Re: Strange asm generated by GCC...

On 12/22/2024 7:49 PM, Chris M. Thomasson wrote:

On 12/21/2024 2:37 AM, aph@littlepinkcloud.invalid wrote:

jseigh <jseigh_es00@xemaps.com> wrote:

On 12/19/24 19:43, Chris M. Thomasson wrote:

Why in the world would GCC use an XCHG instruction for the following
code. The damn XCHG has an implied LOCK prefix! Yikes!
>

>
Speaking of strange code
>
#include <atomic>
>
bool test1(std::atomic<int> var, int addend)
{
     int expected = var.load(std::memory_order_relaxed);
     int update = expected + addend;
     return var.compare_exchange_weak(expected, update,
std::memory_order_acq_rel, std::memory_order_seq_cst);
}
>
This is asm for armv8-a clang 9.0.0
>
test1(std::atomic<int>, int):
         ldr     w8, [x0]
         ldaxr   w9, [x0]
         cmp     w9, w8
         b.ne    .LBB0_3
         add     w8, w8, w1
         stlxr   w9, w8, [x0]
         cbz     w9, .LBB0_4
         mov     w0, wzr
         ret
.LBB0_3:
         clrex
         mov     w0, wzr
         ret
.LBB0_4:
         mov     w0, #1
         ret
>
I picked a version that just did ll/sc to avoid
the question of whether a failed CASAL did a store or not.
>
I don't see anything that forces a store memory barrier
on all the fail paths.  I could be missing something.

>
Why would there be one? If the store does not take place, there's no
need for a memory barrier because there's no store for anyone to
synchronize with. The only effect of a failed weak CAS is a load. If
you really need a store on failure because of its side effect you can
always add one.

 Iirc, the membars for the success and failure can be "useful" for popping from a lock-free stack. Wrt the C++ API the CAS can give you the updated value on a failure. So, there is a load. Depending on what you are doing, it might require an acquire.

Loading the head of the lock-free stack would be an acquire at the start of the CAS loop. The CAS can use relaxed for the success and an acquire for the failure.