I'm certainly not saying that it can't be done or that it isn't useful but that you're understating how much needs to be changed and the impact of it. Memory corruption also doesn't become predictable, just *less* impacted by optimization. It's still always going to be impacted.
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Here's something that's undefined: accessing uninitialized data.
int a; if (cond1) { a = 5; } else { if (cond2) { a = 10; } } use(a); do_other_stuff(); use(a);
Lets say that cond1 & cond2. Should both calls to use(a) be guaranteed to read the same value from uninitialized data?
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Of course they should see the same value. Holy cow you’re throwing the easiest examples at me. It’s like you’re making my point for me: that this is easy to fix but that some people need compiler educations.
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Okay, and that also means that using MADV_FREE in malloc and elsewhere is not possible either, which is a massive performance cost. Uninitialized memory can and does change value at runtime beyond just compiler optimizations avoiding saving uninitialized data via spill / restore.
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That's likely glibc what is going to be doing for their stack cache since MADV_DONTNEED is a significant performance cost for their implementation, and it doesn't become a non-issue if restricted to malloc since it still means that uninitialized memory can change between reads.
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Reading uninit data being undefined instead of locking it to an unspecified value permits massive optimizations like MADV_FREE and more efficient register allocation/spilling. Similarly, other memory safety issues being undefined permits optimization / freedom of implementation.
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Many programs have bugs where they read data that has just been freed, but handle it being an arbitrary value. The issue is often benign with common allocators. However, with other implementations the access will fault and they crash. It's good it's not required to let it work.
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Also, signed overflow being undefined rather than defined as wrapping means that more secure implementations where it traps are permitted. Passing -fsanitize=signed-integer-overflow -fsanitize-trap=signed-integer-overflow is standards compliant and used for hardening in AOSP.
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Similarly, lots of other UB that's easy to catch with simple branches at runtime (not most memory and type safety issues, but lots of other bugs) can be made to trap while remaining standards compliant, including enforcing not dereferencing outside many objects with object-size.
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Implementations of memory safety for C via fat pointers, etc. also depend on these things being undefined. By making it acceptable to index from one object into another and dereference the pointer, you would be forbidding memory safe implementations of C which are very important.
If the fat pointers define new behavior then it’s ok for that definition to contradict “classic C”. C making it defined or not only enables you to “say” that a secure version of C is “compliant”. It’s not a bad thing if you had to say that it was non compliant to classic C.
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They don't define any new behavior. They catch undefined behavior like out-of-bounds accesses and use-after-free. These things are already broken and non-portable. C is deliberately very portable and permits implementations that are memory safe. It even permits using a GC.
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