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

[Richard Smith via cfe-dev]

On Tue, Nov 10, 2015 at 10:11 AM, Jason Haslam via cfe-dev <
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.
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