[PATCH] D57854: [InstCombine] Optimize `atomicrmw <op>, 0` into `load atomic` when possible

[Quentin Colombet via Phabricator via llvm-commits]

qcolombet marked an inline comment as done.
qcolombet added inline comments.


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Comment at: lib/Transforms/InstCombine/InstCombineAtomicRMW.cpp:39
+           Ordering != AtomicOrdering::Unordered &&
+           "AtomicRMWs don't make sense with Unordered or NotAtomic");
+    if (Ordering != AtomicOrdering::Acquire &&
----------------
jfb wrote:
> Agreed NotAtomic is bonkers (`AtomicRMWInst::Init` disallows it), but Unordered might be sensible, @reames would know.
>From the language reference (http://llvm.org/docs/LangRef.html#atomic-memory-ordering-constraints):
Unordered
[...]
This ordering cannot be specified for read-modify-write operations; it is not strong enough to make them atomic in any interesting way.

And if I try to write a test case with unordered I get:
error: atomicrmw cannot be unordered

So I think we are good :).


Repository:
  rL LLVM

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https://reviews.llvm.org/D57854