The history seems to be that clang is following the rules laid out in N3664: Clarifying Memory Allocation which allows the compiler to optimize around memory allocations but as Nick Lewycky points out :
Shafik pointed out that seems to violate causality but N3664 started life as N3433, and I’m pretty sure we wrote the optimization first and wrote the paper afterwards anyway.
So clang implemented the optimization which later on became a proposal that was implemented as part of C++14.
The base question is whether this is a valid optimization prior to N3664, that is a tough question. We would have to go to the as-if rule covered in the draft C++ standard section 1.9 Program execution which says(emphasis mine):
The semantic descriptions in this International Standard define a
parameterized nondeterministic abstract machine. This International
Standard places no requirement on the structure of conforming
implementations. In particular, they need not copy or emulate the
structure of the abstract machine. Rather, conforming implementations
are required to emulate (only) the observable behavior of the abstract
machine as explained below.5
where note 5 says:
This provision is sometimes called the “as-if” rule, because an
implementation is free to disregard any requirement of this
International Standard as long as the result is as if the requirement
had been obeyed, as far as can be determined from the observable
behavior of the program. For instance, an actual implementation need
not evaluate part of an expression if it can deduce that its value is
not used and that no side effects affecting the observable behavior of
the program are produced.
Since new could throw an exception which would have observable behavior since it would alter the return value of the program, that would seem to argue against it being allowed by the as-if rule.
Although, it could be argued it is implementation detail when to throw an exception and therefore clang could decide even in this scenario it would not cause an exception and therefore eliding the new call would not violate the as-if rule.
It also seems valid under the as-if rule to optimize away the call to the non-throwing version as well.
But we could have a replacement global operator new in a different translation unit which could cause this to affect observable behavior, so the compiler would have to have some way a proving this was not the case, otherwise it would not be able to perform this optimization without violating the as-if rule. Previous versions of clang did indeed optimize in this case as this godbolt example shows which was provided via Casey here, taking this code:
#include <cstddef>
extern void* operator new(std::size_t n);
template<typename T>
T* create() { return new T(); }
int main() {
auto result = 0;
for (auto i = 0; i < 1000000; ++i) {
result += (create<int>() != nullptr);
}
return result;
}
and optimizing it to this:
main: # @main
movl $1000000, %eax # imm = 0xF4240
ret
This indeed seems way too aggressive but later versions do not seem to do this.