[RyuJit] Stackspace reservation is for pre-optimized code

[benaadams]

Optimized code reserves stack space for pre-optimized stack

(See also https://github.com/dotnet/coreclr/issues/9068 which I imagine should produce identical output; but produces very different output)

e.g. The following code reserves 1000 bytes of stack even though it only uses registers

; Lcl frame size = 1000

G_M27581_IG01:
       4881ECE8030000       sub      rsp, 0x3E8 // 1000 bytes

Code

[MethodImpl(MethodImplOptions.NoInlining)]
private static byte OneSpan(byte[] array)
{
    var span = new Span<byte>(array);
    span = span.Slice(1);
    span = span.Slice(1);
    span = span.Slice(1);
    span = span.Slice(1);
    span = span.Slice(1);
    span = span.Slice(1);
    span = span.Slice(1);
    span = span.Slice(1);
    span = span.Slice(1);
    span = span.Slice(1);
    span = span.Slice(1);
    span = span.Slice(1);
    span = span.Slice(1);
    span = span.Slice(1);
    span = span.Slice(1);
    span = span.Slice(1);
    span = span.Slice(1);
    span = span.Slice(1);
    span = span.Slice(1);
    span = span.Slice(1);
    span = span.Slice(1);
    span = span.Slice(1);
    span = span.Slice(1);
    span = span.Slice(1);
    span = span.Slice(1);
    span = span.Slice(1);
    span = span.Slice(1);
    span = span.Slice(1);
    span = span.Slice(1);
    span = span.Slice(1);
    return span[0];
}

Asm produced

;
; Lcl frame size = 1000

G_M27581_IG01:
       4881ECE8030000       sub      rsp, 0x3E8

G_M27581_IG02:
       4885C9               test     rcx, rcx
       0F8468010000         je       G_M27581_IG36

G_M27581_IG03:
       8B4108               mov      eax, dword ptr [rcx+8]
       83F801               cmp      eax, 1
       0F8263010000         jb       G_M27581_IG37

G_M27581_IG04:
       FFC8                 dec      eax
       83F801               cmp      eax, 1
       0F8262010000         jb       G_M27581_IG38

G_M27581_IG05:
       FFC8                 dec      eax
       83F801               cmp      eax, 1
       0F8261010000         jb       G_M27581_IG39

G_M27581_IG06:
       FFC8                 dec      eax
       83F801               cmp      eax, 1
       0F8260010000         jb       G_M27581_IG40

G_M27581_IG07:
       FFC8                 dec      eax
       83F801               cmp      eax, 1
       0F825F010000         jb       G_M27581_IG41

G_M27581_IG08:
       FFC8                 dec      eax
       83F801               cmp      eax, 1
       0F825E010000         jb       G_M27581_IG42

G_M27581_IG09:
       FFC8                 dec      eax
       83F801               cmp      eax, 1
       0F825D010000         jb       G_M27581_IG43

G_M27581_IG10:
       FFC8                 dec      eax
       83F801               cmp      eax, 1
       0F825C010000         jb       G_M27581_IG44

G_M27581_IG11:
       FFC8                 dec      eax
       83F801               cmp      eax, 1
       0F825B010000         jb       G_M27581_IG45

G_M27581_IG12:
       FFC8                 dec      eax
       83F801               cmp      eax, 1
       0F825A010000         jb       G_M27581_IG46

G_M27581_IG13:
       FFC8                 dec      eax
       83F801               cmp      eax, 1
       0F8259010000         jb       G_M27581_IG47

G_M27581_IG14:
       FFC8                 dec      eax
       83F801               cmp      eax, 1
       0F8258010000         jb       G_M27581_IG48

G_M27581_IG15:
       FFC8                 dec      eax
       83F801               cmp      eax, 1
       0F8257010000         jb       G_M27581_IG49

G_M27581_IG16:
       FFC8                 dec      eax
       83F801               cmp      eax, 1
       0F8256010000         jb       G_M27581_IG50

G_M27581_IG17:
       FFC8                 dec      eax
       83F801               cmp      eax, 1
       0F8255010000         jb       G_M27581_IG51

G_M27581_IG18:
       FFC8                 dec      eax
       83F801               cmp      eax, 1
       0F8254010000         jb       G_M27581_IG52

G_M27581_IG19:
       FFC8                 dec      eax
       83F801               cmp      eax, 1
       0F8253010000         jb       G_M27581_IG53

G_M27581_IG20:
       FFC8                 dec      eax
       83F801               cmp      eax, 1
       0F8252010000         jb       G_M27581_IG54

G_M27581_IG21:
       FFC8                 dec      eax
       83F801               cmp      eax, 1
       0F8251010000         jb       G_M27581_IG55

G_M27581_IG22:
       FFC8                 dec      eax
       83F801               cmp      eax, 1
       0F8250010000         jb       G_M27581_IG56

G_M27581_IG23:
       FFC8                 dec      eax
       83F801               cmp      eax, 1
       0F824F010000         jb       G_M27581_IG57

G_M27581_IG24:
       FFC8                 dec      eax
       83F801               cmp      eax, 1
       0F824E010000         jb       G_M27581_IG58

G_M27581_IG25:
       FFC8                 dec      eax
       83F801               cmp      eax, 1
       0F824D010000         jb       G_M27581_IG59

G_M27581_IG26:
       FFC8                 dec      eax
       83F801               cmp      eax, 1
       0F824C010000         jb       G_M27581_IG60

G_M27581_IG27:
       FFC8                 dec      eax
       83F801               cmp      eax, 1
       0F824B010000         jb       G_M27581_IG61

G_M27581_IG28:
       FFC8                 dec      eax
       83F801               cmp      eax, 1
       0F824A010000         jb       G_M27581_IG62

G_M27581_IG29:
       FFC8                 dec      eax
       83F801               cmp      eax, 1
       0F8249010000         jb       G_M27581_IG63

G_M27581_IG30:
       FFC8                 dec      eax
       83F801               cmp      eax, 1
       0F8248010000         jb       G_M27581_IG64

G_M27581_IG31:
       FFC8                 dec      eax
       83F801               cmp      eax, 1
       0F8247010000         jb       G_M27581_IG65

G_M27581_IG32:
       FFC8                 dec      eax
       83F801               cmp      eax, 1
       0F8246010000         jb       G_M27581_IG66

G_M27581_IG33:
       FFC8                 dec      eax
       85C0                 test     eax, eax
       0F8646010000         jbe      G_M27581_IG67

G_M27581_IG34:
       488D4108             lea      rax, bword ptr [rcx+8]
       4883C026             add      rax, 38
       0FB600               movzx    rax, byte  ptr [rax]

G_M27581_IG35:
       4881C4E8030000       add      rsp, 0x3E8
       C3                   ret      

G_M27581_IG36:
       33C9                 xor      ecx, ecx
       E871F0FFFF           call     ThrowHelper:ThrowArgumentNullException(int)

G_M27581_IG37:
       B902000000           mov      ecx, 2
       E8B7F0FFFF           call     ThrowHelper:ThrowArgumentOutOfRangeException(int)

G_M27581_IG38:
       B902000000           mov      ecx, 2
       E8ADF0FFFF           call     ThrowHelper:ThrowArgumentOutOfRangeException(int)

G_M27581_IG39:
       B902000000           mov      ecx, 2
       E8A3F0FFFF           call     ThrowHelper:ThrowArgumentOutOfRangeException(int)

G_M27581_IG40:
       B902000000           mov      ecx, 2
       E899F0FFFF           call     ThrowHelper:ThrowArgumentOutOfRangeException(int)

G_M27581_IG41:
       B902000000           mov      ecx, 2
       E88FF0FFFF           call     ThrowHelper:ThrowArgumentOutOfRangeException(int)

G_M27581_IG42:
       B902000000           mov      ecx, 2
       E885F0FFFF           call     ThrowHelper:ThrowArgumentOutOfRangeException(int)

G_M27581_IG43:
       B902000000           mov      ecx, 2
       E87BF0FFFF           call     ThrowHelper:ThrowArgumentOutOfRangeException(int)

G_M27581_IG44:
       B902000000           mov      ecx, 2
       E871F0FFFF           call     ThrowHelper:ThrowArgumentOutOfRangeException(int)

G_M27581_IG45:
       B902000000           mov      ecx, 2
       E867F0FFFF           call     ThrowHelper:ThrowArgumentOutOfRangeException(int)

G_M27581_IG46:
       B902000000           mov      ecx, 2
       E85DF0FFFF           call     ThrowHelper:ThrowArgumentOutOfRangeException(int)

G_M27581_IG47:
       B902000000           mov      ecx, 2
       E853F0FFFF           call     ThrowHelper:ThrowArgumentOutOfRangeException(int)

G_M27581_IG48:
       B902000000           mov      ecx, 2
       E849F0FFFF           call     ThrowHelper:ThrowArgumentOutOfRangeException(int)

G_M27581_IG49:
       B902000000           mov      ecx, 2
       E83FF0FFFF           call     ThrowHelper:ThrowArgumentOutOfRangeException(int)

G_M27581_IG50:
       B902000000           mov      ecx, 2
       E835F0FFFF           call     ThrowHelper:ThrowArgumentOutOfRangeException(int)

G_M27581_IG51:
       B902000000           mov      ecx, 2
       E82BF0FFFF           call     ThrowHelper:ThrowArgumentOutOfRangeException(int)

G_M27581_IG52:
       B902000000           mov      ecx, 2
       E821F0FFFF           call     ThrowHelper:ThrowArgumentOutOfRangeException(int)

G_M27581_IG53:
       B902000000           mov      ecx, 2
       E817F0FFFF           call     ThrowHelper:ThrowArgumentOutOfRangeException(int)

G_M27581_IG54:
       B902000000           mov      ecx, 2
       E80DF0FFFF           call     ThrowHelper:ThrowArgumentOutOfRangeException(int)

G_M27581_IG55:
       B902000000           mov      ecx, 2
       E803F0FFFF           call     ThrowHelper:ThrowArgumentOutOfRangeException(int)

G_M27581_IG56:
       B902000000           mov      ecx, 2
       E8F9EFFFFF           call     ThrowHelper:ThrowArgumentOutOfRangeException(int)

G_M27581_IG57:
       B902000000           mov      ecx, 2
       E8EFEFFFFF           call     ThrowHelper:ThrowArgumentOutOfRangeException(int)

G_M27581_IG58:
       B902000000           mov      ecx, 2
       E8E5EFFFFF           call     ThrowHelper:ThrowArgumentOutOfRangeException(int)

G_M27581_IG59:
       B902000000           mov      ecx, 2
       E8DBEFFFFF           call     ThrowHelper:ThrowArgumentOutOfRangeException(int)

G_M27581_IG60:
       B902000000           mov      ecx, 2
       E8D1EFFFFF           call     ThrowHelper:ThrowArgumentOutOfRangeException(int)

G_M27581_IG61:
       B902000000           mov      ecx, 2
       E8C7EFFFFF           call     ThrowHelper:ThrowArgumentOutOfRangeException(int)

G_M27581_IG62:
       B902000000           mov      ecx, 2
       E8BDEFFFFF           call     ThrowHelper:ThrowArgumentOutOfRangeException(int)

G_M27581_IG63:
       B902000000           mov      ecx, 2
       E8B3EFFFFF           call     ThrowHelper:ThrowArgumentOutOfRangeException(int)

G_M27581_IG64:
       B902000000           mov      ecx, 2
       E8A9EFFFFF           call     ThrowHelper:ThrowArgumentOutOfRangeException(int)

G_M27581_IG65:
       B902000000           mov      ecx, 2
       E89FEFFFFF           call     ThrowHelper:ThrowArgumentOutOfRangeException(int)

G_M27581_IG66:
       B902000000           mov      ecx, 2
       E895EFFFFF           call     ThrowHelper:ThrowArgumentOutOfRangeException(int)

G_M27581_IG67:
       E880EFFFFF           call     ThrowHelper:ThrowIndexOutOfRangeException()
       CC                   int3     

; Total bytes of code 689, prolog size 7 for method Program:OneSpan(ref):ubyte

category:cq
theme:register-allocator
skill-level:expert
cost:medium

[gkhanna79]

CC @RussKeldorph

[RussKeldorph]

@dotnet/jit-contrib

[AndyAyersMS]

Considering for 2.1.

[AndyAyersMS]

This doesn't seem to repro anymore (from CHK jit in REL build):

; Assembly listing for method T:OneSpan(ref):ubyte
; Lcl frame size = 40

G_M24727_IG01:
       4883EC28             sub      rsp, 40

G_M24727_IG02:
       4885C9               test     rcx, rcx
       0F84BC010000         je       G_M24727_IG35

Frame size of 40 is what one would expect from a non-leaf with no locals (8 bytes for align, 32 for out arg area).

Not sure what changed things, maybe struct promotion kicking in now where it didn't before?

[benaadams]

2 field Span vs 3 field Span? Was before the runtime has the two field Span so was using the three field Span. Will see if can repo

[benaadams]

Still does it using a three field "SpanLike" StackSpace.cs though interestingly dotnet/coreclr#9068 and dotnet/coreclr#9066 have switched output StackSpace.asm

[benaadams]

This sample now does a bunch of redundant movs

byte OneSpantype(byte[] array) (in StackSpace.cs/StackSpace.asm)

; Lcl frame size = 40

G_M5322_IG01:
       57                   push     rdi
       56                   push     rsi
       4883EC28             sub      rsp, 40

G_M5322_IG02:
       4885C9               test     rcx, rcx
       0F84AD020000         je       G_M5322_IG36

G_M5322_IG03:
       8B7108               mov      esi, dword ptr [rcx+8]
       488BF9               mov      rdi, rcx
       48B9E852780EF97F0000 mov      rcx, 0x7FF90E7852E8
       33D2                 xor      edx, edx
       E88AFAAD5F           call     CORINFO_HELP_CLASSINIT_SHARED_DYNAMICCLASS
       488B05637AEEFF       mov      rax, qword ptr [reloc classVar[0xe788360]]
       83FE01               cmp      esi, 1
       0F828D020000         jb       G_M5322_IG37

G_M5322_IG04:
       48FFC0               inc      rax
       8D4EFF               lea      ecx, [rsi-1]
       8BF1                 mov      esi, ecx
       8BCE                 mov      ecx, esi
       8BF1                 mov      esi, ecx
       83FE01               cmp      esi, 1
       0F8282020000         jb       G_M5322_IG38

G_M5322_IG05:
       48FFC0               inc      rax
       8D4EFF               lea      ecx, [rsi-1]
       8BF1                 mov      esi, ecx
       8BCE                 mov      ecx, esi
       8BF1                 mov      esi, ecx
       83FE01               cmp      esi, 1
       0F8277020000         jb       G_M5322_IG39

[AndyAyersMS]

The register shuffling could be something similar to dotnet/coreclr#11390.

[AndyAyersMS]

Just to make current status clear...

First example above is now "good":

; Lcl frame size = 40

G_M24727_IG01:
       4883EC28             sub      rsp, 40

G_M24727_IG02:
       4885C9               test     rcx, rcx
       0F84BC010000         je       G_M24727_IG35

G_M24727_IG03:
       488D4110             lea      rax, bword ptr [rcx+16]
       8B4908               mov      ecx, dword ptr [rcx+8]
       83F901               cmp      ecx, 1
       0F82B6010000         jb       G_M24727_IG36
...

In the second example stack space is also contained, but register shuffling is still happening, even after dotnet/coreclr#16028. Will investigate.

[AndyAyersMS]

The root issues are in the complexities of LSRA preferencing -- does not look like there is an easy surgical fix and any larger change here will be quite disruptive. So moving out of 2.1.

[AndyAyersMS]

Here's a kind of similar example:

    public static ReadOnlySpan<char> Y(string s) => s.AsReadOnlySpan();

generates a chain of span copies, which the jit can't coalesce:

G_M47024_IG01:

G_M47024_IG02:
       4885D2               test     rdx, rdx
       7507                 jne      SHORT G_M47024_IG03
       33C0                 xor      rax, rax
       4533C0               xor      r8d, r8d
       EB1A                 jmp      SHORT G_M47024_IG04

G_M47024_IG03:
       488D420C             lea      rax, bword ptr [rdx+12]
       4C8BC0               mov      r8, rax
       8B5208               mov      edx, dword ptr [rdx+8]
       8BC2                 mov      eax, edx
       498BD0               mov      rdx, r8
       4C8BC2               mov      r8, rdx
       8BD0                 mov      edx, eax
       498BC0               mov      rax, r8
       448BC2               mov      r8d, edx

G_M47024_IG04:
       488901               mov      bword ptr [rcx], rax
       44894108             mov      dword ptr [rcx+8], r8d
       488BC1               mov      rax, rcx

G_M47024_IG05:
       C3                   ret

[CarolEidt]

The register shuffling may have been fixed e.g. by dotnet/coreclr/pull/19429