JIT: loop interchange optimization

[ghost]

Source: http://stackoverflow.com/a/11303693/863980

From the link above it is evident that the following two methods are logically equal, due to Loop-Invariant code motion:

public static void UnHoisted(ref int[] data, int arraySize)
{
    long sum = 0;

    for (int i = 0; i < 100000; ++i)
    {
        for (int j = 0; j < arraySize; ++j)
        {
            if (data[j] >= 128)
                sum += data[j];
        }
    }
}

and

public static void Hoisted(ref int[] data, int arraySize)
{
    long sum = 0;

    for (int j = 0; j < arraySize; ++j)
    {
        if (data[j] >= 128)
        {
            sum += data[j] * 100000;
        }
    }
}

The driver (Main) method looks like:

public static void Main (string[] args)
{
    const int arraySize = 32768;
    int[] data = new int[arraySize];
    Random random = new Random();

    for (int c = 0; c < arraySize; ++c)
        data[c] = random.Next() % 256;

    UnHoisted(ref data, arraySize);
    // Hoisted(ref data, arraySize);
}

Diffing the produced disasm of UnHoisted and Hoisted methods results in:

diff --git a/c:/temp/UnHoisted.txt b/c:/temp/Hoisted.txt
index 9f25a6b..e4cbf57 100644
--- a/c:/temp/UnHoisted.txt
+++ b/c:/temp/Hoisted.txt
@@ -1,79 +1,66 @@
-; Assembly listing for method Program:UnHoisted(byref,int)
+; Assembly listing for method Program:Hoisted(byref,int)
 ; Emitting BLENDED_CODE for X64 CPU with AVX
 ; optimized code
 ; rsp based frame
 ; fully interruptible
 ; Final local variable assignments
 ;
-;  V00 arg0         [V00,T04] (  4,  18  )   byref  ->  rcx        
-;  V01 arg1         [V01,T05] (  3,  18  )     int  ->  rdx        
-;  V02 loc0         [V02,T06] (  3,  17  )    long  ->  rax        
-;  V03 loc1         [V03,T07] (  4,  13  )     int  ->   r8        
-;  V04 loc2         [V04,T00] (  8,  66  )     int  ->  r10        
-;  V05 loc3         [V05,T01] (  6,  64  )    bool  ->   r9        
-;  V06 tmp0         [V06,T02] (  3,  48  )     ref  ->   r9        
-;  V07 tmp1         [V07,T03] (  3,  48  )     ref  ->   r9        
-;  V08 OutArgs      [V08    ] (  1,   1  )  lclBlk (32) [rsp+0x00]  
+;  V00 arg0         [V00,T05] (  4,   6  )   byref  ->  rcx        
+;  V01 arg1         [V01,T04] (  3,  10  )     int  ->  rdx        
+;  V02 loc0         [V02,T06] (  3,   5  )    long  ->  rax        
+;  V03 loc1         [V03,T00] (  8,  21  )     int  ->   r8        
+;  V04 loc2         [V04,T01] (  4,  20  )    bool  ->   r9        
+;  V05 tmp0         [V05,T02] (  3,  12  )     ref  ->   r9        
+;  V06 tmp1         [V06,T03] (  3,  12  )     ref  ->   r9        
+;  V07 OutArgs      [V07    ] (  1,   1  )  lclBlk (32) [rsp+0x00]  
 ;
 ; Lcl frame size = 40

-G_M4700_IG01:
+G_M9639_IG01:
        4883EC28             sub      rsp, 40

-G_M4700_IG02:
+G_M9639_IG02:
        33C0                 xor      rax, rax
        4533C0               xor      r8d, r8d
-       EB5D                 jmp      SHORT G_M4700_IG07
+       EB49                 jmp      SHORT G_M9639_IG05

-G_M4700_IG03:
-       4533D2               xor      r10d, r10d
-       EB45                 jmp      SHORT G_M4700_IG06
-
-G_M4700_IG04:
+G_M9639_IG03:
        4C8B09               mov      r9, gword ptr [rcx]
-       458B5908             mov      r11d, dword ptr [r9+8]
-       453BD3               cmp      r10d, r11d
-       7365                 jae      SHORT G_M4700_IG09
-       4D63DA               movsxd   r11d, r10d
-       43817C991080000000   cmp      dword ptr [r9+4*r11+16], 128
+       458B5108             mov      r10d, dword ptr [r9+8]
+       453BC2               cmp      r8d, r10d
+       7352                 jae      SHORT G_M9639_IG07
+       4D63D0               movsxd   r10d, r8d
+       43817C911080000000   cmp      dword ptr [r9+4*r10+16], 128
        410F9CC1             setl     r9b
        450FB6C9             movzx    r9, r9b
        4585C9               test     r9d, r9d
-       751D                 jne      SHORT G_M4700_IG05
+       7521                 jne      SHORT G_M9639_IG04
        4C8B09               mov      r9, gword ptr [rcx]
-       458B5908             mov      r11d, dword ptr [r9+8]
-       453BD3               cmp      r10d, r11d
-       7340                 jae      SHORT G_M4700_IG09
-       4D63DA               movsxd   r11d, r10d
-       478B4C9910           mov      r9d, dword ptr [r9+4*r11+16]
+       458B5108             mov      r10d, dword ptr [r9+8]
+       453BC2               cmp      r8d, r10d
+       732D                 jae      SHORT G_M9639_IG07
+       4D63D0               movsxd   r10d, r8d
+       47694C9110A0860100   imul     r9d, dword ptr [r9+4*r10+16], 0x186A0
        4D63C9               movsxd   r9d, r9d
        4903C1               add      rax, r9

-G_M4700_IG05:
-       41FFC2               inc      r10d
-
-G_M4700_IG06:
-       443BD2               cmp      r10d, edx
-       410F9CC1             setl     r9b
-       450FB6C9             movzx    r9, r9b
-       4585C9               test     r9d, r9d
-       75AE                 jne      SHORT G_M4700_IG04
+G_M9639_IG04:
        41FFC0               inc      r8d

-G_M4700_IG07:
-       4181F8A0860100       cmp      r8d, 0x186A0
+G_M9639_IG05:
+       443BC2               cmp      r8d, edx
        410F9CC1             setl     r9b
        450FB6C9             movzx    r9, r9b
        4585C9               test     r9d, r9d
-       7592                 jne      SHORT G_M4700_IG03
+       75AA                 jne      SHORT G_M9639_IG03

-G_M4700_IG08:
+G_M9639_IG06:
        4883C428             add      rsp, 40
        C3                   ret      

-G_M4700_IG09:
-       E8D152265F           call     CORINFO_HELP_RNGCHKFAIL
+G_M9639_IG07:
+       E8E952245F           call     CORINFO_HELP_RNGCHKFAIL
        CC                   int3     

-; Total bytes of code 132, prolog size 4 for method Program:UnHoisted(byref,int)
+; Total bytes of code 108, prolog size 4 for method Program:Hoisted(byref,int)
 ; ============================================================

The execution time of UnHoisted() is much higher than that of Hoisted().

Do we have a starting point for this kind of optimization; such as some basic heuristics in place which needs to be enhanced or would it require an implementation from ground-up?

Cc @mikedn, @omariom, @cmckinsey

category:cq
theme:loop-opt
skill-level:expert
cost:extra-large

[mikedn]

Have you seen a compiler which implements the optimization described in that StackOverflow post? 😄

@mikedn, ouch! like the Japanese spirit says, "if nobody has done it, I must do it" ㊗️

[Unwarranted] I think while Intel's compiler is good at loop-interchange optimization, it is also leading in LIH arena, but I am not sure what kind of areas are fuzzy and most challenging (research-wise).

[mikedn]

Yes, Intel's compiler does this optimization, all others vectorize the inner loop and don't do anything else interesting. The interesting thing about the Intel compiler is that it does it only with /O2, not with /O1. That may imply that the compiler wasn't specifically doing this optimization but that the result was the fortunate result of other optimizations. Loop interchange requires data dependence analysis and the compiler might have done that for autovectorization purposes.

[OtherCrashOverride]

I don't think this is a good fit for CoreCLR. The runtime's job is to take an intermediate representation (IR) and make it target specific as quickly as possible: JIT = Just In Time (compilation). Additionally, the IR has no concept of loops or other structured programming techniques. It only understands branches (goto). Combine this with the runtime's goal of supporting multiple languages and it makes it very difficult to determine what is a candidate for further optimization and what should not be touched due to side effects that may occur. In the example given above, it is entire possible that another thread is updating the array (from a JIT point of view) and so the order of operation is important.

Given the above, this type of optimization is better suited towards Roslyn, etc. Additionally, compiler optimizations should not be viewed as a substitute for human programming skill.

[TPSResearch]

I completely disagree with OtherCrashOverride. In the 90's, developers were instructed to not try to outsmart the compiler, just write readable code and let the optimizer take care of the details of making it fast. Now, we appear to be back to hand-optimizing code because the JIT'er isn't smart enough. Also, the argument that another thread might modify the the array is flawed. Optimizing compilers simply don't work that way...If they did they could never do "loop fusion" for example.

Finally, the idea of "JIT = Just In Time (compilation)" is, IMHO, the biggest problem. I'd bet that the majority of code in use today runs on servers and does not care how long it takes to compile. Result: the mandate is to have a "sub-optimizing" compiler.

[masonwheeler]

Agreeing with @TPSResearch. We've always known that JITs are kind of a dumb idea, and this is just another reiteration of that principle. Yes, there's a place for them, particularly in debugging and testing when the goal is to iterate as quickly as possible, but for deployment of a working product, a good AOT compiler will beat a good JIT hands down, every time.

An optimization like this is probably not suitable to the JIT, but it should be shunted over towards LLILC, who has the development of an AOT compiler as their long-term goal.

[benpye]

If you are looking to get this in LLILC then wouldn't it really be LLVM where the optimization should be performed?

[OtherCrashOverride]

In the 90's, developers were instructed to not try to outsmart the compiler, just write readable code and let the optimizer take care of the details of making it fast. Now, we appear to be back to hand-optimizing code because the JIT'er isn't smart enough.

There will always be a difference in code written for clarity and maintenance and that written for performance.

The main thing that has changed since the 90's is first class Ahead-Of-Time compilation (AOT) support since Windows on ARM does not allow execution of JIT code (W^X). AOT is where we can spend as much time as needed performing deep optimizations in an off-line processes. Its important that the JIT do its job as fast as possible: a server environment may not care; use cases outside of that will. CoreCLR serves many domains outside of servers and languages outside of C#.

I don't think anyone is under the impression that this is a trivial optimization. Should those that write performant code be penalized by a slower JIT that will not find anything to optimize? With AOT the penalty is a O(1) cost, however with JIT its an O(n) cost.

In the end what is being discussed is outside the scope of JIT. It is modifications to IL, not compilation of it. Someone should put forth a pull request of the optimization feature so that it can be tested to answer how much of a degradation it will have on JIT time and also to show real world performance increases that can be expected. Its entirely possible that it will have no statistically meaningful benefit to majority of existing, real world, code.

[Eyas]

Right. The takeaway here that applies to many JIT optimization issues are:

1. Keep in mind this is a JIT compiler. Many complex optimizations incur a runtime cost that is not worthwhile in many cases. This is actually how RyuJIT came to replace the 64-bit JITter. AOT is coming to C# in the form of .NET Native. When there is an appropriate forum to do so, some of these optimization issues should move from CoreCLR to there.
2. One easy heuristic about when a proposed optimization is certainly out of the scope of a JITter: when very few AOT optimizing compilers actually perform that optimization 😄

[masonwheeler]

@Eyas If "AOT is coming to C# in the form of .NET Native," that really doesn't do much for people using other CLR languages, because it doesn't appear that they're open-sourcing the .NET Native toolchain. So that's kind of an unhelpful observation.

[OtherCrashOverride]

when very few AOT optimizing compilers actually perform that optimization

I believe this type of optimization is outside the scope of both JIT and AOT. This is something for Roslyn. JIT and AOT deal with IL which is analogous to dealing with processor assembly code. Compilers typically optimize at a higher level. Optimizing in Roslyn is analogous to optimizing in a C/C++ compiler. It makes more sense because it has a better view and understanding of what is going on with the code. However, I think this too is probably better served by a compiler warning, Visual Studio 'lightbulb' suggestion, or an offline tool rather than forcing it on everyone. The author of the code should be made aware and also giving the option to alter the source code or ignore it. The option to ignore it becomes important when there is a purposeful order of operation due to the algorithm being multi-threaded.

When there is an appropriate forum to do so, some of these optimization issues should move from CoreCLR to there.

Meanwhile should this be a candidate of Roslyn for IL generation by csc and vbc (where other optimizations are taking place today and where we can afford to spend time for deep analysis)?

@jaredpar, can you please suggest; ideally what should be the plan of attack here?

[leppie]

Easy test, can you prove the array is not being mutated?

[masonwheeler]

@jasonwilliams200OK @OtherCrashOverride The problem with saying "oh, just do it in the front-end compiler" is that, again, there are a lot more than just one (or even two) CLR languages! "Just do it in the compiler" means we're back to the bad old days when every compiler needs to reinvent every wheel every time. Wasn't getting away from that the entire point of having a common language infrastructure in the first place?

[OtherCrashOverride]

Its probably worth mentioning that this is something the LLVM back-end (dotnet/llilc) can provide:
http://llvm.org/docs/Passes.html#licm-loop-invariant-code-motion

Mono has implemented LLVM backend support already. They also retain classic JIT due to the fact that LLVM based JIT adds considerable time to the JIT process which is undesirable to some customers.