```
void *a = malloc(10);
```
Results with:
```
movl $10, %edi
call malloc
movq %rax, -8(%rbp)
```
```
void *a = 0;
__sync_bool_compare_and_swap(&a, 0, malloc(10));
```
Results with:
```
movl $10, %edi
call malloc
movq %rax, %rdx
movl $0, %eax
lock cmpxchgq %rdx, -8(%rbp)
```
```
void *a = 0;
__sync_lock_test_and_set(&a, malloc(10));
```
Results with:
```
movq $0, -8(%rbp)
movl $10, %edi
call malloc
xchgq -8(%rbp), %rax
```
The `LOCK` prefix essentially means that the current CPU has ownership of the cache line for the duration of the operation, so you get the atomicity for the memory access. All the internet says that `xchgq` has implicit `LOCK` if one of it's arguments is a memory address, which is the case here, so indeed `__sync_bool_compare_and_swap` and `__sync_lock_test_and_set` should roughly have the same penalty.
---
Reply to this email directly or view it on GitHub:
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