Comment by gpderetta
3 years ago
> using "a pointer was dereferenced" as evidence "this pointer is safe to dereference" is comically bad evidence
Do you think the compiler would be right to remove the second check here?
if (!x) std::abort();
if (!x) return;
... = *x;
What about changing std::abort with the following?
[[noreturn]] void my_abort();
How's that different form a check after dereferencing a pointer? In both cases the check can be removed because dataflow or control flow analysis.
What if my_abort returns instead? Or another thread changes x after the fact?
If I had written the above code, I had clearly done something wrong. I would not want the compiler to remove the second check. I'd want it to (at the very least) warn me about an unreachable return statement, so that I could remove the actual meaningless code.
It's been long enough since I wrote C that I'm not familiar with that noreturn syntax or the contract I guess it implies, but control flow analysis which can prove the code will never be run, should all ideally warn me about it so that I can remove it in the source code, not quietly remove it from the object code.
I'm not demanding that it should happen in every case, but the cases where it's undecidable whether a statement is reachable or not, obviously it's undecidable for purposes of optimizing away the statement too.
The first check might be in a completely different function in another module (for example a postcondition check before a return). Removing dead code is completely normal and desirable, warning every time it happens would be completely pointless and wrong.
In this case, the compiler should warn that the second statement will never be executed, instead of just silently removing it.
libX_foo from libX gets at some point updated to abort if the return value would be null. After interprocedural analysis (possibly during LTO) the compiler infers that the if statement is redundant.
Should the compiler complain? Should you remove the check?
Consider that libX_foo returning not-null might not be part of the contract and just an implementation detail of this version.
> Should the compiler complain? Should you remove the check?
Yes and yes.
> Consider that libX_foo returning not-null might not be part of the contract and just an implementation detail of this version.
How is it an “implementation detail” whether a procedure can return null? That's always an important part of its interface.
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