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If I understood correctly, the optimization proposed by Antoine was somehow rejected because _PyLong_IS_SMALL_INT() may be optimized "incorrectly".
If a compiler "miscompiles" this macro, can't we disable this optimization on this specific compiler, instead of missing an interesting optimization on all compilers? I bet that no compiler will do insane optimization on such test.
Where in CPython source code do we use directly references to small_ints? The common case is to write PyLong_FromLong(0). Can compiler call PyLong_FromLong() to detect that the result is part of the small_ints array? Or that it is not part of small_ints?
The corner case is very unlikely to me.
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I tested Antoine's patch with GCC 4.6 and CFLAGS="-O3 -flto" (-flto: standard link-time optimizer): the test suite pass. I don't see any magic optimization here, sorry.
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"fwiw I've always found this helpful for undefined behavior: http://blog.regehr.org/archives/213 and, just as it says "x+1 > x" will be optimized to a nop, by the same logic "v >= &array[0] && v < &array[array_len]" will also be optimized to a nop."
"x+1 > x" and "v >= &array[0]" are not the same checks. In the first test, x is used in both parts. I don't understand.