Custom forward zero (rust-lang#913)

* Custom forward zero

* Fix activity analysis of agg

Author: wsmoses

enzyme/Enzyme/ActivityAnalysis.cpp

@@ -1054,7 +1054,7 @@ bool ActivityAnalyzer::isConstantValue(TypeResults const &TR, Value *Val) {
 
   // This value is certainly an integer (and only and integer, not a pointer or
   // float). Therefore its value is constant
-  if (TR.intType(1, Val, /*errIfNotFound*/ false).isIntegral()) {
+  if (TR.query(Val)[{-1}] == BaseType::Integer) {
     if (EnzymePrintActivity)
       llvm::errs() << " Value const as integral " << (int)directions << " "
                    << *Val << " "

enzyme/Enzyme/EnzymeLogic.cpp

@@ -2828,8 +2828,11 @@ void createTerminator(DiffeGradientUtils *gutils, BasicBlock *oBB,
     } else if (!gutils->isConstantValue(ret)) {
       toret = gutils->diffe(ret, nBuilder);
     } else {
+      IRBuilder<> eB(gutils->inversionAllocs);
       Type *retTy = gutils->getShadowType(ret->getType());
-      toret = Constant::getNullValue(retTy);
+      auto al = eB.CreateAlloca(retTy);
+      ZeroMemory(eB, retTy, al, /*isTape*/ false);
+      toret = nBuilder.CreateLoad(al);
     }
 
     break;