adding first autodiff test

Author: ZuseZ4

tests/codegen/autodiff.rs

@@ -0,0 +1,33 @@
+//@ compile-flags: -C opt-level=3  -Clto=fat  -Zllvm-plugins=/home/manuel/prog/rust-middle/build/x86_64-unknown-linux-gnu/enzyme/build/Enzyme/libEnzyme-19.so -Cpasses=enzyme
+//@ no-prefer-dynamic
+//@ needs-enzyme
+#![feature(autodiff)]
+
+use std::autodiff::autodiff;
+
+#[autodiff(d_square, Reverse, Duplicated, Active)]
+#[no_mangle]
+fn square(x: &f64) -> f64 {
+    x * x
+}
+
+// CHECK:define internal fastcc double @diffesquare(double %x.0.val, ptr nocapture align 8 %"x'"
+// CHECK-NEXT:invertstart:
+// CHECK-NEXT:  %_0 = fmul double %x.0.val, %x.0.val
+// CHECK-NEXT:  %0 = fadd fast double %x.0.val, %x.0.val
+// CHECK-NEXT:  %1 = load double, ptr %"x'", align 8, !alias.scope !17816, !noalias !17819
+// CHECK-NEXT:  %2 = fadd fast double %1, %0
+// CHECK-NEXT:  store double %2, ptr %"x'", align 8, !alias.scope !17816, !noalias !17819
+// CHECK-NEXT:  ret double %_0
+// CHECK-NEXT:}
+
+fn main() {
+    let x = 3.0;
+    let output = square(&x);
+    assert_eq!(9.0, output);
+
+    let mut df_dx = 0.0;
+    let output_ = d_square(&x, &mut df_dx, 1.0);
+    assert_eq!(output, output_);
+    assert_eq!(6.0, df_dx);
+}