Invalid codegen with Unsafe.As on FullOpts when accessing a struct field through ulong->struct cast.

[neon-sunset]

Description

Given:

struct Example
{
 object? Value;
 ulong Inner;

 struct ExampleInner { int Offset; int Length }
 
 public int Offset
 {
 get
 {
 var inner = Inner;
 return Unsafe.As<ulong, ExampleInner>(ref inner).Offset;
 }
 }
 
 public int Length
 {
 get
 {
 var inner = Inner;
 return Unsafe.As<ulong, ExampleInner>(ref inner).Length;
 }
 }
}

It appears that accessing ExampleInner fields stored as ulong produces invalid codegen on FullOpts but not on MinOpts or with Unsafe.BitCast: the produced assembly reads the Length field at the offset of 8 bytes instead of 12.

Reproduction Steps

1. Create new console application
2. Paste https://gist.github.com/neon-sunset/fed48339292c0cc1f153bf30ffb1ebca into Program.cs
3. dotnet run -c release

Expected behavior

Output:

--- MinOpts
Offset: 1234
Length: 5678
--- As
Offset: 1234
Length: 5678
--- Bitcast
Offset: 1234
Length: 5678

Actual behavior

Output:

--- MinOpts
Offset: 1234
Length: 5678
--- As
Offset: 1234
Length: 1234 <--- Unexpected
--- Bitcast
Offset: 1234
Length: 5678

Regression?

Yes, does not reproduce on .NET 7 and .NET 6.

Known Workarounds

None, bitcast produces worse codegen.

Configuration

Reproduced on:

.NET SDK:
 Version: 8.0.100-preview.7.23364.31
 Commit: 378e8abf4f

Runtime Environment:
 OS Name: Mac OS X
 OS Version: 14.0
 OS Platform: Darwin
 RID: osx-arm64
 Base Path: /usr/local/share/dotnet/sdk/8.0.100-preview.7.23364.31/

.NET SDK:
 Version: 8.0.100-preview.7.23363.19
 Commit: bef7f10d1d

Runtime Environment:
 OS Name: Windows
 OS Version: 10.0.22621
 OS Platform: Windows
 RID: win10-x64
 Base Path: C:\Program Files\dotnet\sdk\8.0.100-preview.7.23363.19\

Other information

No response

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Issue Details

---

Description

Given:

struct Example
{
 object? Value;
 ulong Inner;

 struct ExampleInner { int Offset; int Length }
 
 public int Offset
 {
 get
 {
 var inner = Inner;
 return Unsafe.As<ulong, ExampleInner>(ref inner).Offset;
 }
 }
 
 public int Length
 {
 get
 {
 var inner = Inner;
 return Unsafe.As<ulong, ExampleInner>(ref inner).Length;
 }
 }
}

It appears that accessing ExampleInner fields stored as ulong produces invalid codegen on FullOpts but not on MinOpts or with Unsafe.BitCast: the produced assembly reads the Length field at the offset of 8 bytes instead of 12.

Reproduction Steps

1. Create new console application
2. Paste https://gist.github.com/neon-sunset/fed48339292c0cc1f153bf30ffb1ebca into Program.cs
3. dotnet run -c release

Expected behavior

Output:

--- MinOpts
Offset: 1234
Length: 5678
--- As
Offset: 1234
Length: 5678
--- Bitcast
Offset: 1234
Length: 5678

Actual behavior

Output:

--- MinOpts
Offset: 1234
Length: 5678
--- As
Offset: 1234
Length: 1234 <--- Unexpected
--- Bitcast
Offset: 1234
Length: 5678

Regression?

Yes, does not reproduce on .NET 7 and .NET 6.

Known Workarounds

None, bitcast produces worse codegen.

Configuration

Reproduced on:

.NET SDK:
 Version: 8.0.100-preview.7.23364.31
 Commit: 378e8abf4f

Runtime Environment:
 OS Name: Mac OS X
 OS Version: 14.0
 OS Platform: Darwin
 RID: osx-arm64
 Base Path: /usr/local/share/dotnet/sdk/8.0.100-preview.7.23364.31/

.NET SDK:
 Version: 8.0.100-preview.7.23363.19
 Commit: bef7f10d1d

Runtime Environment:
 OS Name: Windows
 OS Version: 10.0.22621
 OS Platform: Windows
 RID: win10-x64
 Base Path: C:\Program Files\dotnet\sdk\8.0.100-preview.7.23363.19\

Other information

No response

Author:	neon-sunset
Assignees:	-
Labels:	area-CodeGen-coreclr
Milestone:	-

[EgorBo]

hm.. interesting, sounds like FieldSeq is messed up in FIELDADDR

[EgorBo]

The bug seems to be in SelectLocalIndirTransform

[MichalPetryka]

None, bitcast produces worse codegen.

That should be solved with #85562.