Implement FNeg for Forward Mode (rust-lang#232)

* implement fneg for forward mode

* fix tests

* fail early

Author: tgymnich

enzyme/Enzyme/AdjointGenerator.h

@@ -235,8 +235,6 @@ class AdjointGenerator
   void visitInstruction(llvm::Instruction &inst) {
     // TODO explicitly handle all instructions rather than using the catch all
     // below
-    if (Mode == DerivativeMode::ReverseModePrimal)
-      return;
 
 #if LLVM_VERSION_MAJOR >= 10
     if (auto *FPMO = dyn_cast<FPMathOperator>(&inst)) {
@@ -248,16 +246,34 @@ class AdjointGenerator
         Value *orig_op1 = FPMO->getOperand(0);
         bool constantval1 = gutils->isConstantValue(orig_op1);
 
-        IRBuilder<> Builder2(inst.getParent());
-        getReverseBuilder(Builder2);
+        if (constantval1) {
+          return;
+        }
 
-        Value *idiff = diffe(FPMO, Builder2);
+        switch (Mode) {
+        case DerivativeMode::ReverseModeCombined:
+        case DerivativeMode::ReverseModeGradient: {
+          IRBuilder<> Builder2(inst.getParent());
+          getReverseBuilder(Builder2);
 
-        if (!constantval1) {
+          Value *idiff = diffe(FPMO, Builder2);
           Value *dif1 = Builder2.CreateFNeg(idiff);
           setDiffe(FPMO, Constant::getNullValue(FPMO->getType()), Builder2);
           addToDiffe(orig_op1, dif1, Builder2,
                      dif1->getType()->getScalarType());
+          break;
+        }
+        case DerivativeMode::ForwardMode: {
+          IRBuilder<> Builder2(&inst);
+          getForwardBuilder(Builder2);
+
+          Value *idiff = diffe(orig_op1, Builder2);
+          Value *dif1 = Builder2.CreateFNeg(idiff);
+          setDiffe(FPMO, dif1, Builder2);
+          break;
+        }
+        case DerivativeMode::ReverseModePrimal:
+          return;
         }
         return;
       }

enzyme/test/Enzyme/ForwardMode/fneg.ll

@@ -0,0 +1,31 @@
+; RUN: if [ %llvmver -ge 10 ]; then %opt < %s %loadEnzyme -enzyme -enzyme-preopt=false -mem2reg -instsimplify -simplifycfg -S | FileCheck %s; fi
+
+; extern double __enzyme_fwddiff(void*, double, double);
+;
+; double fneg(double x) {
+;     return -x;
+; }
+; 
+; double dfneg(double x) {
+;     return __enzyme_fwddiff((void*)fneg, x, 1.0);
+; }
+
+
+define double @fneg(double %x) {
+  %fneg = fneg double %x
+  ret double %fneg
+}
+
+define double @dfneg(double %x) {
+  %1 = call double @__enzyme_fwddiff(double (double)* @fneg, double %x, double 1.0)
+  ret double %1
+}
+
+declare double @__enzyme_fwddiff(double (double)*, double, double)
+
+
+; CHECK: define internal { double } @diffefneg(double %x, double %"x'") {
+; CHECK-NEXT:   %1 = fneg fast double %"x'"
+; CHECK-NEXT:   %2 = insertvalue { double } undef, double %1, 0
+; CHECK-NEXT:   ret { double } %2
+; CHECK-NEXT: }