[cfe-dev] Unwind behaviour in Clang/LLVM

[Keith Walker]

> From: Renato Golin [mailto:renato.golin at linaro.org]
>
> We're having some discussions about the behaviour of exception handling and Dwarf
> sharing unwind logic, tables, etc. and it seems that the code around it wasn't
> designed with any particular goal in mind, but evolved (like the EHABI) and now
> we're seeing the results from it.
>
> The problems below are assuming C vs. C++, but it actually apply to any possibly-exceptional
> vs. never-exceptional cases.
>
>
> 1. C vs. C++
>
> We have two unwind flags: nounwind, which flags functions that can't unwind (leaf,
> nothrow, etc) and uwtable, which forces generation of the table regardless of
> nounwind. It seems sensible that C++ code with exceptions enabled should generate
> the tables for all functions, in case they're called by (or call) external functions.
> In C we don't want any of that.
>
> GCC seems to never emit tables, and G++ always do, even on C code (.c files,
> no exception or anything), which is very sensible and in line with my reasoning above.
> Clang, on the other hand, always generates them. I guess it'll have to figure out what
> to do based on its impressions on what language is being used to produce similar results.

You can make GCC emit unwind tables for C if you use the command line option -funwind-tables.

This could be useful if you are compiling C code which is called by C++ code and which itself calls C++ code and you wish the C++ exception to propagate through the C code.

> I believe that emitting the tables on anything that could potentially interact
> with exceptional code makes sense, but that's clearly a front-end decision.
> To LLVM, nounwind and uwtables should be absolute:

> IF (uwtables)
>  IF (nownwind)
>    CantUnwind
>  ELSE
>    Unwind Table
> ELSE
>   do nothing
> ENDIF

This certainly appears to me a sensible sequence of decision.

> 2. .fnstart/.fnend+friends
>
> Another problem is that the emission of unwinding tables (based on
> .fnstart/.fnend symbols and others) is conditional *only* to the existence
> (or not) of an exception handling class being loaded (EHABI, Dwarf). Which
> means that, we can't disable the EH on a per-function basis.
>
> We'll have to change the way these symbols are emitted, at least when using
> ARMException, so that we can emit the tables and honour the uwtable on a per-function basis.
>
> Again, this is a requirement for problem 1, but it'd need to be fixed after 3.
>
>
> 3. Unwinding code
>
> Currently, even when no exception handling are needed, the exception code is
> used to generate Dwarf unwinding directives (CFI) for the debugger.

This is because the 2 are so similar.

> Both DwarfCFIException and ARMException inherit from DwarfException, and they
> are called to do the debug info about the stack unwinding, which is (at least)
> misplaced. The consensus is that this code should be factored out.

There is some interaction in that if you are generating a .eh_frame section for exception handling then there is little sense is generating a DWARF .debug_frame section as well;  I would hope most debuggers could use the .eh_frame section if present as its format is so similar to a .debug_frame section.

> The part that is relevant to this thread is that, today, if -arm-disable-ehabi
> is requested, ARMException will not be used and we won't have a way of
> generating debug stack directives, which is wrong.

What does -arm-disable-ehabi actually mean?

Currently it is effectively "don't generate exception tables".

Maybe is should it rather be "don't generate EHABI exception tables but generate .eh_frame tables instead"?   Obviously this requires the necessary DWARF like support to generate the information for the .eh_frame tables.

> Factoring out this code is a requirement for the unwinding problem (1), since
> if we disable EH today, we'll disable Dwarf stack unwinding altogether. But we
> also need a final solution for problem 4 below before we start.

I think the solution depends on what we mean when we specify -arm-disable-ehabi ... is it

 IF(arm_disable_ehabi)
   USE ARMException
 ELSE
   USE DwarfCFiNoException   <---- Just generates DWARF debugging information
 ENDIF

Or is it

 IF(arm_disable_ehabi)
   USE ARMException
 ELSE
   USE DwarfCFIException
 ENDIF

> 4. Clang EH control
>
> There are a number of Clang/LLVM options to control exception handling:
>  * -fno-excetpion (enable/disable EH on C++ mode, off in C mode)
>  * -fcxx-exception (no idea, is it objC++ specific? does it control tables in any way?)
>  * -funwind-tables (forces uwtable attribute?)
>  * -arm-disable-ehabi (ARM specific bogus flag)
>
> Those options are not always completely exclusive, and they damage different
> parts of the compilation process (as seen recently on the list), so we need
> a clear consensus on what each option mean (or should mean), and translate it
> to the back-end (via function attributes). This would considerably simplify
> the back-end and help us tackle this refactoring.
>
> My main target for this problem is to have the one true option on the front-ends,
> and rely only on function attributes on the back-end (including tools like llc).
> For that, I'd love to be left with -fexception only and infer all the rest from
> the language / conditions of the code.
>
> Ex:
>
> IF (C++ mode)
>   IF (-fno-exception)
>     no uwtable
>     no nounwind
>   ELSE
>     uwtable
>     IF (leaf / nothrow)
>       nounwind
>     ENDIF
>   ENDIF
> ELSE
>   no uwtable
>   no nounwind
> ENDIF
>
> My main goal is to get rid of (at least) -arm-disable-ehabi.

Hmmm!   I'm very nervous about the backend making decisions based on the language.

As I mentioned above I also wonder if you do still need to support -funwind-tables for compiling C code through which you wish to allow exceptions to propogate.

In which case the logic is more like ....

  IF (-funwind-tables || -fexceptions)
    uwtable
    IF (leaf / nothrow)
      nounwind
    ENDIF
  ELSE
    no uwtable
    no nounwind
  ENDIF

The main difference I see between -fexceptions and -funwind-tables is:

-fexceptions allows compiling exception handling constructs in the source language and enables the generation of unwind tables.
-funwind-tables only enables the generation of unwind tables.

Keith



Keith


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