Add support to 'long' switch statements

[shargon]

There is a problem with the switch instruction, it seems that works if we have less entries than 6, with more, it doesn't works.

I made a pull request for test this issue at, #87

if you comment some entries, the test will work

[erikzhang]

It is because the CLR uses hash table to optimize the switch segment. And there is no native support for that hash table in NeoVM.

[shargon]

There is not possible to convert this hash table into a jump table?

Pseudocode:

// With hash table
hash= [ 0x0a, 0x0b, 0x0c ] 
JMP(hash[2]);

// Without hash table

0x00 NOP
0x01 JMP 0x0a
0x02 JMP 0x0b
0x03 JMP 0x0c

JMP 0x02

[shargon]

dnSpy decompile the source as:

using System;
using Neo.SmartContract.Framework;

namespace Neo.Compiler.MSIL.TestClasses
{
	// Token: 0x02000017 RID: 23
	internal class Contract_Switch : SmartContract
	{
		// Token: 0x0600005E RID: 94 RVA: 0x00003C78 File Offset: 0x00001E78
		public static object Main(string method, object[] args)
		{
			if (method != null)
			{
				uint num = <PrivateImplementationDetails>.ComputeStringHash(method);
				if (num <= 518729469u)
				{
					if (num <= 434841374u)
					{
						if (num <= 350953279u)
						{
							if (num != 334175660u)
							{
								if (num == 350953279u)
								{
									if (method == "19")
									{
										return 20;
									}
								}
							}
							else if (method == "18")
							{
								return 19;
							}
						}
						else if (num != 401286136u)
						{
							if (num != 418063755u)
							{
								if (num == 434841374u)
								{
									if (method == "16")
									{
										return 17;
									}
								}
							}
							else if (method == "15")
							{
								return 16;
							}
						}
						else if (method == "14")
						{
							return 15;
						}
					}
					else if (num <= 468396612u)
					{
						if (num != 451618993u)
						{
							if (num == 468396612u)
							{
								if (method == "10")
								{
									return 11;
								}
							}
						}
						else if (method == "17")
						{
							return 18;
						}
					}
					else if (num != 485174231u)
					{
						if (num != 501951850u)
						{
							if (num == 518729469u)
							{
								if (method == "13")
								{
									return 14;
								}
							}
						}
						else if (method == "12")
						{
							return 13;
						}
					}
					else if (method == "11")
					{
						return 12;
					}
				}
				else if (num <= 873244444u)
				{
					if (num <= 822911587u)
					{
						if (num != 806133968u)
						{
							if (num == 822911587u)
							{
								if (method == "4")
								{
									return 5;
								}
							}
						}
						else if (method == "5")
						{
							return 6;
						}
					}
					else if (num != 839689206u)
					{
						if (num != 856466825u)
						{
							if (num == 873244444u)
							{
								if (method == "1")
								{
									return 2;
								}
							}
						}
						else if (method == "6")
						{
							return 7;
						}
					}
					else if (method == "7")
					{
						return 8;
					}
				}
				else if (num <= 923577301u)
				{
					if (num != 890022063u)
					{
						if (num != 906799682u)
						{
							if (num == 923577301u)
							{
								if (method == "2")
								{
									return 3;
								}
							}
						}
						else if (method == "3")
						{
							return 4;
						}
					}
					else if (method == "0")
					{
						return 1;
					}
				}
				else if (num != 1007465396u)
				{
					if (num != 1024243015u)
					{
						if (num == 2381486463u)
						{
							if (method == "20")
							{
								return 21;
							}
						}
					}
					else if (method == "8")
					{
						return 9;
					}
				}
				else if (method == "9")
				{
					return 10;
				}
			}
			return 99;
		}
	}
}

[erikzhang]

You need to implement <PrivateImplementationDetails>.ComputeStringHash() in NeoVM/Interop to support switch.

[shargon]

@lightszero do you think that this is feasible?

[lightszero]

@shargon

as eric said. ComputeStringHash is a sad story.

swtich xxx
{
case "aa":
}

I can't got "aa" form IL in current convert solution(current solution is use Mono.cecil to conv IL -> AVM).

One way is add a ComputeStringHash on NEOVM.
Another way for add switch support need to use another convert solution(maybe mix some rosyln part).

I will working on rosyln way,but Use rosyln to got more info need input csproj file instead input dll file.It is a big change.

I had try add a stringhash way
neo-project/neo-vm#15

[erikzhang]

@lightszero We can add ComputeStringHash() on Interop.

[lightszero]

C# 's stringhash is different with java,so we need many stringhash interop.

[shargon]

I understand the problem ... if we receive a compilation error (user friendly), is enough to me

[shargon]

@lightszero could you check why here https://github.com/neo-project/neo-devpack-dotnet/pull/87/files#diff-38aefe612b09c5f19d1ec205b952b39cR19 is not throwed any exception?

[shargon]

And is possible to generate this method on runtime, without a syscall, and just call it?

internal static uint ComputeStringHash(string s)
	{
		uint num;
		if (s != null)
		{
			num = 2166136261u;
			for (int i = 0; i < s.Length; i++)
			{
				num = ((uint)s.get_Chars(i) ^ num) * 16777619u;
			}
		}
		return num;
	}

[shargon]

@lightszero this is something crazy?

Try to detect the call, and redirect to another one loaded in the framework
shargon#2

https://github.com/shargon/neo-devpack-dotnet/pull/2/files#diff-319fbfaba4982e6265df86aadb9e928bR769-R777

[erikzhang]

Calling the generated ComputeStringHash() in the runtime is too expensive.

[shargon]

I agree with you @erikzhang
Changed #87 to detect and improve the exception and finished the unit tests to make it easy for developers to discover why this does not work

[lock9]

@shargon will #87 close this issue?
I just checked #87 and it seems Erik has agreed to implement this. I will move this to design for two days, just to make sure.

[lightszero]

we can add some simple method for switch.
after of all,we can touch the contract c# sourcecode.
If we can't get info from IL,then we got it from sourcecode.
we can got all string in sourcecode.and save all of it.

then we got a Hash in IL,we can knwon the string.

[lightszero]

no need to touch contract c# sourcecode
I study IL switch pattern，now it worked after delete hashpart。

[shargon]

Thanks @lightszero ! amazing work!

[devhawk]

Can we port this to master-2.x?

[shargon]

Yes, I think so

[shargon]

Also, this issue should be closed and create a new one #157