From a670bafc121c20168624f70a388dbe8556402c7f Mon Sep 17 00:00:00 2001
From: kaizhangNV <149626564+kaizhangNV@users.noreply.github.com>
Date: Wed, 9 Jul 2025 11:25:29 -0500
Subject: 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.
---
 source/slang/slang-ir-check-differentiability.cpp | 46 ++++++++++++++++++++++-
 1 file changed, 45 insertions(+), 1 deletion(-)

(limited to 'source/slang/slang-ir-check-differentiability.cpp')

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,
-- 
cgit v1.2.3