If I raise the price of my product, less people will buy it but I'll make more profit per-unit -- so the amount of money I make is an inverted U with price on the x axis and money on the y axis, and I should set the price at the inflection point.
No, that's actually UB. The important bit here is "compiler defined" -- UB means the compiler is allowed to assume it never happens while compiling.
Consider, for example, an implementation defined function f() -- which can also diverge/crash horribly, etc.
If I write
if p {
print("p is true")
} else {
g()
}
if p {
f()
}
Then either we:
- print p is true and execute f
- do nothing
This is true regardless of if f immediately crashes the computer, nasal demons, whatever -- that's implementation defined.
UB means f may never happen.
And that means the compiler may optimize this to just:
g()
Notice the difference here -- the print never happens!, and g always happens.
You can see why this is concerning when you write code like
if dry_run {
print("would run rm -rf /")
} else {
run("rm -rf /")
}
if dry_run {
// oops: some_debug_string is NULL and will segfault!
print(some_debug_string);
}
I see what you're going for, but I don't see how your example is UB. If `p` is a pointer, and, after your `if (p)` check, `p` is dereferenced unconditionally, then yes, your check for `p == NULL` could be removed, and the code under the `if` would be removed as well. But the example you've constructed is not UB.
If doesn't matter what 'p' is in their example. The point is: if 'f' is undefined behavior (rather than just impl-defined), then the optimizer concludes that the "if p { f() }" can never happen... which means that we're allowed to assume that 'if p { ... } else { ... }' (in the first part of the example) will always take the else branch. The compiler will optimize accordingly and just always call g() unconditionally.
I mean... you can use an encryption scheme compatible with this (if you know the password).
I suppose this makes some sense for home computers (burglars and police raids are rare) but for a laptop, you really don't want thieves getting all your details.
Ironically -- this probably was paranoid a few years ago, but now -- "ChatGPT, use this prepared prompt to extract all useful info from this hard drive"
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