Reverse-mode AD fixes for loops with non-trivial break region (#2671)

* Fix crash when applying autodiff to functions with no arguments

* Fixes for loops where the break region is non-trivial

* Minor fix

* Implement array legalization correctly.

* Fix array legalization.

---------

Co-authored-by: Yong He <yhe@nvidia.com>

2 files changed, 65 insertions, 0 deletions

diff --git a/tests/autodiff/reverse-more-loops.slang b/tests/autodiff/reverse-more-loops.slang
new file mode 100644
index 000000000..173caf963
--- /dev/null
+++ b/tests/autodiff/reverse-more-loops.slang
@@ -0,0 +1,60 @@
+//TEST(compute, vulkan):COMPARE_COMPUTE_EX:-vk -compute -shaderobj -output-using-type
+
+//TEST_INPUT:ubuffer(data=[0 0 0 0], stride=4):out,name=outputBuffer
+
+RWStructuredBuffer<float> outputBuffer;
+
+[BackwardDifferentiable]
+float bsdf()
+{
+    return 0.5;
+}
+
+[ForwardDerivativeOf(bsdf)]
+DifferentialPair<float> d_bsdf()
+{
+    return diffPair(0.5f, 1.0f);
+}
+
+[BackwardDerivativeOf(bsdf)]
+void d_bsdf(float dOut)
+{
+    outputBuffer[0] += dOut;
+}
+
+[BackwardDifferentiable]
+float tracePath()
+{
+    float thp = 1.0;
+    float L = 0.0;
+
+    uint depth = 0;
+
+    for (int i = 0; i < 3; ++i)
+    {
+        if (depth <= 2)
+        {
+            thp = thp * bsdf();
+
+            L = thp * 1.0;
+
+            if (depth >= 2) break;
+
+            depth = depth + 1;
+        }
+        else
+        {
+            L = 0.0;
+        }
+    }
+    
+    return L;
+}
+
+[numthreads(1, 1, 1)]
+void computeMain(uint3 dispatchThreadID: SV_DispatchThreadID)
+{
+    {
+        __bwd_diff(tracePath)(1.0); // Expect: 1.0 in outputBuffer[0]
+    }
+}
diff --git a/tests/autodiff/reverse-more-loops.slang.expected.txt b/tests/autodiff/reverse-more-loops.slang.expected.txt
new file mode 100644
index 000000000..2bf164814
--- /dev/null
+++ b/tests/autodiff/reverse-more-loops.slang.expected.txt
@@ -0,0 +1,5 @@
+type: float
+0.750000
+0.000000
+0.000000
+0.000000