Fix autodiff empty ret regression

compiler/rustc_codegen_llvm/src/builder/autodiff.rs

@@ -378,5 +378,12 @@ pub(crate) fn generate_enzyme_call<'ll, 'tcx>(
 
     let call = builder.call(enzyme_ty, None, None, ad_fn, &args, None, None);
 
-    builder.store_to_place(call, dest.val);
+    let fn_ret_ty = builder.cx.val_ty(call);
+    if fn_ret_ty != builder.cx.type_void() && fn_ret_ty != builder.cx.type_struct(&[], false) {
+        // If we return void or an empty struct, then our caller (due to how we generated it)
+        // does not expect a return value. As such, we have no pointer (or place) into which
+        // we could store our value, and would store into an undef, which would cause UB.
+        // As such, we just ignore the return value in those cases.
+        builder.store_to_place(call, dest.val);
+    }
 }

tests/codegen-llvm/autodiff/abi_handling.rs

@@ -1,7 +1,7 @@
 //@ revisions: debug release
 
-//@[debug] compile-flags: -Zautodiff=Enable -C opt-level=0 -Clto=fat
-//@[release] compile-flags: -Zautodiff=Enable -C opt-level=3 -Clto=fat
+//@[debug] compile-flags: -Zautodiff=Enable,NoTT -C opt-level=0 -Clto=fat
+//@[release] compile-flags: -Zautodiff=Enable,NoTT -C opt-level=3 -Clto=fat
 //@ no-prefer-dynamic
 //@ needs-enzyme
 

tests/codegen-llvm/autodiff/batched.rs

@@ -1,4 +1,4 @@
-//@ compile-flags: -Zautodiff=Enable -C opt-level=3  -Clto=fat
+//@ compile-flags: -Zautodiff=Enable,NoTT -C opt-level=3  -Clto=fat
 //@ no-prefer-dynamic
 //@ needs-enzyme
 //

tests/codegen-llvm/autodiff/scalar.rs

@@ -1,4 +1,4 @@
-//@ compile-flags: -Zautodiff=Enable -C opt-level=3  -Clto=fat
+//@ compile-flags: -Zautodiff=Enable,NoTT -C opt-level=3  -Clto=fat
 //@ no-prefer-dynamic
 //@ needs-enzyme
 #![feature(autodiff)]

tests/codegen-llvm/autodiff/sret.rs

@@ -1,4 +1,4 @@
-//@ compile-flags: -Zautodiff=Enable -C opt-level=3  -Clto=fat
+//@ compile-flags: -Zautodiff=Enable,NoTT -C opt-level=3  -Clto=fat
 //@ no-prefer-dynamic
 //@ needs-enzyme
 

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);
+}