This is a deep misconception, and not at all how most undefined behavior is related to optimization.
Trivial example: Compilers assume that your variables aren't written to randomly from other threads. Without this assumption, virtually no optimization would be possible. Therefore, data races are UB - they violate a hard assumption of the compiler. But at no point did the compiler say "oh, you wrote to this variable without synchronization! Now I'll show you, hehehe!".
This is the same for e.g. removed overflow checks. Compilers can optimize many loops only if they assume signed integer overflow never happens. So, because compilers should be able to assume this, it is UB if it happens in your code. The same logic that deduces "this loop cannot wrap around" deduces "this if condition [an overflow check] cannot ever be true".
But it's easier for a programmer who got their UB-reliant checks optimized out to attribute malice to the compiler than to understand optimization fundamentals, and thus we get people complaining to high heaven and back.
That's because the compiler doesn't detect the situation. If it knew, it could, say, remove all code leading to that point - it's legal and the resulting code would be faster.
>Compilers can optimize many loops only if they assume signed integer overflow never happens.
I don't think this happens. Most loops iterate over arrays, and at least in C tradition unsigned integers are used as array index with no signed integers in sight.
> UB problems happen only when compilers detect it, understand it is an opportunity for optimization, because they are allowed to do anything, then do that optimization.
Perhaps I'm misunderstanding the comment, but I took "detect it, understand it is an opportunity for optimization, because they are allowed to do anything, ..." to mean "detect that undefined behavior occurs and then ..."
Instead, it should be "detect that an optimization can be applied and that the optimized code will do the right thing in all cases that undefined behavior does not occur, and apply the optimization."
I've met programmers who seem to believe that compilers intentionally apply weird transformations when they detect undefined behavior. But really, the compilers are applying sensible transformations for well-defined code and ignoring whether those transformations are sensible for code that isn't well-defined.
The compiler sees a variable x that's allocated on the stack. It looks at all the uses of the address of x, and sees that they are all loads and stores. It therefore decides to promote x into a register variable.
Where's the UB, you ask. Well, since the address of x was never leaked, it is UB to compute another pointer to that address (say, via an integer-to-pointer conversion). The compiler never checked for that possibility to affirm that it was UB; it knew that any code that did that would be UB and simply ignored the possibility.
This makes arithmetic overflow a very poor vehicle for UB because it's unusual in that you can't really take advantage of the UB without pointing to the specific operation that caused the UB to occur. This is why I believe that arithmetic overflow UB is gratuitous, and people objecting to UB because of what happens specifically with arithmetic overflow go on to make suggestions that are completely untenable because they don't have familiarity with how most UB works.
The problem causing this is that C loops are overspecified. If there was a “for…in” loop that didn’t make you declare and increment a variable, then manual increments could trap on overflow without causing so many problems.
If you can turn the loop from upward to downward then you do have a bound for it, and then you can tell if it overflows or not.
Also why is signed overflow a problem and not unsigned?
Surely you want unsigned loop optimized also?
Typically not a problem for i=0...n loops but eg `for (i=0;i<n;i+=2)` could overflow.
No, the OP wrote specifically that "the compiler detects [UB]" and then does optimizations exploiting that. Not "detects potential UB". The former is a common misconception, the latter is basically a truism.
Tell me how these use UB: integer constant folding, unused load removal, optimal use of GP registers to minimize stack spilling
>sane assumptions
"sane" here is doing a lot of work. Assuming overflow won't happen is not a sane assumption, assuming some other thread won't randomly write into your stack memory is.
Oh come on, this is like the most UB-centric thing in the entire compiler. The compiler uses UB to know that it has enumerated all uses of a memory location, as any pointer that didn't get its value from the data dependency tree that compiler used had to use UB to compute the address. (Be it from buffer overflow, use after end-of-lifetime, random integer converted to pointer, etc.)
It isn't; that sentence is even a type error. UB is not a property of compiler output. It's a property of compiler input.
The use of registers to store variables between uses (when those variables never have their address taken) relies on the lack of any defined behavior which would allow the program to determine this is being done. The fact you can't find this in the spec is precisely because it is not defined.
Everybody agrees that compilers should only do sane optimizations. Nobody agrees on which subset is sane.
int a = 10;
int var1;
int b = 20;
foo(&var1);
int c = a + b;
void foo(int* x) { *(x-1) = 0; }
Same idea as above.
> Assuming overflow won't happen is not a sane assumption
If the alternative is loops being much slower when you request optimizations, then maybe it is. Consult your compiler manuals if this default irks you.
My original point was that compilers don't specifically abuse UB to sleight you. You're trying hard to draw a line in the sand between different degrees of UB so that you can label one side as "not sane", with (what appears to be) the sole intent of concluding malice on the compiler author side. Please accept that the related tradeoffs have been discussed, and that a default was chosen (if you are curious for a recent instance of this, see [0]).
If you are ok with leaving performance on the table by disabling overflow being UB, you are literally free to do that. You're also free to lobby the standard bodies to alter their carefully considered choices. But you're not going to get anywhere by labeling any of this "insane".
[0]: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p090...
Most instances of UB are ones that the compiler could not detect. Your compiler isn't going to detect that your integer overflows, it assumes that it won't. Shit blows up if that assumption was wrong.
If you want to detect UB, you should run ubsan and the like. The compiler is not running it.
Programmers do not, in general, want to do the amount of work it takes to safely add two numbers together. You need something "weird". Like an error path for every addition. Or a type system for ranges, where you actually have to specify ranges (because adding two bytes together that you casually left as 0-255 will create a new ranged value 0-510; multiplication is even more fun). Or some other exotic idea. So we just let them overflow and the chips fall where they may.
It absolutely can't if it doesn't know what the values are going to be.
> the compiler will have to flag every integer addition as UB
No, I disagree. If it has no knowledge of my runtime values, it can't flag addition as UB because it isn't. It's UB only if I'm using values that would overflow, and the compiler in general can't know that. If I've done my program right, it will never overflow. There is no UB, any flag would be just absolutely wrong. The compiler couldn't detect UB. There's nothing to flag.
If it knows the values, then of course it can detect it and flag it:
x.c:3:18: warning: integer overflow in expression of type ‘int’ results in ‘-2’ [-Woverflow]
3 | int a=2147483647+2147483647;
I agree that overflow-checking is needlessly sucky in C but that's not "the problem." I usually make sure my arithmetic is safe but the compiler won't know it.
void func(struct x *aptr, struct x *bptr) {
x->a = 5;
y->a = 10;
x->a = 10;
}
The compiler is not really "detecting that your program has undefined behavior". Instead, it is assuming that undefined behavior doesn't happen, and optimizes your program using that assumption. It's unclear what "detecting undefined behavior" would mean here... what kind of diagnostic the compiler should emit, or how to suppress the diagnostic. It's clear that this simple code is pervasive, and the preferred approach for dealing with it is somewhat outside the scope of a normal C compiler... something like static analysis, runtime instrumentation, or some kind of formal methods.
I'm not sure this is a good example, because there's no way for you to read them from the same thread (and reading from another thread would be a race). Reading them from a signal handler would yield unspecified values (unless you're using lock-free atomic types, in which case we're probably not worried about optimizing these assignments).
> When the processing of the abstract machine is interrupted by receipt of a signal, the values of objects that are neither lock-free atomic objects nor of type volatile sig_atomic_t are unspecified, as is the state of the floating-point environment.
void function(struct x *ptr) {
ptr->x = 1;
if (ptr == NULL) {
some_big_chunk_of_code();
}
}
"Warning: dead store detected (file):line clobbered by (file):line. Use keyword volatile to always store or remove (file):line."
This way the optimization is promoted from the resulting binary to the source code, and bugs / typos / etc can be corrected.
Telling the developers to optimize these would be equivalent to telling them to start inlining their code (or creating variants of every function that might be inlined, to optimize it for the specific context where it will be inlined).
The above code isn't necessarily code as the programmer wrote it, but the code after it appears after macroexpansion and various optimization passes. If you were writing C++, the code may be templated.