Fix constant instruction binop in forward mode (rust-lang#680)

* Fix constant instruction binop in forward mode

* Fix llvm7

Author: wsmoses

enzyme/Enzyme/AdjointGenerator.h

@@ -1760,8 +1760,6 @@ class AdjointGenerator
 
   void visitBinaryOperator(llvm::BinaryOperator &BO) {
     eraseIfUnused(BO);
-    if (gutils->isConstantInstruction(&BO))
-      return;
 
     size_t size = 1;
     if (BO.getType()->isSized())
@@ -1778,6 +1776,8 @@ class AdjointGenerator
     switch (Mode) {
     case DerivativeMode::ReverseModeGradient:
     case DerivativeMode::ReverseModeCombined:
+      if (gutils->isConstantInstruction(&BO))
+        return;
       createBinaryOperatorAdjoint(BO);
       break;
     case DerivativeMode::ForwardMode:
@@ -2255,6 +2255,11 @@ class AdjointGenerator
   }
 
   void createBinaryOperatorDual(llvm::BinaryOperator &BO) {
+    if (gutils->isConstantInstruction(&BO)) {
+      forwardModeInvertedPointerFallback(BO);
+      return;
+    }
+
     IRBuilder<> Builder2(&BO);
     getForwardBuilder(Builder2);
 

enzyme/test/Enzyme/ForwardMode/inactivebinop.ll

@@ -0,0 +1,22 @@
+; RUN: %opt < %s %loadEnzyme -enzyme -enzyme-preopt=false -mem2reg -instsimplify -simplifycfg -S | FileCheck %s
+
+declare void @_Z16__enzyme_fwddiff(...)
+
+define void @_Z34testFwdDerivativesRosenbrockEnzymev(i8* %a, i8* %b) {
+  call void (...) @_Z16__enzyme_fwddiff(double (i64*)* @f, metadata !"enzyme_dup", i8* %a, i8* %b)
+  ret void
+}
+
+define double @f(i64* %i10) {
+bb:
+  %i13 = load i64, i64* %i10, align 8
+  %i14 = sub i64 2, %i13
+  %i15 = sdiv exact i64 %i14, 8
+  %a5 = uitofp i64 %i15 to double
+  ret double %a5
+}
+
+; CHECK: define internal double @fwddiffef(i64* %i10, i64* %"i10'")
+; CHECK-NEXT: bb:
+; CHECK-NEXT:   ret double 0.000000e+00
+; CHECK-NEXT: }