Fix diagnostics for [PreferRecompute] (shader-slang#5159)

* Fix diagnostics for [PreferRecompute]

* Update dont-warn-on-simple-prefer-recompute.slang

* Update slang-ir-autodiff.cpp

* Update dont-warn-on-simple-prefer-recompute.slang

* Update warn-on-prefer-recompute-side-effects.slang

Author: saipraveenb25

source/slang/slang-diagnostic-defs.h

@@ -790,7 +790,7 @@ DIAGNOSTIC(41904, Error, unableToAlignOf, "alignof could not be performed for ty
 
 DIAGNOSTIC(42001, Error, invalidUseOfTorchTensorTypeInDeviceFunc, "invalid use of TorchTensor type in device/kernel functions. use `TensorView` instead.")
 
-DIAGNOSTIC(42050, Warning, potentialIssuesWithPreferRecomputeOnSideEffectMethod, "$0 has [PreferRecompute] and may have side effects. side effects may execute multiple times. use [PreferRecompute(SideEffectBehavior.Allow)], or mark function with [NoSideEffect]")
+DIAGNOSTIC(42050, Warning, potentialIssuesWithPreferRecomputeOnSideEffectMethod, "$0 has [PreferRecompute] and may have side effects. side effects may execute multiple times. use [PreferRecompute(SideEffectBehavior.Allow)], or mark function with [__NoSideEffect]")
 
 DIAGNOSTIC(45001, Error, unresolvedSymbol, "unresolved external symbol '$0'.")
 

source/slang/slang-ir-autodiff.cpp

@@ -2416,9 +2416,15 @@ void checkAutodiffPatterns(
         if (auto func = as<IRFunc>(inst))
         {
             if (func->sourceLoc.isValid() && // Don't diagnose for synthesized functions
-                func->findDecoration<IRPreferRecomputeDecoration>() &&
-                !func->findDecoration<IRNoSideEffectDecoration>())
+                func->findDecoration<IRPreferRecomputeDecoration>())
             {
+                // If we don't have any side-effect behavior, we should warn (note: read-none is a stronger 
+                // guarantee than no-side-effect)
+                //
+                if (func->findDecoration<IRNoSideEffectDecoration>() ||
+                    func->findDecoration<IRReadNoneDecoration>())
+                    continue;
+
                 auto preferRecomputeDecor = func->findDecoration<IRPreferRecomputeDecoration>();
                 auto sideEffectBehavior = as<IRIntLit>(preferRecomputeDecor->getOperand(0))->getValue();
 

tests/autodiff/dont-warn-on-simple-prefer-recompute.slang

@@ -0,0 +1,26 @@
+//TEST:SIMPLE(filecheck=CHECK): -target hlsl -line-directive-mode none -stage compute -entry computeMain
+
+//TEST_INPUT:ubuffer(data=[0 0 0 0], stride=4):out,name=outputBuffer
+RWStructuredBuffer<float> outputBuffer;
+
+[BackwardDifferentiable]
+[PreferRecompute]
+float comp(float a, float b)
+{
+    // CHECK: standard error = {
+    // CHECK-NEXT: }
+    return a * b;
+}
+
+
+[shader("compute")]
+[numthreads(128, 1, 1)]
+void computeMain(uint3 group_thread_id: SV_GroupThreadID, uint3 dispatch_thread_id: SV_DispatchThreadID)
+{
+    DifferentialPair<float> value1 = diffPair(3.f, 0.f);
+    DifferentialPair<float> value2 = diffPair(3.f, 0.f);
+
+    bwd_diff(comp)(value1, value2, 1.f);
+
+    outputBuffer[dispatch_thread_id.x] = value1.d;
+}

tests/autodiff/warn-on-prefer-recompute-side-effects.slang

@@ -12,7 +12,7 @@ float get_thread_5_value(float v, uint group_thread_id)
     if(group_thread_id == 5)
     {
         s_shared = detach(v);
-        // CHECK: tests/autodiff/warn-on-prefer-recompute-side-effects.slang(10): warning 42050: get_thread_5_value has [PreferRecompute] and may have side effects. side effects may execute multiple times. use [PreferRecompute(SideEffectBehavior.Allow)], or mark function with [NoSideEffect]
+        // CHECK: tests/autodiff/warn-on-prefer-recompute-side-effects.slang(10): warning 42050: get_thread_5_value has [PreferRecompute] and may have side effects. side effects may execute multiple times. use [PreferRecompute(SideEffectBehavior.Allow)], or mark function with [__NoSideEffect]
         // CHECK: float get_thread_5_value(float v, uint group_thread_id)
         // CHECK:       ^~~~~~~~~~~~~~~~~~
     }