Native Disassembly only shows first basic block

jwdj · February 10

I rather like the Native Disassembly feature in LINQPad and use it often to compare implementations. However it seems to be broken I think in the latest version, because only the first basicblock is shown. For example:

ref T ElementAt(T[] array, int offset)
=> ref array[offset];

Above code generates the following. At line L0008 it refers to label L0017 but that code is not visible anymore. (It used to show: int 3)

L0000 sub rsp, 0x28
L0004 cmp r8d, [rdx+8]
L0008 jae short L0017
L000a mov eax, r8d
L000d lea rax, [rdx+rax*4+0x10]
L0012 add rsp, 0x28
L0016 ret

Does anyone know a workaround? (Other than going back to a previous version of LINQPad)

i2van · February 11

8.8.6 (latest beta), the same:

ref T ElementAt<T>(T[] array, int offset)
=> ref array[offset];

L0000   sub rsp, 0x28
L0004   cmp r8d, [rdx+8]
L0008   jae short L0017
L000a   mov eax, r8d
L000d   lea rax, [rdx+rax*4+0x10]
L0012   add rsp, 0x28
L0016   ret

JoeAlbahari · February 13

Can you provide a full repro? I've tried running a fresh install of LINQPad 8.8.6 (Windows Sandbox) with the following script:

void Main()
{
}

ref T ElementAt<T>(T[] array, int offset)
    => ref array[offset];

This is the X64 shown:

ElementAt<T> (T[], Int32)
L0000   push    rbp
L0001   push    rdi
L0002   sub rsp, 0x28
L0006   lea rbp, [rsp+0x30]
L000b   mov [rbp+0x10], rcx
L000f   mov [rbp+0x18], rdx
L0013   mov [rbp+0x20], r8d
L0017   cmp dword ptr [0x7ffbdfdac258], 0
L001e   je  short L0025
L0020   call    0x00007ffc3e863390
L0025   mov rax, [rbp+0x18]
L0029   mov ecx, [rbp+0x20]
L002c   cmp ecx, [rax+8]
L002f   jb  short L0036
L0031   call    0x00007ffbdeb11728
L0036   mov edx, ecx
L0038   lea rax, [rax+rdx*4+0x10]
L003d   add rsp, 0x28
L0041   pop rdi
L0042   pop rbp
L0043   ret

jwdj · February 19

Here is a full example. A clue might be that on my work-pc it works okay if I select .NET 8, but only shows the first basic block when .NET 9 is selected.
LINQPad version: 8.7.4 (X64)
Host runtime version: 8.0.12
Default query runtime version: 9.0.1
Default query reference assembly version: 9.0.1
Roslyn Version: 4.12.0-3.24572.7
FSharp.Compiler.Service version: 43.8.101.0
NuGet client version: 6.7.1.1

void Main()
{
    var myArray = new int[10];
    ref var a = ref ElementAt(myArray, 5);
    a = 5;
    myArray.Dump();
}

ref T ElementAt<T>(T[] array, int offset)
    => ref array[offset];

This outputs with /o+:

X64
Main ()

L0000   sub rsp, 0x28
L0004   mov rcx, 0x7ffa29498aa8
L000e   mov edx, 0xa
L0013   call    0x00007ffa88fb7a40
L0018   mov dword ptr [rax+0x24], 5
L001f   mov rdx, rax
L0022   mov rcx, 0x7ffa2a02f040
L002c   call    qword ptr [0x7ffa2a0441f8]
L0032   nop 
L0033   add rsp, 0x28
L0037   ret

ElementAt (T[], Int32)

L0000   sub rsp, 0x28
L0004   cmp r8d, [rdx+8]
L0008   jae short L0017
L000a   mov eax, r8d
L000d   lea rax, [rdx+rax*4+0x10]
L0012   add rsp, 0x28
L0016   ret

i2van · February 20

Code generated is the same for .NET 8/9

JoeAlbahari · February 25

Thanks. I'll dig into this some time next week.

JoeAlbahari · March 9

The truncated output is caused by a workaround for a ClrMD bug which appears now to be fixed.

I've removed the workaround in 8.8.8. Let me know how you get along.

Native Disassembly only shows first basic block

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