Handle the case where the parent if-else region's after-block is unreachable. (shader-slang#3241)

Also added a test for this.

Co-authored-by: Yong He <yonghe@outlook.com>

Author: saipraveenb25

source/slang/slang-ir-autodiff-cfg-norm.cpp

@@ -356,6 +356,30 @@ struct CFGNormalizationPass
                     //
                     afterBaseRegion = true;
 
+                    // One case we do check for is if the after block is 'unreachable'
+                    // i.e. the terminator is an `unreachable` instruction.
+                    // In this case, we can safely assume that the after block does not
+                    // have anything to execute. Further, we need to re-wire the
+                    // previously unreachable block to the parent break block.
+                    // Note that this operation is safe because if the after block was 
+                    // originally unreachable, all potential paths to it must have 
+                    // broken out of the region.
+                    // 
+                    if (auto unreachInst = as<IRUnreachable>(afterBlock->getTerminator()))
+                    {
+                        // Link it to the parentAfterBlock.
+                        builder.setInsertInto(afterBlock);
+                        unreachInst->removeAndDeallocate();
+
+                        builder.emitBranch(parentAfterBlock);
+
+                        // We can now safely assume that the after block is empty.
+                        // Set 'afterBaseRegion' to false, which should lead the rest
+                        // of the logic to avoid splitting the after block
+                        // 
+                        afterBaseRegion = false;
+                    }
+
                     // Do we need to split the after region?
                     if (afterBaseRegion && afterBreakRegion)
                     {

tests/autodiff/control-flow-bug.slang

@@ -0,0 +1,60 @@
+//TEST(compute):COMPARE_COMPUTE_EX:-slang -compute -shaderobj -output-using-type
+//TEST(compute, vulkan):COMPARE_COMPUTE_EX:-vk -compute -shaderobj -output-using-type
+
+//TEST_INPUT:ubuffer(data=[0 0 0 0 0], stride=4):out,name=outputBuffer
+RWStructuredBuffer<float> outputBuffer;
+
+[Differentiable] [PreferRecompute]
+float3 fetch(float2 uv)
+{
+    if (uv.x > 0.5f)
+    {
+        if (uv.x > 0.7f)
+            return float3(2.) * uv.y;
+        else
+            return float3(1.) * uv.y;
+    }
+    else
+    {
+        if (uv.x > 0.3f)
+            return float3(4.) * uv.y;
+        else
+            return float3(3.) * uv.y;
+    }
+}
+
+[numthreads(1, 1, 1)]
+void computeMain(uint3 dispatchThreadID: SV_DispatchThreadID)
+{
+    float2 uv = (float2)dispatchThreadID.xy / float2(512, 512);
+    float3 color = fetch(uv);
+    outputBuffer[0] = color.x; // Expect: 0.0
+
+    {
+        DifferentialPair<float2> dpuv = diffPair(float2(0.6f));
+        bwd_diff(fetch)(dpuv, float3(1.f));
+
+        outputBuffer[1] = dpuv.d.y; // Expect: 1.0 * 3 = 3
+    }
+
+    {
+        DifferentialPair<float2> dpuv = diffPair(float2(0.8f));
+        bwd_diff(fetch)(dpuv, float3(1.f));
+
+        outputBuffer[2] = dpuv.d.y; // Expect: 2.0 * 3 = 6
+    }
+
+    {
+        DifferentialPair<float2> dpuv = diffPair(float2(0.1f));
+        bwd_diff(fetch)(dpuv, float3(1.f));
+ 
+        outputBuffer[3] = dpuv.d.y; // Expect: 3.0 * 3 = 9
+    }
+
+    {
+        DifferentialPair<float2> dpuv = diffPair(float2(0.4f));
+        bwd_diff(fetch)(dpuv, float3(1.f));
+
+        outputBuffer[4] = dpuv.d.y; // Expect: 4.0 * 3 = 12
+    }
+}

tests/autodiff/control-flow-bug.slang.expected.txt

@@ -0,0 +1,6 @@
+type: float
+0.000000
+3.000000
+6.000000
+9.000000
+12.000000