[cfe-dev] boost delay.c consumes huge amounts of memory

[Jason Haslam via cfe-dev]

> On Nov 10, 2015, at 2:53 PM, Richard Smith <richard at metafoo.co.uk> wrote:
> 
> On Tue, Nov 10, 2015 at 10:11 AM, Jason Haslam via cfe-dev <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> wrote:
> There’s a test file in boost at libs/preprocessor/doc/examples/delay.c that causes clang to quickly consume large amounts of memory. Has anybody seen this before? Is there some sort of workaround (like maybe a flag to limit macro expansion depth)? I couldn’t find an issue in the bug tracker.
> 
> From that file:
> # /* The time complexity of DELAY(N) is O(2^N).
> #  *
> #  * Handy when recompiles are too fast to take a coffee break. :)
> #  *
> #  * Template metaprogramming can be used for implementing similar
> #  * delays. Unfortunately template instantiation consumes memory,
> #  * therefore compilers usually fail to fully compile long template
> #  * based delays, because they run out of memory.
> #  *
> #  * On many compilers (e.g. g++, MSVC++), this macro takes only a
> #  * small amount of memory to preprocess. On some compilers (e.g.
> #  * MWCW), however, this macro seems to consume huge amounts of
> #  * memory.
> The "problem" is likely to be that we're keeping track of all the macro expansions performed by this ridiculous code, so this will use 2^N memory as well as 2^N time. We don't have any way to turn that off. If you file a bug on this, it's likely to be resolved as invalid unless you can provide a good reason for why we should care about code that is deliberately trying to break the compiler.

Okay, thanks. I don’t necessarily care about this code or expect anything reasonable to happen. The issue arises when users blindly try to slurp up and analyze everything in a directory. The outcome isn’t very friendly (especially on Windows).

Jason

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