GlobalISel: Add assert that legalize mutation makes sense

[arsenm]

Summary

I've repeatedly encountered bugs resulting from custom legalize
mutations returning nonsense legalize results, such as increasing the
number of elements for FewerElements. Add an assert function to make
sure the type to mutate to is consistent with the legalize action.

[paquette]

This seems like a good idea. LGTM.

[arsenm]

r352636

lib/CodeGen/GlobalISel/LegalizerInfo.cpp

raw_ostream &LegalityQuery::print(raw_ostream &OS) const {
   for (const auto &MMODescr : MMODescrs) {
     OS << MMODescr.SizeInBits << ", ";
   }
   OS << "}";
 
   return OS;
 }
 
+#ifndef NDEBUG
+// Make sure the returned mutation makes sense for the match type.
+static bool mutationIsSane(const LegalizeRule &Rule,
+                           const LegalityQuery &Q,
+                           std::pair<unsigned, LLT> Mutation) {
+  const unsigned TypeIdx = Mutation.first;
+  const LLT OldTy = Q.Types[TypeIdx];
+  const LLT NewTy = Mutation.second;
+
+  switch (Rule.getAction()) {
+  case FewerElements:
+  case MoreElements: {
+    if (!OldTy.isVector())
+      return false;
+
+    if (NewTy.isVector()) {
+      if (Rule.getAction() == FewerElements) {
+        // Make sure the element count really decreased.
+        if (NewTy.getNumElements() >= OldTy.getNumElements())
+          return false;
+      } else {
+        // Make sure the element count really increased.
+        if (NewTy.getNumElements() <= OldTy.getNumElements())
+          return false;
+      }
+    }
+
+    // Make sure the element type didn't change.
+    return NewTy.getScalarType() == OldTy.getElementType();
+  }
+  case NarrowScalar:
+  case WidenScalar: {
+    if (OldTy.isVector()) {
+      // Number of elements should not change.
+      if (!NewTy.isVector() || OldTy.getNumElements() != NewTy.getNumElements())
+        return false;
+    } else {
+      // Both types must be vectors
+      if (NewTy.isVector())
+        return false;
+    }
+
+    if (Rule.getAction() == NarrowScalar)  {
+      // Make sure the size really decreased.
+      if (NewTy.getScalarSizeInBits() >= OldTy.getScalarSizeInBits())
+        return false;
+    } else {
+      // Make sure the size really increased.
+      if (NewTy.getScalarSizeInBits() <= OldTy.getScalarSizeInBits())
+        return false;
+    }
+
+    return true;
+  }
+  default:
+    return true;
+  }
+}
+#endif
+
 LegalizeActionStep LegalizeRuleSet::apply(const LegalityQuery &Query) const {
   LLVM_DEBUG(dbgs() << "Applying legalizer ruleset to: "; Query.print(dbgs());
              dbgs() << "\n");
   if (Rules.empty()) {
     LLVM_DEBUG(dbgs() << ".. fallback to legacy rules (no rules defined)\n");
     return {LegalizeAction::UseLegacyRules, 0, LLT{}};
   }
-  for (const auto &Rule : Rules) {
+  for (const LegalizeRule &Rule : Rules) {
     if (Rule.match(Query)) {
       LLVM_DEBUG(dbgs() << ".. match\n");
       std::pair<unsigned, LLT> Mutation = Rule.determineMutation(Query);
       LLVM_DEBUG(dbgs() << ".. .. " << (unsigned)Rule.getAction() << ", "
                         << Mutation.first << ", " << Mutation.second << "\n");
+      assert(mutationIsSane(Rule, Query, Mutation) &&
+             "legality mutation invalid for match");
+
       assert((Query.Types[Mutation.first] != Mutation.second ||
               Rule.getAction() == Lower ||
               Rule.getAction() == MoreElements ||
               Rule.getAction() == FewerElements) &&
              "Simple loop detected");
       return {Rule.getAction(), Mutation.first, Mutation.second};
     } else
       LLVM_DEBUG(dbgs() << ".. no match\n");