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Re: [Mingw-users] GCC 4.3.0 20080502 (alpha) Released - Please help us test it!

ATS wrote:

> One thing that I've found very useful is to translate Unix signals 
> into C++ exceptions (from GCC 4.2 this is supported on Linux). One
> gets semantics very close to what MS VC++: 
> 
>    try {
>       *(int*)0 = 0;
>    } catch ( SignalException* pse ) {
>       printf( "Exception condition: %s\n", pse->GetReason() );
>    }
> 

Thanks for pointing this out.  Just off the top of my head, there is 
some chance we can make this work with MinGW GCC as well.  I'll look 
into it.

> This provides a safety net when loading plugins or other running other 
> code that one has not built oneself. Without a mechanism like this, the
> app is doomed whenever 3rd party code does a NULL pointer access or 
> divide by zero. For me this is a crucial feature. 

MinGW can do it presently using the 'manual' SEH interface in <expt.h>, 
but it isn't nearly as pretty.


> As I see it the either-or debate (SJLJ/DWARF) is a bit limiting. SJLJ
> is slow because it does a complete CPU state save (FPU, MMX?, SSE?). 
> This is needed when saving thread context. However, jumping into a 
> catch block, it is way too much.
...
> My vote would be for a light weight state exception mechanism for Windows. 
> It is not affected by foreign frames, is quite fast and small. It would
> be per thread. A decent interface / binding with SEH on top of that would 
> be a bonus.

I think SEH might be close that 'light weight' mechanism you propose, as 
it doesn't save state, and uses a per-thread mechanism.

Unfortunately, MinGW maintainers do not have the resources to write a 
new exception model; we don't even have resources to implement SEH, a 
frequently-demanded feature.


However, the problem is not that the exception throwing or catching is 
slow.  The problem is that merely passing through a try {} block or 
exception specification such as throw(std::bad_alloc) has a small 
penalty, which can be measurable in tight loops.  DW2 completely solves 
the problem, by having no direct runtime cost at all.


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