Fix loop inversion issue that caused ordinary blocks with multiple predecessors (#3211)

* Add test case for while loop

* Fix loop inversion issue that caused ordinary blocks with multiple predecessors

The original version can leave the CFG in an invalid state with `e4` not being a break block or merge point, but having multiple predecessors.

The updated version creates a separate jump block for each break instruction to avoid this issue.

* Fixup tests

2 files changed, 72 insertions, 0 deletions

diff --git a/tests/autodiff/reverse-while-loop-3.slang b/tests/autodiff/reverse-while-loop-3.slang
new file mode 100644
index 000000000..67f030030
--- /dev/null
+++ b/tests/autodiff/reverse-while-loop-3.slang
@@ -0,0 +1,66 @@
+//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;
+
+// This test isn't actually testing the output, but rather that the compiler doesn't crash upon 
+// encountering a specific loop pattern. ('Data' is non-differentiable here, so the expected output is 0)s
+// 
+
+typedef DifferentialPair<float> dpfloat;
+typedef float.Differential dfloat;
+
+struct P
+{
+    bool terminated;
+    bool isTerminated() { return terminated; }
+    bool isHit() { return !terminated; }
+};
+
+struct Data
+{
+    float t;
+};
+
+void updateData(Data data)
+{
+    data.t = data.t * data.t;
+}
+
+[BackwardDifferentiable]
+float test_simple_while(float y, uint n)
+{
+    Data d = { y };
+    P p;
+    p.terminated = false;
+    int i = n;
+
+    if (p.isTerminated())
+        return d.t;
+
+    [MaxIters(4)]
+    while (!p.isTerminated())
+    {
+        updateData(d);
+        p.terminated = (i-- == 0);
+        if (p.isTerminated())
+            break;
+
+        if (!p.isHit())
+            break;
+    }
+    return d.t;
+}
+
+[numthreads(1, 1, 1)]
+void computeMain(uint3 dispatchThreadID : SV_DispatchThreadID)
+{
+    {
+        dpfloat dpa = dpfloat(1.0, 0.0);
+
+        __bwd_diff(test_simple_while)(dpa, 2, 1.0f);
+        outputBuffer[0] = dpa.d; // Expect: 8.0
+    }
+}
diff --git a/tests/autodiff/reverse-while-loop-3.slang.expected.txt b/tests/autodiff/reverse-while-loop-3.slang.expected.txt
new file mode 100644
index 000000000..ca54c9afe
--- /dev/null
+++ b/tests/autodiff/reverse-while-loop-3.slang.expected.txt
@@ -0,0 +1,6 @@
+type: float
+1.000000
+0.000000
+0.000000
+0.000000
+0.000000