4f658dc851
The immediate impetus for this is that Noah Misch's patch to elide unnecessary table and index rebuilds when changing typmod for temporal types uses it; and this is extracted from that patch, with some further commentary by me. But it seems logically separate from the remainder of the patch, so I'm committing it separately; this is not the first time someone has wanted fls() in the backend and probably won't be the last. If we end up using this in more performance-critical spots it may be worthwhile to add some architecture-specific optimizations to our src/port version of fls() - e.g. any x86 platform can implement this using the assembly instruction BSRL. But performance won't matter a bit for assessing typmod changes, so I'm not worried about that right now.
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