This is an interesting problem I saw when investigating #12322
Context: Debug mode has "/tailcalls-" by default. However, users can turn on "/tailcalls+" explicitly for debug code, in which case we do expect tailcalls to be taken.
Now, for F# 6, fewer tailcalls are taken for Debug code "/tailcalls+". Consider the following continuation calling code:
let g x cont = (x+2) |> cont
- In F# 5 with "/tailcalls+", the "|> cont" was a tailcall
- In F# 6 with "/tailcalls+", the "|> cont" is not a tailcall, I presume because of pipeline debugging on by default
Does this matter? I guess it does since Debug code now takes fewer tailcalls even when /tailcalls+ is specified.
Possible solutions
- We could disable pipeline debugging for pipelines in tailcall-position when "/tailcalls+" is specified, or
- We could completely disable pipeline debugging when "/tailcalls+" is specified.
The first is a little more work.
Symptom
Stack overflow in debug code when using F# 6 where it doesn't occur when using F# 5.
Workarounds
As workaround, you can enable optimizations in debug code which has "/tailcalls+" enabled. This suppresses pipeline debugging. However it makes debugging much worse.
Context
As context: I saw an instance of this in FSharp.Data when activating "/tailcalls+" in a design-time type provider - the overall projects compiled with F# 5 toolchain but didn't with F# 6 toolchain. However this only affected Debug code, including the Debug instance of the type provider. Turning optimizations on for the design-time type provider code would have disabled pipeline debugging and the tailcalls would ahve been recovered.
This is an interesting problem I saw when investigating #12322
Context: Debug mode has "/tailcalls-" by default. However, users can turn on "/tailcalls+" explicitly for debug code, in which case we do expect tailcalls to be taken.
Now, for F# 6, fewer tailcalls are taken for Debug code "/tailcalls+". Consider the following continuation calling code:
Does this matter? I guess it does since Debug code now takes fewer tailcalls even when /tailcalls+ is specified.
Possible solutions
The first is a little more work.
Symptom
Stack overflow in debug code when using F# 6 where it doesn't occur when using F# 5.
Workarounds
As workaround, you can enable optimizations in debug code which has "/tailcalls+" enabled. This suppresses pipeline debugging. However it makes debugging much worse.
Context
As context: I saw an instance of this in FSharp.Data when activating "/tailcalls+" in a design-time type provider - the overall projects compiled with F# 5 toolchain but didn't with F# 6 toolchain. However this only affected Debug code, including the Debug instance of the type provider. Turning optimizations on for the design-time type provider code would have disabled pipeline debugging and the tailcalls would ahve been recovered.