a139ed6d56
Fix the postorder traversal computed by the `DominatorTree`. It was
recording nodes in the wrong order depending on the order child nodes
were visited. Consider the following program:
```
function %foo2(i8) -> i8 {
block0(v0: i8):
brif v0, block1, block2
block1:
return v0
block2:
jump block1
}
```
The postorder produced by the previous implementation was:
```
block2
block1
block0
```
Which is incorrect, as `block1` is branched to by `block2`. Changing the
branch order in the function would also change the postorder result,
yielding the expected order with `block1` emitted first.
The problem was that when pushing successor nodes onto the stack, the
old implementation would also mark them SEEN. This would then prevent
them from being pushed on the stack again in the future, which is
incorrect as they might be visited by other nodes that have not yet been
pushed. This causes nodes to potentially show up later in the postorder
traversal than they should.
This PR reworks the implementation of `DominatorTree::compute` to
produce an order where `block1` is always returned first, regardless of
the branch order in the original program.
Co-authored-by: Jamey Sharp <jsharp@fastly.com>
This crate contains the core Cranelift code generator. It translates code from an intermediate representation into executable machine code.