`latomic` flag sometimes not required?

Question

I know that the -latomic flag is required for the atomic library to be linked in by GCC. However, for some reason std::atomic<int> doesn't require it to build

build without latomic

while structs do

build requires latomic

what is this difference caused by?

Alvera

Please post the code and the errors in the question in form of a minimal reproducible example. – Cobbett

@273K all of my examples are linked through Godbolt and are minimally reproducible. – Alvera

A plausible explanation is that on your target architecture, atomic operations on int-sized operands are directly mapped onto processor instructions (via intrinsics) and don't need any library code. – Gammadion

External links are not searchable on SO, the second code can be reduced to a few lines #include <atomic> struct PropBoardSensorData { float tcTemp1, tcTemp2, tcTemp3; }; int main(){ std::atomic<PropBoardSensorData> atomicData{}; auto newData = atomicData.load(); } – Cobbett

@273K "External links are not searchable on SO" Ironically, the external link does contain the code, so the posted message w/ just links is no shorter than one with full code. – Foolproof

Answer 1

If the compiler inlines the __atomic builtins that std::atomic uses, the object file won't contain any references to functions that libatomic defines.

This is normal for most architectures with atomic<size_t> or smaller, at least with optimization enabled, and sometimes for structs the size of two pointers on some architectures. (But not x86-64 since GCC7, where it chooses not to inline lock cmpxchg16b even if you compile with -mcx16, and chooses not to report it as is_lock_free. https://gcc.gnu.org/ml/gcc-patches/2017-01/msg02344.html)

In your case, as you can see by mousing over the source and looking at the asm, auto newData = atomicData.load(); compiled to mov DWORD PTR [rbp-4], eax even in your debug built. (With a huge amount of clutter since you disabled optimization.)

Papageno