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atomicrmw on pointers: move integer-pointer cast hacks into backend - #144192
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Conceptually, we want to have atomic operations on pointers of the form
fn atomic_add(ptr: *mut T, offset: usize, ...). However, LLVM does not directly support such operations (llvm/llvm-project#120837), so we have to cast theoffsetto a pointer somewhere.This PR moves that hack into the LLVM backend, so that the standard library, intrinsic, and Miri all work with the conceptual operation we actually want. Hopefully, one day LLVM will gain a way to represent these operations without integer-pointer casts, and then the hack will disappear entirely.
Cc @nikic -- this is the best we can do right now, right?
Fixes #134617