Compute LDS beyond the device's shared-memory limit lives in a storage
buffer, and its 64-bit atomics took a lock in that buffer. AMD's Windows
driver compiles the lock's acquire and release to s_barrier, which inside
the divergent lock loop pairs with the program's own barriers, so the
device-memory case of agc_driver_lds_lock_budget read half its sum.
With shaderBufferInt64Atomics the device-memory LDS now uses a 64-bit view
of the buffer instead: 64-bit atomics are compare-exchange loops on it and
32-bit atomics stay native, so no lock is taken. Without the feature the
lock is kept. The per-workgroup slice is rounded up to an even dword count
so that the 64-bit view stays aligned.
Crimson
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