Treat global variables and parameters as non-differentiable when checking derivative data-flow (#4526)

Global parameters are by-default not differentiable (even if they are of a differentiable type), because our auto-diff passes do not touch anything outside of function bodies.

The solution is to use wrapper objects with differentiable getter/setter methods (and we should provide a few such objects in the stdlib).

Fixes: #3289

This is a potentially breaking change: User code that was previously working with global variables of a differentiable type will now throw an error (previously the gradient would be dropped without warning). The solution is to use `detach()` to keep same behavior as before or rewrite the access using differentiable getter/setter methods.

1 files changed, 32 insertions, 0 deletions

diff --git a/tests/autodiff/warn-on-shared-memory-access.slang b/tests/autodiff/warn-on-shared-memory-access.slang
new file mode 100644
index 000000000..bccf8b1fa
--- /dev/null
+++ b/tests/autodiff/warn-on-shared-memory-access.slang
@@ -0,0 +1,32 @@
+//TEST:SIMPLE(filecheck=CHECK): -target hlsl -line-directive-mode none
+
+//TEST_INPUT:ubuffer(data=[0 0 0 0], stride=4):out,name=outputBuffer
+RWStructuredBuffer<float> outputBuffer;
+
+groupshared float s_shared;
+
+[BackwardDifferentiable]
+float get_thread_5_value(float v, uint group_thread_id)
+{
+    if(group_thread_id == 5)
+    {
+        // Using 'detach(v)' makes the error go away
+        s_shared = v;
+        // CHECK: tests/autodiff/warn-on-shared-memory-access.slang(14): error 41024: derivative is lost during assignment to non-differentiable location, use 'detach()' to clarify intention.
+        // CHECK: s_shared = v;
+        // CHECK:          ^
+    }
+    GroupMemoryBarrierWithGroupSync();
+    return s_shared;
+}
+
+[shader("compute")]
+[numthreads(128, 1, 1)]
+void computeMain(uint3 group_thread_id: SV_GroupThreadID, uint3 dispatch_thread_id: SV_DispatchThreadID)
+{
+    DifferentialPair<float> value = diffPair(3.f, 0.f);
+
+    bwd_diff(get_thread_5_value)(value, group_thread_id.x, 1.0f);
+
+    outputBuffer[dispatch_thread_id.x] = value.d;
+}
\ No newline at end of file