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Looks like the if (function pointer) is not optimized. Both branches are present in all builds (float and non-float). That gives a 554-byte function in both cases. In normal mode, ~500 of those bytes are never used (the __dtostrf inlined code), and in non-float mode ~50 of those aren't used (the printf() branch).

I think this may be the best we can hope for, without adding a new #define like your original PR, so I'm approving as-is. If someone else has any ideas, I'd be interested...