Extra tail instruction corrupts stack?

Repro steps

Clone https://github.com/kerams/repro/, open Program.fs, target either net6.0 or net7.0, run in Release mode.

Expected behavior

Byte - 53, in total 1
[|146uy; 1uy; 53uy|]

Printed 10 times.

Actual behavior

This is printed the first 2 times:

Byte - 53, in total 1
[|146uy; 1uy; 0uy|]

Byte - 53, in total 1
[|146uy; 1uy; 112uy|]

Known workarounds

Remove [<Struct>] from MyResult.

OR

Put () at the end of the writeString function. The before and after diff is here. That extra tail instruction is a bit suspicious.

Related information

dotnet --info

.NET SDK: Version: 7.0.100-preview.5.22307.18 Commit: bd8b088037

Runtime Environment: OS Name: Windows OS Version: 10.0.19044 OS Platform: Windows RID: win10-x64 Base Path: C:\Program Files\dotnet\sdk\7.0.100-preview.5.22307.18\

global.json file: Not found

Host: Version: 7.0.0-preview.5.22301.12 Architecture: x64 Commit: 425fedc0fb

.NET SDKs installed: 6.0.400-preview.22301.10 [C:\Program Files\dotnet\sdk] 7.0.100-preview.5.22307.18 [C:\Program Files\dotnet\sdk]

.NET runtimes installed: Microsoft.AspNetCore.App 6.0.5 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App] Microsoft.AspNetCore.App 7.0.0-preview.5.22303.8 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App] Microsoft.NETCore.App 6.0.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App] Microsoft.NETCore.App 7.0.0-preview.5.22301.12 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App] Microsoft.WindowsDesktop.App 6.0.5 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App] Microsoft.WindowsDesktop.App 7.0.0-preview.5.22302.5 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

[jakobbotsch]

The call is passing a Span<byte> that is pointing to local stack memory. Looks like F#'s address exposure analysis is incorrect here, it should not be marking the call as tail. when it takes byrefs that might point to the local frame.

[jakobbotsch]

The same argument applies, SysV ABI (used on Intel Macs) probably does not need the portable tailcall helper for the call to writeString while windows-x64 ABI does.