In PowerPC, the AtomicCmpXchgInst is lowered to
ISD::ATOMIC_CMP_SWAP_WITH_SUCCESS. However, this node does not handle
the weak attribute of AtomicCmpXchgInst. As a result, when compiling C++
atomic_compare_exchange_weak_explicit, the generated assembly includes a
"reservation lost" loop — i.e., it branches back and retries if the
stwcx. (store-conditional) fails. This differs from GCC’s codegen, which
does not include that loop for weak compare-exchange.
Since PowerPC uses LL/SC-style atomic instructions, the patch enables
AtomicExpandImpl::expandAtomicCmpXchg for PowerPC. With this, the weak
attribute is properly respected, and the "reservation lost" loop is
removed for weak operations.
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Co-authored-by: Matt Arsenault <arsenm2@gmail.com>
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