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Updated ABI docs available + Exception Handling info

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They're not marvelously better, but they include the introduction that I wrote a couple years ago (and have mirrored in this blog twice, now). So, check out https://msdn2.microsoft.com/en-us/library/7kcdt6fy(VS.80).aspx to see the x64 ABI stuff in all it's MSDN formatted glory.

In other news, I presented a talk at the Developer Division 'brown bag' performance series (a lunch time lecture for MS developers) about the performance cost of exception handling. I had never really sat down and looked at how various EH constructs impact performance. I actually came to a rather interesting conclusion: for x86, EH is costly, and really does hurt performance, even if you're primarily using C++ objects with destructors. For x64, however, the performance impact of using C++ objects with destructors and throwing exceptions for errors is far outweighed by the much-less complex flowgraph of a function that doesn't have to deal with both return values & error conditions. I'll be giving a pretty similar talk at the Longhorn SDR next week. (https://www.longhornsdr.com/agenda.htm). I'll be talking in the CF07 slot (which was apparently vacated by someone else with a different talk). I'll be focusing on x64 much more so that x86.  Perhaps I'll stick my slide deck up here after the talk...

And finally, I've been approached by about 4 different ISV's over the past few months about various questions about x64 & unwinding. I'm going to spend some of my free time writing a little [useless] JITter to figure out how to do this stuff right. If only I had a 64 bit laptop, I could do this while sitting watching my 2 girls do Tae Kwon Do twice a week...

-Kev

Comments

  • Anonymous
    April 10, 2006
    Kevin,

    Could you explain one thing in unwind procedure? There is said:
    3. If a function table entry is found, RIP can lie within the following three regions:
    ...
    In the prolog
    If the RIP lies within the prolog, then control has not entered the function. There can be no exception handler associated with this exception for this function and the effects of the prolog must be undone to compute the context of the caller function. The RIP is within the prolog if the distance from the function start to the RIP is less than or equal to the prolog size that is encoded in the UNWIND_INFO Structure. The effects of the prolog are unwound by scanning forward through the unwind codes array for the first entry with an offset less than or equal to the offset of the RIP from the function start, then undoing the effect of all remaining items in the unwind code array. Step 1 is then repeated.
    ...

    My question is: after the effect of prolog is undone, what RIP is used to repeat step 1?
    Are there any differences for chained unwind codes?

  • Anonymous
    April 25, 2006
    After the prolog is unwound, the return address sitting on the stack is used as a new RIP, and the process resumes with the calling function.

    Chained unwind just impacts how unwind occurs.  We still unwind each function, but each function may require multiple xdata pieces to completely unwind.

  • Anonymous
    September 10, 2006
    The comment has been removed

  • Anonymous
    September 18, 2006
    Chained regions cannot have handlers associated with them.  Only parent PData entries can have handlers.  The unwinder looks up the pdata chain to find out if you have a handler, though, so you can have chained regions & EH regions together.  This is a little beyond the scope of a comment - perhaps I'll elaborate on this in a future post...

  • Anonymous
    September 18, 2006
    Then, I suppose that step 3.b of unwind procedure should be changed from:
    • Case b) If the RIP lies within the prologue, then control has not entered the function, there can be no exception handler associated with this exception for this function, and the effects of the prolog must be undone to compute the context of the caller function. The RIP is within the prolog if the distance from the function start to the RIP is less than or equal to the prolog size encoded in the unwind info. The effects of the prolog are unwound by scanning forward through the unwind codes array for the first entry with an offset less than or equal to the offset of the RIP from the function start, then undoing the effect of all remaining items in the unwind code array. Step 1 is then repeated.

    To something like that:
    ... The RIP is within the prolog if the distance from the function start to the RIP is less than or equal to the prolog size encoded in the MAIN PARENT unwind info...

    Where MAIN PARENT uwnind info is one from the chain of parent unwind info structures with unset UNW_CHAINED_INFO flag.

    Slava

  • Anonymous
    October 09, 2006
    Kevin, By the way, do you know anything about version #2 of the unwind data - I noticed that VS2005 uses it instead of #1, and there are some changes not only in the unwind data, but in the CEH tables as well. For example, different C++ specific exception handler is used by VS2005: __CxxFrameHandler3, while it used to be __CxxFrameHandler. Looking forward for any information from you! Thanks! Slava

  • Anonymous
    October 12, 2006
    I'm not sure which version number you're referring to - VS2005 is using version #1 (1-based indexing :-)  There is not a 2nd version of this stuff for AMD64 [yet]. The C++ exception handler, however, is a big deal.  Version 3 only invokes destructors and calls catch blocks when a non-C++ exception is thrown if you compiled your code -EHa, which basically makes it much more obvious when people really should have used -EHa, but instead used -GX or -EHs/-EHsc.

  • Anonymous
    January 25, 2007
    The comment has been removed

  • Anonymous
    March 13, 2007
    Slava, Before answering questions, I thought I'd suggest that you try my e-mail for faster response:  kfrei@microsoft.com.  Also, I checked your Bio, and discovered you're in Novosibirsk.  A coworker of mine is also from there, and I'm interviewing someone else who's originally from Novosibirsk in a couple weeks.  Small world (at least when it comes to compilers). Now to the questions.   #1 regarding runtime differences - I'd really have to see the actual code.  I've never seen code that's just straightforward have different behavior based on block layout, but then I've also seen very little code that uses shrink-wrapping be straightforward :-) #2 The NOP after a call when the call is on a region boundary is not always necessary, but falls into the 'better safe than sorry' category in my book. The NOP is only needed when the unwinder needs to be aware of the difference between unwinding through a 'live' stack frame where the instruction after the call is the return address, and when you're in the function itself, and the call has returned.  For unwinding (non-volatile register restoring), this is almost never necessary.  I can't actually come up with a single example where it is necessary, actually :-(  If you start talking about EH regions, then I can provide examples... Regaring JMP encoding, that MSDN documentation is actually wrong:  Any non-direct jmp, be in [rax], rax, or any other encoding of an indirect JMP can be encoded in an epilogue, but only if it includes the REX prefix. The corollary to that is if you EVER use the REX prefix on a jmp, the unwinder assumes you're in an epilogue, so don't use it unless you're doing a tail-call. Hope that helps!