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Comment by ot | Hacker News Reader

Another (obvious) optimization is to make functions that have to be inline smaller. For example, std::string is used ubiquitously in C++ programs; this change shaved a few MBs of .text from a bunch of large services at FB: https://github.com/facebook/folly/commit/be4c6d6b3e21914df8a... .

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That is a smart optimization but it does make you wonder why were they using a size_t to hold the category (only 3 possible values) in the first place...

You should check out how SSO (small string optimization) is implemented. size_t is actually the natural choice, because it is shared with the capacity (or with part of the string in the small case).

Ah I see. In size() and c_str(), strictly speaking it's not the category constants that become 8 byte immediates but rather the category mask. Since the category constants and masks are all compile time constants I wonder if a smarter compiler could have performed this optimization for you.

Exactly. And yes, a "sufficiently smart compiler" [1] could have discovered the optimization, the old and new methods are semantically equivalent. It would be interesting to test a superoptimizer on this, but I suspect it wouldn't go very far because superoptimizers usually operate at the basic block level (this has a branch) and optimize for number of instructions, not bytes.

[1] http://wiki.c2.com/?SufficientlySmartCompiler

Nice. That 64-bit 0x1700000000000000 constant is pretty painful to see.

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