3 ms·

Not the first discussion of this topic, by any means. In this case, I've tried to boil it down to the essential points a practical programmer needs to know, but

Not the first discussion of this topic, by any means. In this case, I've tried to boil it down to the essential points a practical programmer needs to know, but the article still ended up longer than I initially aimed for.

wrs 3y ago
One hopefully constructive comment… I didn’t find this a motivating example as intended: int foo_or_bar(int which) { // Assumes you don't mind both functions being called int x = foo(); int y = bar(); return *(&x + which); } The argument being (if I understood right) if x has to have an address, it can’t be put in a register, so that must be UB or we can’t use registers. Well, how about the rule is that if I take the address of x, it can’t be put in a register? That seems like an obvious rule, and I seem to remember that was a safe assumption before the “great UBification” of compilers. I’m sure there’s a better example of why UB helps optimization, but this one didn’t work for me.
- josephcsible 3y ago
  > Well, how about the rule is that if I take the address of x, it can’t be put in a register? The issue is that y might end up in a register, and you didn't take the address of y.
  - wrs 3y ago
    Ohhh. I misunderstood the example. That would not only depend on a lack of register usage, but depend on where the variables are stored on the stack, which I don’t think anyone could reasonably demand even in 1976. So I still feel there must be a more reasonable and therefore motivating example of optimizations one would want that are enabled by surprising you with UB. (Which I think was the idea behind this example.)
    - int_19h 3y ago
      > depend on where the variables are stored on the stack, which I don’t think anyone could reasonably demand even in 1976 You'd be surprised. A typical compiler of that era would be single-pass, and allocating variables on the stack in order in which they were declared was not uncommon. Don't forget, we're talking about a language that actually had a "register" keyword solely to tell the compiler to enregister the variable!
      - wrs 3y ago
        Well, now that you mention it, that’s true, I was there. :) But I just want to go back to 1992, not 1976.

wrs 3y ago

One hopefully constructive comment… I didn’t find this a motivating example as intended: int foo_or_bar(int which) { // Assumes you don't mind both functions being called int x = foo(); int y = bar(); return *(&x + which); } The argument being (if I understood right) if x has to have an address, it can’t be put in a register, so that must be UB or we can’t use registers. Well, how about the rule is that if I take the address of x, it can’t be put in a register? That seems like an obvious rule, and I seem to remember that was a safe assumption before the “great UBification” of compilers. I’m sure there’s a better example of why UB helps optimization, but this one didn’t work for me.

josephcsible 3y ago
> Well, how about the rule is that if I take the address of x, it can’t be put in a register? The issue is that y might end up in a register, and you didn't take the address of y.
- wrs 3y ago
  Ohhh. I misunderstood the example. That would not only depend on a lack of register usage, but depend on where the variables are stored on the stack, which I don’t think anyone could reasonably demand even in 1976. So I still feel there must be a more reasonable and therefore motivating example of optimizations one would want that are enabled by surprising you with UB. (Which I think was the idea behind this example.)
  - int_19h 3y ago
    > depend on where the variables are stored on the stack, which I don’t think anyone could reasonably demand even in 1976 You'd be surprised. A typical compiler of that era would be single-pass, and allocating variables on the stack in order in which they were declared was not uncommon. Don't forget, we're talking about a language that actually had a "register" keyword solely to tell the compiler to enregister the variable!
    - wrs 3y ago
      Well, now that you mention it, that’s true, I was there. :) But I just want to go back to 1992, not 1976.