[Python-Dev] [Python-checkins] cpython: Issue #14716: Change integer overflow check in unicode_writer_prepare() (original) (raw)

Antoine Pitrou solipsis at pitrou.net
Mon May 7 18:38:58 CEST 2012

On Mon, 7 May 2012 12:35:27 +0100 Mark Dickinson <dickinsm at gmail.com> wrote:

Hmm. Very clever, but it's not obvious that that overflow check is mathematically sound. As it turns out, the maths works provided that PYSSIZETMAX isn't congruent to 4 modulo 5; since PYSSIZETMAX will almost always be one less than a power of 2 and powers of 2 are always congruent to 1, 2 or 4 modulo 5, we're safe. Is the gain from this kind of micro-optimization really worth the cost of replacing obviously correct code with code whose correctness needs several minutes of thought?

Agreed that the original code is good enough. Dividing by 4 is fast, and this particular line of code is followed by a memory reallocation.

In general, "clever" micro-optimizations that don't produce significant performance improvements should be avoided, IMHO :-)

Regards

Antoine.

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