no_diff diagnostics improvement (#7655)

close #6286.

This PR is to improve the diagnostics for no_diff usage.

In a differentiable function, any calls to a non-diff function with constant arguments should not require no_diff attribute.
This PR adds this extra check at `checkAutoDiffUsages` where it checks the differentiability on IR.

In a differentiable method, we will force to use `[NoDiffThis]` attribute if there is access to non-differentiable `This` type. Once this access is detected we will report a warning to bring users attention that this access won't generate any derivative, they have to use `[NoDiffThis]` to suppress that warning.
This PR adds this check at type checking stage, because it's the easiest way to find out all the `This` accesses.

5 files changed, 95 insertions, 1 deletions

diff --git a/source/slang/diff.meta.slang b/source/slang/diff.meta.slang
index 542983049..b22d91595 100644
--- a/source/slang/diff.meta.slang
+++ b/source/slang/diff.meta.slang
@@ -992,10 +992,12 @@ struct DiffTensorView
 
     [BackwardDerivative(__load_backward)]
     [ForwardDerivative(__load_forward)]
+    [NoDiffThis]
     T load(uint i) { return primal.load(i); }
 
     [BackwardDerivative(__load_backward)]
     [ForwardDerivative(__load_forward)]
+    [NoDiffThis]
     __generic<let N : int>
     T load(vector<uint, N> i) { return primal.load(i); }
 
@@ -1026,10 +1028,12 @@ struct DiffTensorView
 
     [BackwardDerivative(__store_backward)]
     [ForwardDerivative(__store_forward)]
+    [NoDiffThis]
     void store(uint x, T val) { primal.store(x, val); }
 
     [BackwardDerivative(__store_backward)]
     [ForwardDerivative(__store_forward)]
+    [NoDiffThis]
     __generic<let N : int>
     void store(vector<uint, N> x, T val) { primal.store(x, val); }
 
@@ -1135,10 +1139,12 @@ struct DiffTensorView
 
     [BackwardDerivative(__loadOnce_backward)]
     [ForwardDerivative(__loadOnce_forward)]
+    [NoDiffThis]
     T loadOnce(uint i) { return primal.load(i); }
 
     [BackwardDerivative(__loadOnce_backward)]
     [ForwardDerivative(__loadOnce_forward)]
+    [NoDiffThis]
     __generic<let N : int>
     T loadOnce(vector<uint, N> i) { return primal.load(i); }
 
@@ -1168,10 +1174,12 @@ struct DiffTensorView
 
     [BackwardDerivative(__storeOnce_backward)]
     [ForwardDerivative(__storeOnce_forward)]
+    [NoDiffThis]
     void storeOnce(uint x, T val) { primal.store(x, val); }
 
     [BackwardDerivative(__storeOnce_backward)]
     [ForwardDerivative(__storeOnce_forward)]
+    [NoDiffThis]
     __generic<let N : int>
     void storeOnce(vector<uint, N> x, T val) { primal.store(x, val); }
 
diff --git a/source/slang/slang-check-expr.cpp b/source/slang/slang-check-expr.cpp
index b28b458da..c7e58a888 100644
--- a/source/slang/slang-check-expr.cpp
+++ b/source/slang/slang-check-expr.cpp
@@ -1632,6 +1632,31 @@ void SemanticsVisitor::maybeRegisterDifferentiableTypeImplRecursive(ASTBuilder*
     }
 }
 
+// This checks that if a differentiable function access a non-diff type "This", in such case we
+// want to provide a non-error diagnostic to the user to notify that there could be an unexpected
+// behavior because every member access will not have derivative computed for it. User can use
+// [NoDiffThis] to clarify that this is intended.
+void SemanticsVisitor::maybeCheckMissingNoDiffThis(Expr* expr)
+{
+    if (auto memberExpr = as<MemberExpr>(expr))
+    {
+        auto thisExpr = as<ThisExpr>(memberExpr->baseExpression);
+        if (thisExpr && isTypeDifferentiable(memberExpr->type.type))
+        {
+            if (isTypeDifferentiable(calcThisType(thisExpr->type.type)) ||
+                this->m_parentFunc->findModifier<NoDiffThisAttribute>())
+            {
+                return;
+            }
+
+            getSink()->diagnose(
+                memberExpr->loc,
+                Diagnostics::noDerivativeOnNonDifferentiableThisType,
+                memberExpr->declRef.getDecl(),
+                this->m_parentFunc);
+        }
+    }
+}
 
 Expr* SemanticsVisitor::CheckTerm(Expr* term)
 {
@@ -1649,7 +1674,13 @@ Expr* SemanticsVisitor::CheckTerm(Expr* term)
     if (this->m_parentFunc && this->m_parentFunc->findModifier<DifferentiableAttribute>())
     {
         maybeRegisterDifferentiableType(getASTBuilder(), checkedTerm->type.type);
+
+        if (!this->m_parentFunc->findModifier<TreatAsDifferentiableAttribute>())
+        {
+            maybeCheckMissingNoDiffThis(checkedTerm);
+        }
     }
+
     return checkedTerm;
 }
 
diff --git a/source/slang/slang-check-impl.h b/source/slang/slang-check-impl.h
index 4a6ccfe17..75d0bfc90 100644
--- a/source/slang/slang-check-impl.h
+++ b/source/slang/slang-check-impl.h
@@ -2041,6 +2041,8 @@ public:
     // Check and register a type if it is differentiable.
     void maybeRegisterDifferentiableType(ASTBuilder* builder, Type* type);
 
+    void maybeCheckMissingNoDiffThis(Expr* expr);
+
     // Find the default implementation of an interface requirement,
     // and insert it to the witness table, if it exists.
     bool findDefaultInterfaceImpl(
diff --git a/source/slang/slang-diagnostic-defs.h b/source/slang/slang-diagnostic-defs.h
index bf0e91150..249bc4f88 100644
--- a/source/slang/slang-diagnostic-defs.h
+++ b/source/slang/slang-diagnostic-defs.h
@@ -1399,6 +1399,15 @@ DIAGNOSTIC(
     primalSubstituteTargetMustHaveHigherDifferentiabilityLevel,
     "primal substitute function for differentiable method must also be differentiable. Use "
     "[Differentiable] or [TreatAsDifferentiable] (for empty derivatives)")
+DIAGNOSTIC(
+    31159,
+    Warning,
+    noDerivativeOnNonDifferentiableThisType,
+    "There is no derivative calculated for member '$0' because the parent struct is not "
+    "differentiable. "
+    "If this is intended, consider using [NoDiffThis] on the function '$1' to suppress this "
+    "warning. Alternatively, users can mark the parent struct as [Differentiable] to propagate "
+    "derivatives.")
 
 DIAGNOSTIC(31200, Warning, deprecatedUsage, "$0 has been deprecated: $1")
 DIAGNOSTIC(31201, Error, modifierNotAllowed, "modifier '$0' is not allowed here.")
diff --git a/source/slang/slang-ir-check-differentiability.cpp b/source/slang/slang-ir-check-differentiability.cpp
index e9cb7e1f1..d83d7bb76 100644
--- a/source/slang/slang-ir-check-differentiability.cpp
+++ b/source/slang/slang-ir-check-differentiability.cpp
@@ -82,6 +82,49 @@ public:
             callInst->findDecoration<IRDifferentiableCallDecoration>());
     }
 
+    // If a function call takes all literals as arguments, it will implies that this function will
+    // not be expected to any gradients, in this case, this call should be treated as no_diff even
+    // there is no 'no_diff' decorated on it explicitly. In the actual check, we only need to check
+    // the argument corresponding to the differentiable parameters, because non-differentiable
+    // parameter are not expected to produce any gradients anyway.
+    bool shouldCallImpliesNoDiff(
+        DifferentiableTypeConformanceContext& diffTypeContext,
+        IRCall* callInst)
+    {
+        if (shouldTreatCallAsDifferentiable(callInst))
+        {
+            return true;
+        }
+
+        auto calleeFuncType = as<IRFuncType>(callInst->getCallee()->getFullType());
+        if (!calleeFuncType)
+            return false;
+
+        SLANG_RELEASE_ASSERT(calleeFuncType->getParamCount() == callInst->getArgCount());
+
+        bool doesImplyNoDiff = true;
+        UInt paramIndex = 0;
+        for (auto paramType : calleeFuncType->getParamTypes())
+        {
+            if (isDifferentiableType(diffTypeContext, paramType))
+            {
+                auto arg = callInst->getArg(paramIndex);
+                if (!as<IRConstant>(arg))
+                {
+                    doesImplyNoDiff = false;
+                }
+            }
+            paramIndex++;
+        }
+
+        if (doesImplyNoDiff)
+        {
+            IRBuilder irBuilder(callInst->getModule());
+            irBuilder.addDecoration(callInst, kIROp_TreatCallAsDifferentiableDecoration);
+        }
+        return doesImplyNoDiff;
+    }
+
     bool isDifferentiableFunc(IRInst* func, DifferentiableLevel level)
     {
         switch (func->getOp())
@@ -497,7 +540,8 @@ public:
                         // No need to fail here if the function is no_diff in
                         // both inputs and all outputs, this is equivalent of
                         // inserting no_diff on this inst.
-                        if (!isNeverDiffFuncType(cast<IRFuncType>(callee->getDataType())))
+                        if (!isNeverDiffFuncType(cast<IRFuncType>(callee->getDataType())) &&
+                            !shouldCallImpliesNoDiff(diffTypeContext, call))
                         {
                             sink->diagnose(
                                 inst,