From d40c143eb4f19f1dfd0d0dcf9b718be6e495ca27 Mon Sep 17 00:00:00 2001
From: Sai Praveen Bangaru <31557731+saipraveenb25@users.noreply.github.com>
Date: Tue, 27 Aug 2024 15:00:43 -0400
Subject: Explicitly detach derivative when forming a non-differentiable struct
 out of differentiable args (#4901)

* Explicitly detach derivative when forming a non-differentiable struct out of differentiable args

This fixes an issue where initializer lists get optimized out and lose information about non-differentiability.

There are 2 places where this could have been fixed:
1. When coercing initializer-list exprs, we can check for non-differentiable aggregate types and use a detach derivative on all the args.
2. Add an extra case in the peephole optimization step that adds detach-derivative when simplifying a make-struct of a non-differentiable type.

Even though solution 2 is more elegant, this PR goes with solution 1 simply to avoid having to use a differentiable-type-conformance-context that is used in the auto-diff IR passes to check for differentiability.

* Change test name + add expected vals
---
 source/slang/slang-ast-expr.h           |  8 ++++++++
 source/slang/slang-check-conversion.cpp | 21 +++++++++++++++++++++
 source/slang/slang-check-expr.cpp       |  7 +++++++
 source/slang/slang-check-impl.h         |  2 ++
 source/slang/slang-lower-to-ir.cpp      | 10 ++++++++++
 5 files changed, 48 insertions(+)

(limited to 'source')

diff --git a/source/slang/slang-ast-expr.h b/source/slang/slang-ast-expr.h
index c07f7f5b9..f9b8831f1 100644
--- a/source/slang/slang-ast-expr.h
+++ b/source/slang/slang-ast-expr.h
@@ -544,6 +544,13 @@ class OpenRefExpr : public Expr
     Expr* innerExpr = nullptr;
 };
 
+class DetachExpr : public Expr
+{
+    SLANG_AST_CLASS(DetachExpr)
+
+    Expr* inner = nullptr;
+};
+
     /// Base class for higher-order function application
     /// Eg: foo(fn) where fn is a function expression.
     ///
@@ -563,6 +570,7 @@ class DifferentiateExpr : public HigherOrderInvokeExpr
 {
     SLANG_ABSTRACT_AST_CLASS(DifferentiateExpr)
 };
+
     /// An expression of the form `__fwd_diff(fn)` to access the 
     /// forward-mode derivative version of the function `fn`
     ///
diff --git a/source/slang/slang-check-conversion.cpp b/source/slang/slang-check-conversion.cpp
index c135ada8d..85040dc55 100644
--- a/source/slang/slang-check-conversion.cpp
+++ b/source/slang/slang-check-conversion.cpp
@@ -498,6 +498,27 @@ namespace Slang
             toInitializerListExpr->type = QualType(toType);
             toInitializerListExpr->args = coercedArgs;
 
+            // Wrap initalizer list args if we're creating a non-differentiable struct within a 
+            // differentiable function.
+            // 
+            if (auto func = getParentFuncOfVisitor())
+            {
+                if (func->findModifier<DifferentiableAttribute>() && 
+                    !isTypeDifferentiable(toType))
+                {
+                    for (auto &arg : toInitializerListExpr->args)
+                    {
+                        if (isTypeDifferentiable(arg->type.type))
+                        {
+                            auto detachedArg = m_astBuilder->create<DetachExpr>();
+                            detachedArg->inner = arg;
+                            detachedArg->type = arg->type;
+                            arg = detachedArg;
+                        }
+                    }
+                }
+            }
+
             *outToExpr = toInitializerListExpr;
         }
 
diff --git a/source/slang/slang-check-expr.cpp b/source/slang/slang-check-expr.cpp
index 4d36299bb..3072c3257 100644
--- a/source/slang/slang-check-expr.cpp
+++ b/source/slang/slang-check-expr.cpp
@@ -3242,6 +3242,13 @@ namespace Slang
         return expr;
     }
 
+    Expr* SemanticsExprVisitor::visitDetachExpr(DetachExpr* expr)
+    {
+        expr->inner = CheckTerm(expr->inner);
+        expr->type = expr->inner->type;
+        return expr;
+    }
+
 
     static bool _isSizeOfType(Type* type)
     {
diff --git a/source/slang/slang-check-impl.h b/source/slang/slang-check-impl.h
index 82ac8de59..f997abc57 100644
--- a/source/slang/slang-check-impl.h
+++ b/source/slang/slang-check-impl.h
@@ -2806,6 +2806,8 @@ namespace Slang
 
         Expr* visitDefaultConstructExpr(DefaultConstructExpr* expr);
 
+        Expr* visitDetachExpr(DetachExpr* expr);
+
         Expr* visitSPIRVAsmExpr(SPIRVAsmExpr*);
 
             /// Perform semantic checking on a `modifier` that is being applied to the given `type`
diff --git a/source/slang/slang-lower-to-ir.cpp b/source/slang/slang-lower-to-ir.cpp
index 02c4fae68..b9d7a898f 100644
--- a/source/slang/slang-lower-to-ir.cpp
+++ b/source/slang/slang-lower-to-ir.cpp
@@ -3929,6 +3929,16 @@ struct ExprLoweringVisitorBase : public ExprVisitor<Derived, LoweredValInfo>
                 baseVal.val));
     }
 
+    LoweredValInfo visitDetachExpr(DetachExpr* expr)
+    {
+        auto baseVal = lowerRValueExpr(context, expr->inner);
+
+        return LoweredValInfo::simple(
+            getBuilder()->emitDetachDerivative(
+                lowerType(context, expr->type),
+                getSimpleVal(context, baseVal)));
+    }
+
     LoweredValInfo visitPrimalSubstituteExpr(PrimalSubstituteExpr* expr)
     {
         auto baseVal = lowerSubExpr(expr->baseFunction);
-- 
cgit v1.2.3