Re: [TUHS] Unix stories

On Wed, Jan 4, 2017, at 08:04, Steffen Nurpmeso wrote: I assume you're imagining it as being equivalent to i = j + i + 1, with a redundant store operation. But why couldn't it equally well mean i = 0; i += j; i+= ++i i = 0; i += j; i += (i += 1) If an architecture were to the most efficient way to assign an additive expression to a variable to zero it out and add each successive operand to it. The example seems contrived, because it's honestly hard to make a reasonable-sounding case for the prefix operators, and my usual go-to examples require postfix operators and/or pointers. But to be fair, your example is contrived too; why wouldn't you just do i += j + 1? But for a better example, I was in a discussion a couple weeks ago with someone who thought it was clear what they meant by an expression like this: *a++ = *a And not only was I easily able to come up with two reasonable-looking implementations where it means different things, I guessed wrong on which one they thought it should mean. My examples were stack-based architectures with a "store to address" instruction taking operands in each of the two possible orders, making it natural to evaluate either the RHS or the address-of-LHS first. A more realistic register-based architecture with pipelining might make it more efficient to evaluate one or the other first, or parts of each [assuming more complex expressions than in the example] mixed together, depending on the exact data dependencies in both expressions.