It would be fine if I could opt into new semantics — something like Rust's "editions" would resolve my objections about these compiler optimizations. But that doesn't seem to be on offer in the C ecosystem.
C culture and to certain extent Objective-C and C++ ones are tainted by microptimizaitons while typing, where the compilers are the worst examples.
Unfortunely UNIX and C go together, so those that want to keep UNIX like platforms around bettter fix C somehow.
Yet Azure Sphere, despite its security message, uses C only SDK.
Meanwhile Visual Studio now supports C11 and C17.
Market pressure, their customers weren't willing to buy into it, and the new Microsoft also wants all those POSIX FOSS packages written in C running on Windows.
Hopefully the newer compiled languages will kill C++.
Quoting from another of my comments:
> > [What are you objecting to?]
> Inferring any propositional statement about the program (eg "this pointer is not null") from the fact that its negation would imply undefined behaviour.
That is what the problem is. Undefined behaviour is a licence to implement operations without regard for unusual corner cases, not to infer the absence of said corner cases from those operations and then apply that 'knowledge' elsewhere.
> I fail to see why one would consider a compiler evolving while conforming to language specification defective or malicious
It's https://en.wikipedia.org/wiki/Malicious_compliance in near-textbook form.
I'm not completely sure I understand that correctly, but do you mean that statements that are constant unless considering a possible (and "credible"?) implementation of UB shouldn't be fair-game for the compiler to optimize out? EDIT: I think I see a more tricky case that may be one of those you're referring to. Dereferencing a pointer further in the code shouldn't be a valid justification for optimizing out previous tests of it being null. I can relate with that but I suspect that it would prevent many classes of branch pruning.
I get what you suggest while describing malicious compliance but I can imagine that it could be a false impression resulting from trade-offs that favor optimization opportunities to "out-of-spec but canonical/natural" implementations.
Not further. Anywhere. If the implementation wishes to rewrite pointer dereferences from `use(*p)` to:
if(!p) abort();
use(*p);
use(*p);
if(!p) return NOPE;
if(!p) abort();
use(*p);
//if(!p) return NOPE; // unreachable because of if, not because of use
use(*p);
//if(!p) return NOPE; // CVE-20XX-XXXXX
> it would prevent many classes of branch pruning.
Yes, that's the general idea.
It dereferences a null pointer, invoking undefined behaviour, then calls the function `use` with the resulting value.
> what should the compiler emit?
Probably something to the effect of:
ld r0 [sp+.p] # if p is not already in a register
ld r0 [r0] # *p
jsr use
ld r0 [sp+.p] # if p is not already in a register
jz r0 .panic
ld r0 [r0] # *p
jsr use
Conforming to the spec is not a virtue. We want the compiler to be reasonable, regardless of whether the spec is. When the spec is malicious, conforming to the spec is malicious behavior.
For example, the Java spec says that the << operator (bit shift left) will accept any right operand, but performs a modulo-32 operation on the right operand before doing the shift. So `a << 4` is `a` shifted 4 places left, `a << 20` is `a` shifted 20 places left, and `a << 36` is `a` shifted 4 places left again.
This is absurd, and I'm comfortable calling it a bug in the spec. `a << 40` needs to have 0 in the lowest 40 bits. It does not need to have random values in bits 8-31.
This behavior is documented, but that doesn't make things better, it makes them worse.
But the philosophy that says "if it's documented, then it's OK" doesn't even allow for the concept of a bug in the spec.
Do you want compilers to stop adding optimizations while staying within the bounds defined by the spec? That they somehow guess that a given piece of code that may trigger UB is too important for them to optimize it based on the assumption that the developer knew what she was doing and ensured that it wouldn't?
The case of a compiler update breaking the test sounds like desirable to me. It pinpoints a critical piece of code that was relying on a specific implementation's behavior that is not specified. This could have been triggered by a compiler or architecture change. If this is something that is a hassle to fix immediately, you can temporarily downgrade to the version of the compiler used earlier or disable some optimizations.
I fail to see why one would consider a compiler evolving while conforming to language specification defective or malicious (however, with that definition I fear that finding one that isn't may indeed be difficult).