diff options
Diffstat (limited to 'src')
| -rw-r--r-- | src/passes/OptimizeInstructions.cpp | 31 |
1 files changed, 27 insertions, 4 deletions
diff --git a/src/passes/OptimizeInstructions.cpp b/src/passes/OptimizeInstructions.cpp index e6854cf24..18fbf6d83 100644 --- a/src/passes/OptimizeInstructions.cpp +++ b/src/passes/OptimizeInstructions.cpp @@ -3127,21 +3127,44 @@ private: ChildIterator ifTrueChildren(curr->ifTrue); if (ifTrueChildren.children.size() == 1) { // ifTrue and ifFalse's children will become the direct children of - // curr, and so there must be an LUB for curr to have a proper new + // curr, and so they must be compatible to allow for a proper new // type after the transformation. // - // An example where that does not happen is this: + // At minimum an LUB is required, as shown here: // // (if // (condition) // (drop (i32.const 1)) // (drop (f64.const 2.0)) // ) + // + // However, that may not be enough, as with nominal types we can + // have things like this: + // + // (if + // (condition) + // (struct.get $A 1 (..)) + // (struct.get $B 1 (..)) + // ) + // + // It is possible that the LUB of $A and $B does not contain field + // "1". With structural types this specific problem is not possible, + // and it appears to be the case that with the GC MVP there is no + // instruction that poses a problem, but in principle it can happen + // there as well, if we add an instruction that returns the number + // of fields in a type, for example. For that reason, and to avoid + // a difference between structural and nominal typing here, disallow + // subtyping in both. (Note: In that example, the problem only + // happens because the type is not part of the struct.get - we infer + // it from the reference. That is why after hoisting the struct.get + // out, and computing a new type for the if that is now the child of + // the single struct.get, we get a struct.get of a supertype. So in + // principle we could fix this by modifying the IR as well, but the + // problem is more general, so avoid that.) ChildIterator ifFalseChildren(curr->ifFalse); auto* ifTrueChild = *ifTrueChildren.begin(); auto* ifFalseChild = *ifFalseChildren.begin(); - bool validTypes = - Type::hasLeastUpperBound(ifTrueChild->type, ifFalseChild->type); + bool validTypes = ifTrueChild->type == ifFalseChild->type; // In addition, after we move code outside of curr then we need to // not change unreachability - if we did, we'd need to propagate |