Allow simplifying self-referential Phi parameters (#6870)

3 files changed, 56 insertions, 4 deletions

diff --git a/source/slang/slang-ir-simplify-cfg.cpp b/source/slang/slang-ir-simplify-cfg.cpp
index 280874f74..2310a5cce 100644
--- a/source/slang/slang-ir-simplify-cfg.cpp
+++ b/source/slang/slang-ir-simplify-cfg.cpp
@@ -728,9 +728,9 @@ static bool simplifyBoolPhiParams(IRBlock* block)
 
 static bool removeTrivialPhiParams(IRBlock* block)
 {
-    // We can remove a phi parmeter if:
-    // 1. all arguments to a parameter is the same (not really a phi).
-    // 2. the arguments to the parameter is always the same as arguments to another existing
+    // We can remove a phi parameter if:
+    // 1. all non-self-referential arguments to a parameter are the same (not really a phi).
+    // 2. the arguments to the parameter are always the same as arguments to another existing
     // parameter (duplicate phi).
 
     bool changed = false;
@@ -765,7 +765,10 @@ static bool removeTrivialPhiParams(IRBlock* block)
         termInsts.add(termInst);
         for (UInt i = 0; i < termInst->getArgCount(); i++)
         {
-            if (args[i].areKnownValueSame)
+            // Self-referential parameters can be skipped, as they cannot
+            // introduce a new value. The phi can only have multiple different
+            // values if non-self-referential arguments differ.
+            if (args[i].areKnownValueSame && termInst->getArg(i) != params[i])
             {
                 if (args[i].knownValue == nullptr)
                     args[i].knownValue = termInst->getArg(i);
diff --git a/tests/bugs/gh-6860.slang b/tests/bugs/gh-6860.slang
new file mode 100644
index 000000000..13a7e329a
--- /dev/null
+++ b/tests/bugs/gh-6860.slang
@@ -0,0 +1,20 @@
+//TEST:SIMPLE(filecheck=CHECK): -target spirv
+// CHECK: OpEntryPoint
+func breaker()->float {
+    var x: float;
+    for (int i = 0; i < 1; ++i) {
+        if (true) {
+        } else {
+            x = 0.0;
+        }
+    }
+    return x;
+}
+
+[shader("fragment")]
+float4 fragment(float4 in: SV_Position)
+    : SV_Target
+{
+    let res = breaker();
+    return float4(0, 0, 0, 0);
+}
diff --git a/tests/bugs/gh-6862.slang b/tests/bugs/gh-6862.slang
new file mode 100644
index 000000000..8b65b2f2a
--- /dev/null
+++ b/tests/bugs/gh-6862.slang
@@ -0,0 +1,29 @@
+//TEST:SIMPLE(filecheck=CHECK):-stage fragment -entry fragment -target wgsl
+
+func dummy(b: StructuredBuffer<float>)->float {
+    return 0;
+}
+
+func breaker(b: StructuredBuffer<float>)->float {
+    // CHECK-NOT: var<storage, read> {{.*}} : array<f32> = {{.*}};
+    var x: float = 0;
+    for (int i = 0; i < 1; ++i) {
+        x = dummy(b);
+        if (true) {
+        } else {
+            return 0;
+            x = 0;
+        }
+    }
+    return x;
+}
+
+StructuredBuffer<float> b;
+
+[shader("fragment")]
+float4 fragment(float4 in: SV_Position)
+    : SV_Target
+{
+    let res = breaker(b);
+    return float4(res, 0, 0, 0);
+}