replace STSFLD with PUSH const

[Hecate2]

We have been always doing STSFLD in _initialize method for all static variables. They can be replaced with PUSH consts when the static variables are read. The major changes are in Peephole.cs

Future steps:

• For expensive PUSHDATA instructions, if the pushed static variable is read very often, store it to static field at the beginning of method.

[steven1227]

Nice step

[shargon]

Love it!!!

[shargon]

I have different concerns:

• There are cases where it can be more expensive, some pushes are more expensive. Can we ensure that we choose the cheap one?
• the script will be bigger, we have a limmit script size. If this limit is reached, how can we avoid this mechanism?
• whats happene in this case:

class myClass
{
public int MyVar { get; } = 7 ;
public myClass()
   {
   MyVar = 3;
   }
}

[Hecate2]

    public class MyClass : SmartContract.Framework.SmartContract
    {
        public static int MyVar = 7;
        public MyClass()
        {
            MyVar = 3;
        }
        public int Main() => MyVar;
    }

# Method Start Neo.Compiler.CSharp.TestContracts.MyClass.Main()
00 INITSLOT 00-01 # 0 local variables, 1 arguments
# Code test-compiler.cs line 13: "MyVar"
03 LDSFLD0
# Method End Neo.Compiler.CSharp.TestContracts.MyClass.Main()
04 RET
# Method Start Neo.Compiler.CSharp.TestContracts.MyClass.MyClass()
05 INITSLOT 00-01 # 0 local variables, 1 arguments
08 LDARG0
09 CALL 05 # pos: 14 (offset: 5)
# Code test-compiler.cs line 11: "3"
# Code test-compiler.cs line 11: "MyVar = 3"
11 PUSH3
# Code test-compiler.cs line 11: "MyVar = 3"
# Code test-compiler.cs line 11: "MyVar = 3"
12 STSFLD0
# Method End Neo.Compiler.CSharp.TestContracts.MyClass.MyClass()
# Code test-compiler.cs line 12: "}"
13 RET
14 INITSLOT 00-01 # 0 local variables, 1 arguments
17 RET
18 INITSSLOT 01 # 1 static variables
20 PUSH7
21 STSFLD0
22 RET
23 NEWARRAY0
24 DUP
25 CALL EC # pos: 5 (offset: -20)
27 JMP E5 # pos: 0 (offset: -27)
29 RET

My optimizer is designed to do nothing on a static variable that is written in the main execution. main executes INITSSLOT at offset 18, and starts at offset 23, and returns 3.

[Hecate2]

* There are cases where it can be more expensive, some pushes are more expensive. Can we ensure that we choose the cheap one?

            [OpCode.PUSHINT128] = 1 << 2,
            [OpCode.PUSHINT256] = 1 << 2,
            [OpCode.PUSHDATA1] = 1 << 3,
            [OpCode.PUSHDATA2] = 1 << 9,
            [OpCode.PUSHDATA4] = 1 << 12,

These OpCodes are more expensive than LDSFLD and STSFLD (1 << 1).

For PUSHDATA2 and 4, if they are read twice in a method, it can be more expensive than LDSFLD. I have no immediate solution in a short time, but I have not really seen any contract in production that has to read a single huge piece of static DATA for multiple times. And if such a huge piece of data has to be pushed for every method, other methods become significantly more expensive. Also, developers can write local variables to temporarily store the static variable as a workaround.

For PUSHDATA1, we spend 16+8+2 (INITSSLOT + PUSHDATA1 + STSFLD) to prepare it in the static fields, and get it at a price of 2 (LDSFLD) each time. If we always use PUSHDATA1 when we read it, we can have (16+8+2)/(8-2) = 4.3333 times before it get more expensive than INITSSLOT.

Also we can have (16+4+2)/(4-2) = 11 PUSHINT128s if we do not push it in the _initialize method.

[Jim8y]

I have different concerns:

• There are cases where it can be more expensive, some pushes are more expensive. Can we ensure that we choose the cheap one?
• the script will be bigger, we have a limmit script size. If this limit is reached, how can we avoid this mechanism?
• whats happene in this case:

class myClass
{
public int MyVar { get; } = 7 ;
public myClass()
   {
   MyVar = 3;
   }
}

this can be further optimized, for instance do a script size check.

[Jim8y]

following this path, you can further optimize static fileds that are only updated and used in one method as the local const variable of that method.

[Jim8y]

assign myself @Jim8y a task as updating analyzer to apply relevent check.

[Hecate2]

following this path, you can further optimize static fileds that are only updated in one method as the local const variable of that method. cause when i was imolememting out keyword, i use global static variable to store the out value, many things like this can only be done post compile

Complex optimizations require delicate data flow analysis on the nef. Maybe I need to transform all writes (to eval stack, slot and storage) to static single assignment (SSA) form. I may be able to prepare it in half a year.

[Hecate2]

* There are cases where it can be more expensive, some pushes are more expensive. Can we ensure that we choose the cheap one?

Is it OK that I do not optimize the static fields that are too expensive to push, and are pushed in a loop or for too many times?

[Jim8y]

@shargon

[vncoelho]

There is no coverage for this method.

Also, maybe something like (!operand.HasValue || operand.Value.Span.SequenceEqual(i.Operand.Span))

[Hecate2]

Ok. I did plan to use this method but I found it meaningless later.