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"I'm okay with undefined behavior, as long as its behavior is well-defined." You keep using that word. I don't think it means what you think it means. :)

Dylan16807 3y ago
A behavior like "corrupts whatever is in memory at that location" when your code screws up a pointer is not exactly "well-defined", but it does disallow "pretends it could have corrupted RAM and does whatever it feels like instead". To put that another way, you can treat certain behaviors as if they're opaque functions until after you're done optimizing. Similar to how you might handle volatile. Some of these changes would be very easy. For example, instead of treating "dereference zero" as unreachable code, treat it the same as "dereference unknown location".
- stouset 3y ago
  Your platform almost certainly does do something sane and reasonable for UB-exhibiting code. Where your problem lies is what happens when you optimize that code. And guaranteeing optimizations do something superficially sane on unsound code while still performing reasonable and obvious optimizations for well-formed code is insanely hard. The fundamental problem is that the design of C guarantees that lots and lots and lots of syntactically valid and intuitively written code is semantically total nonsense. Part of this is that C is low-level and part is just that it was designed 50 years ago before anyone had the faintest idea what kinds of terrible mistakes were being made. If there were simple fixes here it wouldn’t still be a problem.
  - Dylan16807 3y ago
    > Where your problem lies is what happens when you optimize that code. And guaranteeing optimizations do something superficially sane on unsound code while still performing reasonable and obvious optimizations for well-formed code is insanely hard. It really depends on the type of optimization. A lot of the scariest results come when the compiler treats code as dead, and in very many cases you could go ahead and emit the code anyway and performance would be fine. > If there were simple fixes here it wouldn’t still be a problem. Nobody is trying to reduce the scope of UB, though. Even things that are very easy to define or implementation-define like syntax errors or left shifting with certain parameters are still sitting on the list.
torstenvl 3y ago
Plenty of things are undefined, yet bounded. And being bounded does not mean that something is well-defined. The number of elements of the set of real numbers between 1 and 2 is undefined, but we can say with absolute certainty that 3.5 isn't one of them.

Dylan16807 3y ago

A behavior like "corrupts whatever is in memory at that location" when your code screws up a pointer is not exactly "well-defined", but it does disallow "pretends it could have corrupted RAM and does whatever it feels like instead". To put that another way, you can treat certain behaviors as if they're opaque functions until after you're done optimizing. Similar to how you might handle volatile. Some of these changes would be very easy. For example, instead of treating "dereference zero" as unreachable code, treat it the same as "dereference unknown location".

stouset 3y ago
Your platform almost certainly does do something sane and reasonable for UB-exhibiting code. Where your problem lies is what happens when you optimize that code. And guaranteeing optimizations do something superficially sane on unsound code while still performing reasonable and obvious optimizations for well-formed code is insanely hard. The fundamental problem is that the design of C guarantees that lots and lots and lots of syntactically valid and intuitively written code is semantically total nonsense. Part of this is that C is low-level and part is just that it was designed 50 years ago before anyone had the faintest idea what kinds of terrible mistakes were being made. If there were simple fixes here it wouldn’t still be a problem.
- Dylan16807 3y ago
  > Where your problem lies is what happens when you optimize that code. And guaranteeing optimizations do something superficially sane on unsound code while still performing reasonable and obvious optimizations for well-formed code is insanely hard. It really depends on the type of optimization. A lot of the scariest results come when the compiler treats code as dead, and in very many cases you could go ahead and emit the code anyway and performance would be fine. > If there were simple fixes here it wouldn’t still be a problem. Nobody is trying to reduce the scope of UB, though. Even things that are very easy to define or implementation-define like syntax errors or left shifting with certain parameters are still sitting on the list.

torstenvl 3y ago

Plenty of things are undefined, yet bounded. And being bounded does not mean that something is well-defined. The number of elements of the set of real numbers between 1 and 2 is undefined, but we can say with absolute certainty that 3.5 isn't one of them.