The future of 32-bit Linux [LWN.net]

The future of 32-bit Linux

Posted Dec 5, 2020 14:57 UTC (Sat) by khim (subscriber, #9252)
In reply to: The future of 32-bit Linux by warrax
Parent article: The future of 32-bit Linux

Surprisingly enough no. It's not an UB to convert from signed to unsigned type and back. When number is representable in both — conversion is guaranteed to keep it's value, if not — it's implementation-defined behavior, not undefined behavior.

This being said it's not all peach and roses. Unsigned type is “dominant”, if you mix signed and unsigned in the same expression results can be surprising, e.g. “u > -1” is almost always true because -1 is interpreted as 0xFFFFFFFF.

But no, there are no problems with UB.

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The future of 32-bit Linux

Posted Dec 6, 2020 9:58 UTC (Sun) by niner (guest, #26151) [Link] (1 responses)

Wouldn't u > -1 be always false then?

The future of 32-bit Linux

Posted Dec 8, 2020 22:01 UTC (Tue) by khim (subscriber, #9252) [Link]

Good catch. I was think about u >= -1. But "always false" also probably for what people expect when they write. And note: compilers are happy to “optimize” that code — but offer no warnings (except for specialized static analysis tools).