Handle Enzyme tape size of zero (rust-lang#368)

Author: LeilaGhaffari

Diff for: enzyme/Enzyme/Enzyme.cpp

@@ -978,6 +978,12 @@ class Enzyme : public ModulePass {
                          ? aug->fn->getReturnType()
                          : cast<StructType>(aug->fn->getReturnType())
                                ->getElementType(tapeIdx);
+        } else {
+          if (sizeOnly) {
+            CI->replaceAllUsesWith(ConstantInt::get(CI->getType(), 0, false));
+            CI->eraseFromParent();
+            return true;
+          }
         }
         if (sizeOnly) {
           auto size = DL.getTypeSizeInBits(tapeType) / 8;

Diff for: enzyme/test/Enzyme/ReverseMode/splitSize5.ll

@@ -0,0 +1,32 @@
+; RUN: %opt < %s %loadEnzyme -enzyme -enzyme-preopt=false -mem2reg -instsimplify -simplifycfg -S | FileCheck %s
+
+; Function Attrs: noinline nounwind readnone uwtable
+define double @tester(double* %x) {
+entry:
+  %gep = getelementptr double, double* %x, i32 1
+  %y = load double, double* %x
+  %z = load double, double* %gep
+  %res = fadd fast double %y, %z
+  ret double %res
+}
+
+define void @test_derivative(double* %x, double* %dx) {
+entry:
+  %size = call i64 (double (double*)*, ...) @__enzyme_augmentsize(double (double*)* nonnull @tester, metadata !"enzyme_dup")
+  %cache = alloca i8, i64 %size, align 1
+  call void (double (double*)*, ...) @__enzyme_augmentfwd(double (double*)* nonnull @tester, metadata !"enzyme_allocated", i64 %size, metadata !"enzyme_tape", i8* %cache, double* %x, double* %dx)
+  tail call void (double (double*)*, ...) @__enzyme_reverse(double (double*)* nonnull @tester, metadata !"enzyme_allocated", i64 %size, metadata !"enzyme_tape", i8* %cache, double* %x, double* %dx)
+  ret void
+}
+
+; Function Attrs: nounwind
+declare void @__enzyme_augmentfwd(double (double*)*, ...)
+declare i64 @__enzyme_augmentsize(double (double*)*, ...)
+declare void @__enzyme_reverse(double (double*)*, ...)
+
+; CHECK: define void @test_derivative(double* %x, double* %dx)
+; CHECK-NEXT: entry:
+; CHECK-NEXT:  %0 = call fast double @augmented_tester(double* %x, double* %dx)
+; CHECK-NEXT:  call void @diffetester(double* %x, double* %dx, double 1.000000e+00)
+; CHECK-NEXT:  ret void
+; CHECK-NEXT:}