Added support for simple while loops (#2667)

* Added support for simple while loops

* Fix support for while loops by changing logic to grab the loop update block

4 files changed, 81 insertions, 12 deletions

diff --git a/source/slang/slang-ir-autodiff-cfg-norm.cpp b/source/slang/slang-ir-autodiff-cfg-norm.cpp
index 4014473ea..d64c6d1f6 100644
--- a/source/slang/slang-ir-autodiff-cfg-norm.cpp
+++ b/source/slang/slang-ir-autodiff-cfg-norm.cpp
@@ -473,14 +473,18 @@ struct CFGNormalizationPass
                     loopEndPoint = falseEndPoint;
                     isLoopOnTrueSide = false;
                 }
+
+                // Right now, we only support loops where the loop is on the true side of
+                // the condition. If we ever encounter the other case, fill in logic to 
+                // flip the condition.
+                //
+                SLANG_RELEASE_ASSERT(isLoopOnTrueSide);
                 
+                // Expect atleast one basic block (other than the condition block), in
+                // the loop.
+                //
                 SLANG_RELEASE_ASSERT(loopEndPoint.exitBlock);
-
-                // Special case.. the if-else of a loop needs it's
-                // after block to be pointing at the last block before
-                // it loops back to the if-else.
-                // 
-                // ifElse->afterBlock.set(loopEndPoint.exitBlock);
+                SLANG_RELEASE_ASSERT(!loopEndPoint.isRegionEmpty);
 
                 // Does the loop endpoint have both 'break' and 'base'
                 // control flows?
diff --git a/source/slang/slang-ir-autodiff-unzip.h b/source/slang/slang-ir-autodiff-unzip.h
index 944df2c81..2ccb8d8e2 100644
--- a/source/slang/slang-ir-autodiff-unzip.h
+++ b/source/slang/slang-ir-autodiff-unzip.h
@@ -277,14 +277,26 @@ struct DiffUnzipPass
 
     IRBlock* getUpdateBlock(IndexedRegion* region)
     {
-        // TODO: What if the 'continue' region has multiple
-        // blocks?
-        // We ideally want the _last_ block before control loops back.
+        auto initBlock = getInitializerBlock(region);
+
+        auto condBlock = region->firstBlock;
+
+        IRBlock* lastLoopBlock = nullptr;
+
+        for (auto predecessor : condBlock->getPredecessors())
+        {
+            if (predecessor != initBlock)
+                lastLoopBlock = predecessor;
+        }
+
+        // Should find atleast one predecessor that is _not_ the 
+        // init block (that contains the loop info). This 
+        // predecessor would be the last block in the loop
+        // before looping back to the condition.
         // 
-        SLANG_RELEASE_ASSERT(as<IRUnconditionalBranch>(
-            region->continueBlock->getTerminator())->getTargetBlock() == region->firstBlock);
+        SLANG_RELEASE_ASSERT(lastLoopBlock);
 
-        return region->continueBlock;
+        return lastLoopBlock;
     }
 
     IRBlock* getFirstLoopBodyBlock(IndexedRegion* region)
diff --git a/tests/autodiff/reverse-while-loop.slang b/tests/autodiff/reverse-while-loop.slang
new file mode 100644
index 000000000..c8d2c542a
--- /dev/null
+++ b/tests/autodiff/reverse-while-loop.slang
@@ -0,0 +1,47 @@
+//TEST(compute):COMPARE_COMPUTE_EX:-slang -compute -shaderobj -output-using-type
+//TEST(compute, vulkan):COMPARE_COMPUTE_EX:-vk -compute -shaderobj -output-using-type
+//TEST(compute):COMPARE_COMPUTE_EX:-cpu -compute -output-using-type -shaderobj
+
+//TEST_INPUT:ubuffer(data=[0 0 0 0 0], stride=4):out,name=outputBuffer
+RWStructuredBuffer<float> outputBuffer;
+
+typedef DifferentialPair<float> dpfloat;
+typedef float.Differential dfloat;
+
+[BackwardDifferentiable]
+float test_simple_while(float y)
+{
+    float t = y;
+
+    bool keepGoing = true;
+    int i = 2;
+
+    [MaxIters(3)]
+    while (keepGoing)
+    {
+        i++;
+        t = t * t;
+
+        keepGoing = (i < 5);
+    }
+
+    return t;
+}
+
+[numthreads(1, 1, 1)]
+void computeMain(uint3 dispatchThreadID : SV_DispatchThreadID)
+{
+    {
+        dpfloat dpa = dpfloat(1.0, 0.0);
+
+        __bwd_diff(test_simple_while)(dpa, 1.0f);
+        outputBuffer[0] = dpa.d; // Expect: 8.0
+    }
+
+    {
+        dpfloat dpa = dpfloat(0.4, 0.0);
+
+        __bwd_diff(test_simple_while)(dpa, 1.0f);
+        outputBuffer[1] = dpa.d; // Expect: 0.0131072
+    }
+}
diff --git a/tests/autodiff/reverse-while-loop.slang.expected.txt b/tests/autodiff/reverse-while-loop.slang.expected.txt
new file mode 100644
index 000000000..76b7cf779
--- /dev/null
+++ b/tests/autodiff/reverse-while-loop.slang.expected.txt
@@ -0,0 +1,6 @@
+type: float
+8.000000
+0.013107
+0.000000
+0.000000
+0.000000