| Issue |
137693
|
| Summary |
Address of empty function is not unique
|
| Labels |
|
| Assignees |
|
| Reporter |
fekir
|
Consider following snippet (https://godbolt.org/z/beGsh8zoj) compiled at least with `-O1`:
~~~~
#include <iostream>
void foo() { __builtin_unreachable(); }
void bar() {}
int main() {
auto ap = size_t(&foo);
auto bp = size_t(&bar);
std::cout
<< ap << "\n"
<< bp << "\n"
<< ap-bp << "\n"
<< (ap == bp ? "true\n" : "false\n");
}
~~~~
the output of this programs look like
~~~~
95027430187360
95027430187360
0
false
~~~~
clang manages to creates two equal numbers (the difference is `0`), that are not equal
I believe this issue is related to https://github.com/llvm/llvm-project/issues/60588 (the oldest one I could find), which was (IMHO wrongly) rejected.
`void foo() { __builtin_unreachable(); }` does not have undefined behavior as long as it is not called, and taking the address of `foo` should thus not trigger undefined behavior.
The argument "The problematic code doesn't need to be executed for UB to take effect" is strangely worded.
The function `int baz(int i){return 1/i;}` also has UB when called as `baz(0)`, but as long as no one calls `baz` with the parameter `0`, there is no UB.
Contrary to most linked bug reports linked to https://github.com/llvm/llvm-project/issues/60588, the problematic function is not called.
As to why one would write something like `foo`; an example would be to write code for a compiler that does not support `__builtin_unreachable`; for example
~~~~
#ifdef PLATFORM_WITH_BUILTIN_UNREACHABLE
void foo() { __builtin_unreachable(); }
#else
void foo() { abort(); }
#endif
// some code that uses foo instead of
// __builtin_unreachable directly to ensure portability
// with different compilers
~~~~
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