[PATCH] D12338: Make `llvm::expandAtomicRMWToCmpXchg`'s initial load atomic.

[Jeffrey Yasskin via llvm-commits]

jyasskin added a comment.

In http://reviews.llvm.org/D12338#245398, @jfb wrote:

> @jyasskin I took a look at implementing what we discussed <http://reviews.llvm.org/D12338#239615>, and it's quite a bit uglier than I though it would be:
>
> - We want the same value that factors into `ATOMIC_*_LOCK_FREE`, which is `MaxAtomicInlineWidth` from `tools/clang/lib/Basic/Targets.cpp` (each `Target` should define a value there, or gets `0` by default).


Richard and I don't think you want that value. On x86-64, the maximum width here is actually 128 bits, but you want to split loads down to 64 bits.

> - We can pass this to the backend through LLVM's `TargetOptions`, which clang's `BackendUtil.cpp` can initialize.

> - `AtomicExpandPass` has a `TargetMachine`, which has `TargetOptions`.

> 

>   This will work for code that come straight from C++, but won't work for code that only uses opt: clang has the information we need, and LLVM doesn't. This is pretty much something that should be in the `DataLayout` instead, which is a *very* involved change! WDYT? Am I missing something obvious?


You're probably right, but I don't know enough about the backends to be sure. It does seem like the backend should know the widths of atomics it supports.


http://reviews.llvm.org/D12338