[riot-notifications] [RIOT-OS/RIOT] fmt: fix fmt_s32_dec() and fmt_s64_dec() sign bit handling (#10784)

Kaspar Schleiser notifications at github.com
Wed Jan 16 16:49:43 CET 2019

> Wouldn't make any sense either, because you are clearly changing behavior. How about adding a unittest for the corner case pointed out in OP?

There actually is one, which is how ubsan got triggered. The tests executes fine.

I did some digging. IMO the compiler gets confused here:

- If only fmt_s32_dec() is "fixed", code size stays the same.
- If only fmt_s64_dec() is "fixed", code size increases by 4 bytes.
- if both are "fixed", code size increases by 8 bytes

(tested compiling for samr21-xpro).

Hm, I'd really like to see all the unittests complete without triggering undefined behaviour. Is that worth 8 bytes?

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