May the compiler replace that with puts("7") and free()? Recall that free() is a no-op when the pointer is NULL.
Are you arguing that each function may reduce pointer accesses down to one but not down to zero? What about
int foo2() {
int *p = malloc(400);
*p = 0;
/* ... code here was inlined then optimized away ... */
ret = 7;
free(p);
return ret;
}
? May we remove '*p = 0;', whether we remove the malloc+free or not?
Generally speaking the compiler tries not to reason about the whole function but just to look at the smallest possible collection of instructions, like how add(x, x) can be pattern matched to mul(x, 2) and so on. Having to reduce to one but not zero memory accesses per function is not easily compatible with that model. We would have to model branches that make the access conditional, the length of accesses may differ (what if 'p' is cast to char* and loaded), read vs. write, multiple accesses with different alignment, and maybe other things I haven't considered.
Both gcc and clang provide -fsanitize=null which checks all pointer accesses for null-ness before performing them. These can be surprising, your code (libraries you didn't write, headers you included) may be dereferencing NULL and relying on the optimizer to remove the invalid access. IMO there should be a "pointer is null-checked after use", it's a clear example where the programmer wrote something other than what they intended.
I think compilers should emit code that make writes to memory when the program asks them to, even if the pointer is then freed. Not doing so too easily leads to security issues. If the pointer turns out to be NULL at runtime, then let it crash. Using 'volatile' often works as a workaround, though.
I have no strong opinion on substituting puts() for printf(). I think incorporating the standard library into the spec was unhelpful, but isn't a major issue either way. It occasionally makes tracing slightly more difficult, but presumably helps with performance enough to offset this, and it's never bitten me as badly as the UB optimizations have.
The existence of volatile is a strong argument that was never the intention that C should map every pointer dereference at the source level to load and stores at the asm level.
It was a workaround (unreliable at that, IIRC) for compilers getting more aggressive in ignoring the intentions of C, including the early standards.
"The Committee kept as a major goal to preserve the traditional spirit of C. There are many facets of the spirit of C, but the essence is a community sentiment of the underlying principles upon which the C language is based. Some of the facets of the spirit of C can be summarized in phrases like
- Trust the programmer.
- Don't prevent the programmer from doing what needs to be done.
..."
> It was a workaround (unreliable at that, IIRC) for compilers getting more aggressive in ignoring the intentions of C, including the early standards.
But volatile was in C89? So how can it be a response to compilers "ignoring the intentions of C" in the early standards?
If anything, an example in the C89 standard makes it very explicit that an implementation may do exactly what gpderetta said (emphasis added):
> An implementation might define a one-to-one correspondence between abstract and actual semantics: at every sequence point, the values of the actual objects would agree with those specified by the abstract semantics. The keyword volatile would then be redundant.
> Alternatively, an implementation might perform various optimizations within each translation unit, such that the actual semantics would agree with the abstract semantics only when making function calls across translation unit boundaries. In such an implementation, at the time of each function entry and function return where the calling function and the called function are in different translation units, the values of all externally linked objects and of all objects accessible via pointers therein would agree with the abstract semantics. Furthermore, at the time of each such function entry the values of the parameters of the called function and of all objects accessible via pointers therein would agree with the abstract semantics. In this type of implementation, objects referred to by interrupt service routines activated by the signal function would require explicit specification of volatile storage, as well as other implementation-defined restrictions.
Going from trust the programmer to requiring volatile semantics for each access is quite a stretch. I would say you are part of an extremely small minority.
Don't compilers already have ways to mark variables and dereferences in a way to say 'I really want access to this value happen'?
They are free to optimize away access to any result that is not used based on code elimination, and these annotations limit what can be eliminated.
However, basing code elimination on UB, as pointed out earlier, would basically eliminate all code if you were rigorous enough because you basically cannot eliminate UB in C code, which is not in any way useful.
> Don't compilers already have ways to mark variables and dereferences in a way to say 'I really want access to this value happen'?
The standard defines it, it's `volatile` I believe.
But it does not help with the examples above as far as I understand (removing the log, removing the early return, time-travel...).
? May we remove '*p = 0;', whether we remove the malloc+free or not?
Sure, it does not solve the question when arbitrarily removing NULL pointer checks is OK.
It is true that when the compiler is inlining code or expanding a macro it may have a NULL check that is spurious in environments that do not map page 0 based on the observation that the pointer was dereferenced previously.
And this assumption is incorrect in environments that do map page 0 causing wrong code generation.
Are you arguing that each function may reduce pointer accesses down to one but not down to zero? What about
? May we remove '*p = 0;', whether we remove the malloc+free or not?Generally speaking the compiler tries not to reason about the whole function but just to look at the smallest possible collection of instructions, like how add(x, x) can be pattern matched to mul(x, 2) and so on. Having to reduce to one but not zero memory accesses per function is not easily compatible with that model. We would have to model branches that make the access conditional, the length of accesses may differ (what if 'p' is cast to char* and loaded), read vs. write, multiple accesses with different alignment, and maybe other things I haven't considered.
Both gcc and clang provide -fsanitize=null which checks all pointer accesses for null-ness before performing them. These can be surprising, your code (libraries you didn't write, headers you included) may be dereferencing NULL and relying on the optimizer to remove the invalid access. IMO there should be a "pointer is null-checked after use", it's a clear example where the programmer wrote something other than what they intended.