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JIT: Improve rangecheck for certain binops #79257
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.
Activity
jakobbotsch commentedon Dec 6, 2022
Related issue that could be fixed at the same time: #65550
drieseng commentedon Jan 15, 2023
When fixed, the following code can be updated:
runtime/src/libraries/System.Private.CoreLib/src/System/Buffers/Text/FormattingHelpers.CountDigits.cs
Lines 147 to 149 in 1555786
Just trying to be helpful, let me know if it's not.
aromaa commentedon Feb 10, 2023
Wrote something to catch this b04892f5f1cef198e3d4c9aa41188d4fbe271954.
Its quite a bit of LOC for no hits in CoreLib other than the potential use case in FormattingHelpers.CountDigits.
EgorBo commentedon Feb 10, 2023
I didn't expect a lot of hits, but it's a good JIT exercise 🙂 with a good test coverage can be merged (haven't looked deeply your branch yet)
xakep139 commentedon Apr 20, 2023
Once fixed, this file needs to be updated as well:
runtime/src/libraries/System.Private.CoreLib/src/System/Buffers/Text/FormattingHelpers.CountDigits.cs
Lines 69 to 70 in 6887fc4