JIT should use boxing helper to box structs with many GC pointers

[MichalStrehovsky]

Unoptimized code is faster than optimized code: #101605 (comment)

#101605 (comment)

Prototype with some extra discussion at #101669.

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[EgorBo]

Marking as Future as we already have a lot of issues for 9.0, but I think I can try to land it this week

[EgorBo]

Working prototype (CoreCLR): EgorBo@1c9f3a4

[EgorBo]

@jkotas what would be the best way to handle possible gc starvation for huge structs - can we handle that on VM's side inside the JIT_StructWriteBarrier call?

[jkotas]

• The JIT helper implementation should look like Buffer::BulkMoveWithWriteBarrier. It polls for GC among other things.
• Null reference exceptions for null source/destination may need an extra care.
• The bulk copy is great when the target is in recently allocated object - unlikely to introduce extra work for GC. It can be mixed bag for arbitrary assignments - it may introduce extra work for GC. I think we can give it a try and see whether it shows up anywhere. (cc @dotnet/gc).

[huoyaoyuan]

I have some questions about further optimizing GC pointer copying:

Currently the actual copy is done in FCall so that GC can only happen at chunk boundaries. Large copy are split in to chunks and implicit GC poll happens when transiting between managed and FCall every time.

The actual GCSafeCopyHelpers are implemented in native code with SSE enabled only (_mm_loadu_ps/_mm_store_ps) since it's in baseline instruction set. Can it achieve significantly higher throughput if other SIMD instruction sets are used (AVX2/512 and ARM intrinsics)? How can the instruction set be dynamically selected, by special managed code block that runs in cooperated mode, or by choosing native stubs dynamically?

[EgorBo]

and ARM intrinsics

I'd expect NEON to kick in here implicitly as is.

Can it achieve significantly higher throughput if other SIMD instruction sets are used

Probably? Depends on how often we see large inputs here, we, probably, do see them when we do various Array/Span CopyTo, but in case of the struct copy discussed here - unlikely (a struct must be really big to see impact from AVX).

How can the instruction set be dynamically selected, by special managed code block that runs in cooperated mode, or by choosing native stubs dynamically?

I'd guess that the simplest option would be to have a dynamic ISA check in the existing code? Just like we e.g. check for atomics on arm64 in C++ code. Obviously, it means + one more condition overhead for small inputs

PS: one imprortant thing we have to keep in mind is the atomicity guarantees, e.g. SIMD loads/store do have them when input is 16 byte aligned (and the alignment is not changed) on x86/64

[EgorBo]

hm, I finished my prototype but for some reason I am not seeing the expected perf improvements compared to raw box helper (on Windows-x64) 🤔

GcStruct9 s9;

[Benchmark] 
public object BoxS9() => s9;

record struct GcStruct9(
    string a1, string a2, string a3, 
    string a4, string a5, string a6,
    string a7, string a8, string a9);

Codegen in Main:

; Method MyBench:BoxS9():System.Object:this (FullOpts)
G_M000_IG01:
       push     rdi
       push     rsi
       push     rbx
       sub      rsp, 32
       mov      rsi, rcx

G_M000_IG02:
       mov      rcx, 0x7FFBDD35BE20
       call     CORINFO_HELP_NEWSFAST
       mov      rbx, rax
       add      rsi, 40
       lea      rdi, bword ptr [rbx+0x08]
       call     CORINFO_HELP_ASSIGN_BYREF
       call     CORINFO_HELP_ASSIGN_BYREF
       call     CORINFO_HELP_ASSIGN_BYREF
       call     CORINFO_HELP_ASSIGN_BYREF
       call     CORINFO_HELP_ASSIGN_BYREF
       call     CORINFO_HELP_ASSIGN_BYREF
       call     CORINFO_HELP_ASSIGN_BYREF
       call     CORINFO_HELP_ASSIGN_BYREF
       call     CORINFO_HELP_ASSIGN_BYREF
       mov      rax, rbx

G_M000_IG03:
       add      rsp, 32
       pop      rbx
       pop      rsi
       pop      rdi
       ret      
; Total bytes of code: 92

Codegen in the prototype:

; Method MyBench:BoxS9():System.Object:this (FullOpts)
G_M5744_IG01:
       push     rsi
       push     rbx
       sub      rsp, 40
       mov      rbx, rcx

G_M5744_IG02:
       mov      rcx, 0x7FFB87E5B0C8      ; GcStruct9
       call     CORINFO_HELP_NEWSFAST
       mov      rsi, rax
       lea      rcx, bword ptr [rsi+0x08]
       lea      rdx, bword ptr [rbx+0x28]
       mov      r8d, 72
       call     CORINFO_HELP_ASSIGN_STRUCT
       mov      rax, rsi

G_M5744_IG03:
       add      rsp, 40
       pop      rbx
       pop      rsi
       ret      
; Total bytes of code: 56

Main: 13.5 ns
PR: 12.5 ns
BoxHelper: 6.5 ns

At first, I thought that the box helper is faster because it knows that the pointers never overlap, but it seems to be calling the same routine as my prototype...

[jkotas]

You have not done this part: The JIT helper implementation should look like Buffer::BulkMoveWithWriteBarrier. It polls for GC among other things.

[huoyaoyuan]

a struct must be really big to see impact from AVX

How about inline array?

I did an experiment about using AVX in copy, and disabling SSE.
Touching arraynative.inl requires a full rebuild for me, which wastes hours.

Benchmark code:

        [Params(1024, 200000, 4000000)]
        public int ArraySize { get; set; }

        [GlobalSetup]
        public void Setup()
        {
            src_str = Enumerable.Range(1, ArraySize).Select(i => $"abcd{i}").ToArray();
            src_str_unordered = [.. src_str];
            new Random(20240430).Shuffle(src_str_unordered);
            dst_str = new string[ArraySize];
            src_nint = Enumerable.Range(1, ArraySize).Select(i => (nint)i).ToArray();
            dst_nint = new nint[ArraySize];
        }

        private string[] src_str;
        private string[] src_str_unordered;
        private string[] dst_str;
        private nint[] src_nint;
        private nint[] dst_nint;

        [Benchmark]
        public string[] StringArray()
        {
            src_str.CopyTo(dst_str, 0);
            return dst_str;
        }

        [Benchmark]
        public string[] StringArray_Unordered()
        {
            src_str_unordered.CopyTo(dst_str, 0);
            return dst_str;
        }

        [Benchmark]
        public nint[] NintArray()
        {
            src_nint.CopyTo(dst_nint, 0);
            return dst_nint;
        }
    }

Results:

Method	Job	Toolchain	ArraySize	Mean	Error	StdDev	Ratio	RatioSD
StringArray	Job-NTZEWB	\AVX256\corerun.exe	1024	46.51 ns	0.221 ns	0.206 ns	0.60	0.00
StringArray	Job-GRYOMD	\NoSSE\corerun.exe	1024	108.67 ns	0.414 ns	0.387 ns	1.40	0.01
StringArray	Job-IGDLAQ	\main\corerun.exe	1024	77.41 ns	0.277 ns	0.231 ns	1.00	0.00
StringArray_Unordered	Job-NTZEWB	\AVX256\corerun.exe	1024	50.76 ns	0.290 ns	0.272 ns	0.65	0.01
StringArray_Unordered	Job-GRYOMD	\NoSSE\corerun.exe	1024	111.31 ns	0.627 ns	0.586 ns	1.43	0.01
StringArray_Unordered	Job-IGDLAQ	\main\corerun.exe	1024	77.82 ns	0.534 ns	0.474 ns	1.00	0.00
NintArray	Job-NTZEWB	\AVX256\corerun.exe	1024	44.01 ns	0.164 ns	0.153 ns	1.01	0.00
NintArray	Job-GRYOMD	\NoSSE\corerun.exe	1024	43.83 ns	0.141 ns	0.125 ns	1.00	0.01
NintArray	Job-IGDLAQ	\main\corerun.exe	1024	43.62 ns	0.207 ns	0.194 ns	1.00	0.00
StringArray	Job-NTZEWB	\AVX256\corerun.exe	200000	44,634.81 ns	248.081 ns	219.917 ns	1.03	0.01
StringArray	Job-GRYOMD	\NoSSE\corerun.exe	200000	47,465.84 ns	213.360 ns	178.165 ns	1.09	0.00
StringArray	Job-IGDLAQ	\main\corerun.exe	200000	43,425.92 ns	207.534 ns	194.127 ns	1.00	0.00
StringArray_Unordered	Job-NTZEWB	\AVX256\corerun.exe	200000	46,459.96 ns	353.088 ns	330.278 ns	1.03	0.01
StringArray_Unordered	Job-GRYOMD	\NoSSE\corerun.exe	200000	46,701.84 ns	301.490 ns	282.014 ns	1.03	0.01
StringArray_Unordered	Job-IGDLAQ	\main\corerun.exe	200000	45,213.14 ns	228.236 ns	202.325 ns	1.00	0.00
NintArray	Job-NTZEWB	\AVX256\corerun.exe	200000	35,568.12 ns	704.679 ns	811.509 ns	1.01	0.02
NintArray	Job-GRYOMD	\NoSSE\corerun.exe	200000	34,635.92 ns	686.125 ns	641.801 ns	0.98	0.02
NintArray	Job-IGDLAQ	\main\corerun.exe	200000	35,296.74 ns	684.155 ns	702.577 ns	1.00	0.00
StringArray	Job-NTZEWB	\AVX256\corerun.exe	4000000	2,067,140.31 ns	41,252.291 ns	67,778.698 ns	1.00	0.05
StringArray	Job-GRYOMD	\NoSSE\corerun.exe	4000000	2,309,220.21 ns	44,077.554 ns	50,759.777 ns	1.11	0.05
StringArray	Job-IGDLAQ	\main\corerun.exe	4000000	2,078,519.09 ns	37,302.752 ns	62,324.518 ns	1.00	0.00
StringArray_Unordered	Job-NTZEWB	\AVX256\corerun.exe	4000000	2,108,473.91 ns	41,784.542 ns	69,812.582 ns	1.01	0.04
StringArray_Unordered	Job-GRYOMD	\NoSSE\corerun.exe	4000000	2,324,801.37 ns	45,282.636 ns	64,943.004 ns	1.11	0.02
StringArray_Unordered	Job-IGDLAQ	\main\corerun.exe	4000000	2,109,031.42 ns	41,459.311 ns	44,360.998 ns	1.00	0.00
NintArray	Job-NTZEWB	\AVX256\corerun.exe	4000000	1,317,257.37 ns	26,272.227 ns	70,125.893 ns	1.00	0.07
NintArray	Job-GRYOMD	\NoSSE\corerun.exe	4000000	1,314,460.98 ns	26,219.591 ns	76,483.778 ns	0.99	0.06
NintArray	Job-IGDLAQ	\main\corerun.exe	4000000	1,324,209.74 ns	26,438.074 ns	71,476.872 ns	1.00	0.00

It seems that using longer vector size has more impact on relatively smaller size. Huge size are more limited by CPU cache I think?