Re: Strange asm generated by GCC...

On 12/20/24 14:59, jseigh wrote:

On 12/20/24 12:46, jseigh 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!
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Speaking of strange code
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That should be a ref paramter.  I though I updated the pasted 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);
}

Alright, my bad.  I should have double checked the docs.
It's undefined behavior in this case.
It seems like the success/failure form of compare_exchange
is redundant unless you want it for self documentation.
Joe Seigh