Deriving a bit-twiddling hack: signed integer overflow
Matt Mastracci looks at the math and programming in deriving a bit-twiddling hack – signed integer overflow.
As a thin layer on top of assembly language, C’s integer arithmetic APIs have always been minimal, effectively just mapping the underlying assembly opcodes to the C arithmetic operators. In addition, while unsigned arithmetic can safely overflow in C, signed arithmetic overflow is considered undefined behaviour, and UB can end up in heartache for C developers.
More modern languages like Rust have a much richer integer API, however. By default, using the standard addition and subtraction operators in debug mode, integer APIs will panic! on overflow or underflow. The same operators will wrap in two’s-complement mode in release mode, though the behaviour is considered defined. If the developer wants to specify carrying, wrapping, checked, or saturating operations, APIs for each of these modes are available.
We don’t have these convenient APIs available in C yet, but it would be great to have them. Given that signed arithmetic overflow is undefined behaviour, can we build a function with the following C signature that works?
Check out the progression to develop an algorithm with two’s complement numbers in the post here.
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