[PATCH] D88104: [GlobalISel] Combine (xor (and x, y), y) -> (and (not x), y)

[Jessica Paquette via Phabricator via llvm-commits]

paquette added inline comments.


================
Comment at: llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp:2679
+
+  if (Y != SharedReg)
+    std::swap(X, Y);
----------------
aemerson wrote:
> This code is a bit big-brain for me this morning. Simplify this to `return X == SharedReg || Y == SharedReg`?
I don't think that would be correct. At this point, `Y` is set to the RHS of the G_AND. This is passed along as the second element of the pair when we get to `applyXorOfAndWithSameReg`.

So we'd end up folding

```
xor (and x, y), x) -> (and (not x), y)
```

if you let `x =1` and `y = 0` then

```
xor (and 1, 0), 1 = 1
and (not 1), 0 = 0
```


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  https://reviews.llvm.org/D88104/new/

https://reviews.llvm.org/D88104