[TUHS] Unix stories
steffen at sdaoden.eu
Thu Jan 5 02:22:38 AEST 2017
Random832 <random832 at fastmail.com> wrote:
|On Wed, Jan 4, 2017, at 08:04, Steffen Nurpmeso wrote:
|> terrible aliasing and "sequence point" rules, where i think it is
|> clear what i mean when i write "i = j + ++i" (i believe this is
|> undefined behaviour).
|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
No i don't, and the thing is that it could definetely not equally
mean anything. That is exactly the point.
I skip quite a lot here.
|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
So if we agree that a high level language should abstract such
problems from the programmer, with a right hand side that is
evaluated and stored in the target of the left hand side, then all
|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.
It is all right by me. "Can't say that on the radio."
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