What is the meaning of ARM LDAXR/STLXR instructions?

Question

Often while disassembling ARM files, I see some code snippet with the following pattern:
loc_BB30:
.text:C0 FE 5F 88 LDAXR           WZR, W0, [X22]
.text:00 04 00 51 SUB             W0, W0, #1
.text:C0 FE 01 88 STLXR           W1, W0, [X22]
.text:A1 FF FF 35 CBNZ            W1, loc_BB30

Which is translated to the following decompile code:
  do
  {
    v2 = __ldaxr((unsigned int *)v1);
    v1 = (unsigned int)(v1 - 1);
  }
  while ( __stlxr(v1, v3) );

What is the meaning of this code? What kind of c code actually produces this kind of snippet?

Ian Cook · Accepted Answer

This general pattern of exclusive-access instructions is usually seen when atomic variables are modified.

C++ Example (C++11 or later)

  #include <atomic>

  void release( std::atomic<int>& refcount ) {
      refcount--;
  }

You can see here on godbolt that GCC's ARM64 compilation of the above produces your assembly code.

C Example (C11)

#include <stdatomic.h>

void release( _Atomic int* refcount ) {
    (*refcount)--;
}

Godbolt version here

C Example (prior to C11, using GCC built-ins)

void release( int* refcount ) {
    __atomic_sub_fetch( refcount, 1, __ATOMIC_ACQ_REL );
}

Godbolt version here

What is the meaning of ARM LDAXR/STLXR instructions?

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