add empty struct ret testcase

Author: ZuseZ4

tests/codegen-llvm/autodiff/void_ret.rs

@@ -0,0 +1,41 @@
+//@ compile-flags: -Zautodiff=Enable,NoTT,NoPostopt -C no-prepopulate-passes -C opt-level=3  -Clto=fat
+//@ no-prefer-dynamic
+//@ needs-enzyme
+
+#![feature(autodiff)]
+use std::autodiff::*;
+
+// Usually we would store the return value of the differentiated function.
+// However, if the return type is void or an empty struct,
+// we don't need to store anything. Verify this, since it caused a bug.
+
+// CHECK:; void_ret::main
+// CHECK-NEXT:    ; Function Attrs:
+// CHECK-NEXT:    define internal
+// CHECK-NOT: store {} undef, ptr undef
+// CHECK: ret void
+
+#[autodiff_reverse(bar, Duplicated, Duplicated)]
+pub fn foo(r: &[f64; 10], res: &mut f64) {
+    let mut output = [0.0; 10];
+    output[0] = r[0];
+    output[1] = r[1] * r[2];
+    output[2] = r[4] * r[5];
+    output[3] = r[2] * r[6];
+    output[4] = r[1] * r[7];
+    output[5] = r[2] * r[8];
+    output[6] = r[1] * r[9];
+    output[7] = r[5] * r[6];
+    output[8] = r[5] * r[7];
+    output[9] = r[4] * r[8];
+    *res = output.iter().sum();
+}
+fn main() {
+    let inputs = Box::new([3.1; 10]);
+    let mut d_inputs = Box::new([0.0; 10]);
+    let mut res = Box::new(0.0);
+    let mut d_res = Box::new(1.0);
+
+    bar(&inputs, &mut d_inputs, &mut res, &mut d_res);
+    dbg!(&d_inputs);
+}