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

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GCC 4.3.0 20080502 (alpha) Released - Please he...
\ Aaron W. LaFramboise (5 May 2008)
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Subject:	Re: GCC 4.3.0 20080502 (alpha) Released - Please help us test it!
Group:	Mingw-users
From:	Aaron W. LaFramboise
Date:	8 May 2008

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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