On 11/12/2010 04:24 PM, Stefan Behnel wrote: > Hi, > > one of the CPython regression tests (test_long_future in Py2.7) failed > because it used the constant expression "1L<< 40000". We had this problem > before, Cython currently calculates the result in the compiler and writes > it literally into the C source. When I disable the folding for constants of > that size, it actually writes "PyInt_FromLong(1L<< 40000)", which is not a > bit better. > > I found this old thread related to this topic but not much more > > http://comments.gmane.org/gmane.comp.python.cython.devel/2449 > > The main problem here is that we cannot make hard assumptions about the > target storage type in C. We currently assume (more or less) that a 'long' > is at least 32bit, but if it happens to be 64bit, it can hold much larger > constants natively, and we can't know that at code generation time. So our > best bet is to play safe and use Python computation for things that may not > necessarily fit the target type. And, yes, my fellow friends of the math, > this implies a major performance regression in the case that Cython cannot > know that it actually will fit at C compilation time. > > However, instead of changing the constant folding here, I think it would be > better to implement type inference for integer literals. It can try to find > a suitable type for a (folded or original) literal, potentially suggesting > PyLong if we think there isn't a C type to handle it. > > The main problem with this approach is that disabling type inference > explicitly will bring code back to suffering from the above problem, which > would surely be unexpected for users. So we might have to implement > something similar at least for the type coercion of integer literals (to > change literals into PyLong if a large constant coerces to a Python type). > > Does this make sense? Any better ideas? >
This isn't really another proposal, just a related thought: For my own code, I would like to have a directive @cython.hugeints(False) or similar, that says that all integer values can be assumed to stay within the machine range, whatever that is. "range(n)" would be assumed to also stay within 32/64 bit, and so on. I almost exclusively use integers for accessing items in arrays or count number of samples or such things. I never use large integers, and I could rather safely just set this directive at the top of my files. I think this may fit very many programmers. Dag Sverre _______________________________________________ Cython-dev mailing list [email protected] http://codespeak.net/mailman/listinfo/cython-dev
