Rafael H. Schloming commented on PROTON-57:
So I think most of the uses of VLAs in the current codebase are due to
expediency/laziness on my part and should be eliminated in favor of a dedicated
buffer held by an appropriate object. I think I'm pretty comfortable overall
saying that we should get to a place where there is no variable stack
allocation in the code whatsoever (by alloca or any other means).
On the general topic I think one of the key things we need to do is ensure that
whatever patterns/practices are causing problems with C++ are addressed in a
way that we don't keep reintroducing more issues. It's fine to take a snapshot
and get it to work, but if upstream practices keep reintroducing more problems
then there will be continual catchup. I think one of the important things here
will be to have some kind of CI build so that those of us who aren't using
visual studio as our primary dev environment get early feedback if we
accidentally break something.
> Proton porting problems between current codebase and Visual Studio 2010
> Key: PROTON-57
> URL: https://issues.apache.org/jira/browse/PROTON-57
> Project: Qpid Proton
> Issue Type: Improvement
> Components: proton-c
> Environment: Windows using Visual Studio 2010
> Reporter: Mary hinton
> Labels: build
> This thread will be used to discuss the porting problems encountered using
> Visual Studio 2010
> Here’s the first one to discuss:
> 1. Visual Studio doesn’t support variable length arrays.
> a. Currently using malloc()/realloc() in my port just to get it to
> compile and be able to report memory allocation errors. This is not what I
> want to submit to the proton group for memory allocation.
> b. Cliff had a good method that included setting up macros and replace
> the VLAs with alloca() in the Windows version, but it could still cause
> problems when the stack overflowed. VLAs can also run out of stack space.
> c. _malloca() should be a better way than _alloca() on Visual Studio.
> Any messages under 1K would be allocated out of the stack. Over 1K will use
> heap. If the average messages are under 1K, this may be the most efficient
> way in Visual Studio. _malloca() also has new security enhancements.
> 1. Using _malloca() in the Windows version and VLA in Linux would
> require two macros. The major difference for the Linux version would be to
> use the new macro for VLA and to include the new free macro even though there
> is nothing to free using VLA. In Visual Studio, _freea(buf) will not free
> anything if it is allocating off the stack.
> Linux can continue to use VLAs.
> #ifdef C99
> #define PN_VLA(TYPE, buf, size) TYPE buf[size]
> #define PN_VLA_FREE
> #define PN_VLA(TYPE, buf, size) TYPE *buf = (TYPE*)
> #define PN_VLA_FREE(buf) _freea(buf)
> d. If the average size messages to allocate out of the stack needs to be
> increased for performance reasons, we can set up a new memory model. The 1K
> is not adjustable for _malloca().
> We can set up new macros along the lines of Microsoft’s suggestion below.
> “I would suggest something similar to what ATL does. Use alloca for small
> (where you define what that is) sizes and use heap allocation for larger
> ones. You can wrap the logic inside of a class or macros. To work around
> the fact that alloca isn't cleaned up at block scope, rewrite the block into
> functions or use lambdas. (I think alloca works inside of lambdas.)”
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