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

3 files changed, 45 insertions, 0 deletions

diff --git a/tests/codegen/autodiffv.rs b/tests/codegen/autodiff/batched.rs
index e0047116405..e0047116405 100644
--- a/tests/codegen/autodiffv.rs
+++ b/tests/codegen/autodiff/batched.rs
diff --git a/tests/codegen/autodiff.rs b/tests/codegen/autodiff/scalar.rs
index 85358f5fcb6..85358f5fcb6 100644
--- a/tests/codegen/autodiff.rs
+++ b/tests/codegen/autodiff/scalar.rs
diff --git a/tests/codegen/autodiff/sret.rs b/tests/codegen/autodiff/sret.rs
new file mode 100644
index 00000000000..5ead90041ed
--- /dev/null
+++ b/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);
+}