Rollup merge of #139465 - EnzymeAD:autodiff-sret, r=oli-obk

add sret handling for scalar autodiff

r? `@oli-obk`

Fixing one of the todo's which I left in my previous batching PR.
This one handles sret for scalar autodiff.  `sret` mostly shows up when we try to return a lot of scalar floats.
People often start testing autodiff which toy functions which just use a few scalars as inputs and outputs, and those were the most likely to be affected by this issue. So this fix should make learning/teaching hopefully a bit easier.

Tracking:

- https://github.com/rust-lang/rust/issues/124509

compiler/rustc_ast/src/expand/autodiff_attrs.rs

@@ -92,6 +92,12 @@ pub struct AutoDiffAttrs {
     pub input_activity: Vec<DiffActivity>,
 }
 
+impl AutoDiffAttrs {
+    pub fn has_primal_ret(&self) -> bool {
+        matches!(self.ret_activity, DiffActivity::Active | DiffActivity::Dual)
+    }
+}
+
 impl DiffMode {
     pub fn is_rev(&self) -> bool {
         matches!(self, DiffMode::Reverse)

compiler/rustc_codegen_llvm/src/builder/autodiff.rs

@@ -201,7 +201,23 @@ fn compute_enzyme_fn_ty<'ll>(
         }
 
         if attrs.width == 1 {
-            todo!("Handle sret for scalar ad");
+            // Enzyme returns a struct of style:
+            // `{ original_ret(if requested), float, float, ... }`
+            let mut struct_elements = vec![];
+            if attrs.has_primal_ret() {
+                struct_elements.push(inner_ret_ty);
+            }
+            // Next, we push the list of active floats, since they will be lowered to `enzyme_out`,
+            // and therefore part of the return struct.
+            let param_tys = cx.func_params_types(fn_ty);
+            for (act, param_ty) in attrs.input_activity.iter().zip(param_tys) {
+                if matches!(act, DiffActivity::Active) {
+                    // Now find the float type at position i based on the fn_ty,
+                    // to know what (f16/f32/f64/...) to add to the struct.
+                    struct_elements.push(param_ty);
+                }
+            }
+            ret_ty = cx.type_struct(&struct_elements, false);
         } else {
             // First we check if we also have to deal with the primal return.
             match attrs.mode {
@@ -388,7 +404,11 @@ fn generate_enzyme_call<'ll>(
                 // now store the result of the enzyme call into the sret pointer.
                 let sret_ptr = outer_args[0];
                 let call_ty = cx.val_ty(call);
-                assert_eq!(cx.type_kind(call_ty), TypeKind::Array);
+                if attrs.width == 1 {
+                    assert_eq!(cx.type_kind(call_ty), TypeKind::Struct);
+                } else {
+                    assert_eq!(cx.type_kind(call_ty), TypeKind::Array);
+                }
                 llvm::LLVMBuildStore(&builder.llbuilder, call, sret_ptr);
             }
             builder.ret_void();

tests/codegen/autodiff/sret.rs

@@ -0,0 +1,45 @@
+//@ compile-flags: -Zautodiff=Enable -C opt-level=3  -Clto=fat
+//@ no-prefer-dynamic
+//@ needs-enzyme
+
+// This test is almost identical to the scalar.rs one,
+// but we intentionally add a few more floats.
+// `df` would ret `{ f64, f32, f32 }`, but is lowered as an sret.
+// We therefore use this test to verify some of our sret handling.
+
+#![feature(autodiff)]
+
+use std::autodiff::autodiff;
+
+#[no_mangle]
+#[autodiff(df, Reverse, Active, Active, Active)]
+fn primal(x: f32, y: f32) -> f64 {
+    (x * x * y) as f64
+}
+
+// CHECK:define internal fastcc void @_ZN4sret2df17h93be4316dd8ea006E(ptr dead_on_unwind noalias nocapture noundef nonnull writable writeonly align 8 dereferenceable(16) initializes((0, 16)) %_0, float noundef %x, float noundef %y)
+// CHECK-NEXT:start:
+// CHECK-NEXT:  %0 = tail call fastcc { double, float, float } @diffeprimal(float %x, float %y)
+// CHECK-NEXT:  %.elt = extractvalue { double, float, float } %0, 0
+// CHECK-NEXT:  store double %.elt, ptr %_0, align 8
+// CHECK-NEXT:  %_0.repack1 = getelementptr inbounds nuw i8, ptr %_0, i64 8
+// CHECK-NEXT:  %.elt2 = extractvalue { double, float, float } %0, 1
+// CHECK-NEXT:  store float %.elt2, ptr %_0.repack1, align 8
+// CHECK-NEXT:  %_0.repack3 = getelementptr inbounds nuw i8, ptr %_0, i64 12
+// CHECK-NEXT:  %.elt4 = extractvalue { double, float, float } %0, 2
+// CHECK-NEXT:  store float %.elt4, ptr %_0.repack3, align 4
+// CHECK-NEXT:  ret void
+// CHECK-NEXT:}
+
+fn main() {
+    let x = std::hint::black_box(3.0);
+    let y = std::hint::black_box(2.5);
+    let scalar = std::hint::black_box(1.0);
+    let (r1, r2, r3) = df(x, y, scalar);
+    // 3*3*1.5 = 22.5
+    assert_eq!(r1, 22.5);
+    // 2*x*y = 2*3*2.5 = 15.0
+    assert_eq!(r2, 15.0);
+    // x*x*1 = 3*3 = 9
+    assert_eq!(r3, 9.0);
+}