 8589934582
0xF6, 0xFF, 0xFF, 0xFF, 0x01, 0x00, 0x00, 0x00, // 8589934582
0xF6, 0xFF, 0xFF, 0xFF, 0x01, 0x00, 0x00, 0x00, // 8589934582
0x9C, 0xFF, 0xFF, 0xFF, 0x02, 0x00, 0x00, 0x00, // 12884901788
0x9C, 0xFF, 0xFF, 0xFF, 0x02, 0x00, 0x00, 0x00, // 12884901788
0x9C, 0xFF, 0xFF, 0xFF, 0x02, 0x00, 0x00, 0x00, // 12884901788
0x18, 0xFC, 0xFF, 0xFF, 0x03, 0x00, 0x00, 0x00, // 17179868184
0x18, 0xFC, 0xFF, 0xFF, 0x03, 0x00, 0x00, 0x00, // 17179868184
0x18, 0xFC, 0xFF, 0xFF, 0x03, 0x00, 0x00, 0x00, // 17179868184
0xF0, 0xD8, 0xFF, 0xFF, 0x04, 0x00, 0x00, 0x00, // 21474826480
0xF0, 0xD8, 0xFF, 0xFF, 0x04, 0x00, 0x00, 0x00, // 21474826480
0xF0, 0xD8, 0xFF, 0xFF, 0x04, 0x00, 0x00, 0x00, // 21474826480
0xF0, 0xD8, 0xFF, 0xFF, 0x04, 0x00, 0x00, 0x00, // 21474826480
0x60, 0x79, 0xFE, 0xFF, 0x05, 0x00, 0x00, 0x00, // 25769703776
0x60, 0x79, 0xFE, 0xFF, 0x05, 0x00, 0x00, 0x00, // 25769703776
0x60, 0x79, 0xFE, 0xFF, 0x05, 0x00, 0x00, 0x00, // 25769703776
0xC0, 0xBD, 0xF0, 0xFF, 0x06, 0x00, 0x00, 0x00, // 30063771072
0xC0, 0xBD, 0xF0, 0xFF, 0x06, 0x00, 0x00, 0x00, // 30063771072
0xC0, 0xBD, 0xF0, 0xFF, 0x06, 0x00, 0x00, 0x00, // 30063771072
0x80, 0x69, 0x67, 0xFF, 0x07, 0x00, 0x00, 0x00, // 34349738368
0x80, 0x69, 0x67, 0xFF, 0x07, 0x00, 0x00, 0x00, // 34349738368
0x80, 0x69, 0x67, 0xFF, 0x07, 0x00, 0x00, 0x00, // 34349738368
0x80, 0x69, 0x67, 0xFF, 0x07, 0x00, 0x00, 0x00, // 34349738368
0x00, 0x1F, 0x0A, 0xFA, 0x08, 0x00, 0x00, 0x00, // 38554705664
0x00, 0x1F, 0x0A, 0xFA, 0x08, 0x00, 0x00, 0x00, // 38554705664
0x00, 0x1F, 0x0A, 0xFA, 0x08, 0x00, 0x00, 0x00, // 38554705664
0x00, 0x36, 0x65, 0xC4, 0x09, 0x00, 0x00, 0x00, // 41949672960
0x00, 0x36, 0x65, 0xC4, 0x09, 0x00, 0x00, 0x00, // 41949672960
0x00, 0x36, 0x65, 0xC4, 0x09, 0x00, 0x00, 0x00, // 41949672960
0x00, 0x00, 0x00, 0x00, 0x0A, 0x00, 0x00, 0x00, // 42949672960
0x00, 0x00, 0x00, 0x00, 0x0A, 0x00, 0x00, 0x00, // 42949672960
};
return table[(int)(uint.Log2(index))];
}While it looks to be quite niche, it showcases multiple ways we can improve bound check elimination for a general case, so basically this is lowered to:
array[Lzcnt.LeadingZeroCount(index | 1) ^ 31]We should handle:
-
OR
index | CNS, e.g.X | 1means lower bound is at least1. -
XOR
index ^ CNS, e.g.X ^ 31means that we should merge X's range with0-b11111range -
HWINTRINSIC - we know that certain intrinsics such as LeadingZeroCount return value in a range of
0..64+ merged with its argument's range
OR and XOR should be handled here + "does overflow" part.
Each of these separately might find uses in the wild.