Handle more scenarios for loop cloning

[kunalspathak]

• Increasing loops that are incremented by more than 1 like "i += 2"

TypeHashingAlgorithm.cs

runtime/src/coreclr/tools/Common/TypeSystem/Common/TypeHashingAlgorithms.cs

Lines 44 to 49 in 7bac4e8

	for (int i = startIndex; i < src.Length; i += 2)
	{
	_hash1 = (_hash1 + _rotl(_hash1, 5)) ^ src[i];
	if ((i + 1) < src.Length)
	_hash2 = (_hash2 + _rotl(_hash2, 5)) ^ src[i + 1];
	}

• Decreasing loops like "i--", "i -= 2", etc.

runtime/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/EqualityComparer.cs

Lines 50 to 63 in 7bac4e8

	internal virtual int LastIndexOf(T[] array, T value, int startIndex, int count)
	{
	int endIndex = startIndex - count + 1;
	for (int i = startIndex; i >= endIndex; i--)
	{
	if (Equals(array[i], value))
	{
	return i;
	}
	}
	return -1;
	}
	#endif
	}

PriorityQueue's Heapify method.

runtime/src/libraries/System.Collections/src/System/Collections/Generic/PriorityQueue.cs

Lines 588 to 591 in 7bac4e8

	for (int index = lastParentWithChildren; index >= 0; --index)
	{
	MoveDownDefaultComparer(nodes[index], index);
	}

Contributes to #65342

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Issue Details

---

• Increasing loops that are incremented by more than 1 like "i += 2"
• Decreasing loops like "i--", "i -= 2", etc.

Author:	kunalspathak
Assignees:	kunalspathak
Labels:	area-CodeGen-coreclr
Milestone:	-

[kunalspathak]

Failures are related to #67878

[kunalspathak]

@dotnet/jit-contrib , @BruceForstall

[BruceForstall]

You need to worry about increment/decrement overflowing a 32-bit integer and wrapping. E.g.:

public static int test_up_big(int[] a, int s)
    {
        int r = 0;
        int i;
        for (i = 1; i < s; i += 2147483647)
        {
            r += a[i];
        }
        return r;
    }

with your code will clone, but generate an access violation, not System.IndexOutOfRangeException.

In the case of for ( ; i < x; i += c), c needs to be strictly less than int.MaxValue - (Array.MaxLength - 1) + 1, which is 0x7fffffff - 0x7fffffc7 + 2 = 0x3a = 58. The idea being that if i was the maximum allowed legal array index of 0x7fffffc6 and you added 0x3a, the result is negative (-2,147,483,648).

Similar applies to down counted loops.

[kunalspathak]

public static int test_up_big(int[] a, int s)
{
int r = 0;
int i;
for (i = 1; i < s; i += 2147483647)
{
r += a[i];
}
return r;
}

Thanks for pointing that out. Let me try to fix that.

[kunalspathak]

@BruceForstall - I tried measuring some of the benchmarks that showed up in asmdiffs and they all show improvements.

Faster	base/diff	Base Median (ns)	Diff Median (ns)	Modality
BilinearTest.Interpol_Scalar	1.23	12591.21	10245.69
BilinearTest.Interpol_Vector	1.08	27857.16	25908.72
BenchmarksGame.FannkuchRedux_5.RunBench(n: 10, expectedSum: 38)	1.06	16428450.00	15492476.47
BenchmarksGame.ReverseComplement_1.RunBench	1.13	1021397.94	903558.01
ByteMark.BenchEmFloatClass	1.05	771413700.00	734018150.00
ByteMark.BenchEmFloat	1.03	3737632200.00	3641513150.00
System.Collections.Tests.Perf_PriorityQueue<String, String>.HeapSort(Size: 100)	10.80	898710.07	83248.11
System.Collections.Tests.Perf_PriorityQueue<String, String>.HeapSort(Size: 10)	10.44	39025.89	3739.26
System.Collections.Tests.Perf_PriorityQueue<String, String>.HeapSort(Size: 1000)	10.34	14949560.00	1445691.48
System.Collections.Tests.Perf_PriorityQueue<Int32, Int32>.HeapSort(Size: 10)	1.04	203.82	195.96
Benchstone.BenchI.HeapSort.Test	1.03	294538.92	287004.51

Slower	diff/base	Base Median (ns)	Diff Median (ns)	Modality
System.Collections.Tests.Perf_PriorityQueue<Guid, Guid>.HeapSort(Size: 100)	1.02	9744.88	9900.55
System.Collections.Tests.Perf_PriorityQueue<Guid, Guid>.HeapSort(Size: 10)	1.01	396.27	401.45

Here is an example asmdiff from Heapify:

[kunalspathak]

ping.

[BruceForstall]

LGTM.

I think you should run a jitstress job before merging.

[kunalspathak]

/azp run runtime-coreclr jitstress

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[kunalspathak]

Improvements on windows/x64: dotnet/perf-autofiling-issues#5072
Improvements on windows/arm64: dotnet/perf-autofiling-issues#5145
Improvements on windows/x64: dotnet/perf-autofiling-issues#5114 (Reverse complement)
Improvements on Ubuntu/x64: dotnet/perf-autofiling-issues#5092
Improvements on Windows/x86: dotnet/perf-autofiling-issues#5028