Miscellaneous backward autodiff fixes. (#2665)

* Fix differentiable type registration

* Fix use of non-differentiable return value in a differentiable func.

* Fix use of primal inst that does not dominate the diff block.

* Fix primal inst hoisting, and add missing type legalization logic.

* Make `detach` defined on all differentiable T.

---------

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

1 files changed, 11 insertions, 0 deletions

diff --git a/source/slang/slang-ir-check-differentiability.cpp b/source/slang/slang-ir-check-differentiability.cpp
index c750b2d3d..1ee94e67e 100644
--- a/source/slang/slang-ir-check-differentiability.cpp
+++ b/source/slang/slang-ir-check-differentiability.cpp
@@ -353,6 +353,17 @@ public:
             auto loop = as<IRLoop>(block->getTerminator());
             if (!loop)
                 continue;
+            bool hasBackEdge = false;
+            for (auto use = loop->getTargetBlock()->firstUse; use; use = use->nextUse)
+            {
+                if (use->getUser() != loop)
+                {
+                    hasBackEdge = true;
+                    break;
+                }
+            }
+            if (!hasBackEdge)
+                continue;
             if (loop->findDecoration<IRLoopMaxItersDecoration>() || loop->findDecoration<IRForceUnrollDecoration>())
             {
                 // We are good.