Signed overflow optimization hazards in the kernel [LWN.net]

Signed overflow optimization hazards in the kernel

Posted Aug 17, 2012 21:57 UTC (Fri) by pdewacht (subscriber, #47633)
In reply to: Signed overflow optimization hazards in the kernel by wahern
Parent article: Signed overflow optimization hazards in the kernel

But given that Linux is only intended to be compiled by gcc, we can rely on its implementation-defined behavior:

The result of, or the signal raised by, converting an integer to a signed integer type when the value cannot be represented in an object of that type (C90 6.2.1.2, C99 6.3.1.3).
For conversion to a type of width N, the value is reduced modulo 2^N to be within range of the type; no signal is raised.

(and I don't see how any compiler for a two's complement computer could define different behavior.)

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Signed overflow optimization hazards in the kernel

Posted Aug 18, 2012 19:48 UTC (Sat) by PaulMcKenney (✭ supporter ✭, #9624) [Link]

True enough!

However, the Linux kernel's code can be legitimately used in any GPLv2 project, including those that might run on systems with non-twos-complement signed integer arithmetic. This sharing of code among compatibly licensed projects is a very good thing, in my view.

Which means in this case, where there is a solution that meets the C standard, and which loses nothing by doing so (at least on x86 and Power), it only makes sense to use that C-standard solution.

Signed overflow optimization hazards in the kernel

Posted Aug 18, 2012 23:58 UTC (Sat) by giraffedata (guest, #1954) [Link]

The result of ... converting an integer to a signed integer type when the value cannot be represented in an object of that type ...
For conversion to a type of width N, the value is reduced modulo 2^N to be within range of the type;

I must be reading that wrongly, because that's not at all what GCC does. With -m32, int is a signed integer type of width 32. UINT_MAX reduced modulo 2^32 is UINT_MAX, which is not within the range of int. So this does not describe what (int)UINT_MAX does.

Rather, what GCC does appears to be the opposite of what the standard requires for conversion from negative number to unsigned integer (add UINT_MAX+1 until it fits): it subtracts UINT_MAX+1 until the value is within the range of int (in this case -1).