VLA: Ignore llvm.stacksave/stackrestore if no SP

If stack is not initialized (no SP), which means there's no VLA,
we can safely remove llvm.stacksave and stackrestore

Change-Id: I252ff3ce5383ba23bcd3514591e8b9fc159d61f1

Author: DianaChen

IGC/Compiler/CISACodeGen/EmitVISAPass.cpp

@@ -8326,11 +8326,17 @@ void EmitPass::EmitIntrinsicMessage(llvm::IntrinsicInst* inst)
         // do nothing
         break;
     case Intrinsic::stacksave:
-        emitLLVMStackSave(inst);
+        // If stack is not initialized (no SP), we can assume there's no VLA.
+        // We can ignore llvm.stacksave and llvm.stackrestore intrinsics
+        if (m_currShader->hasSP())
+            emitLLVMStackSave(inst);
         break;
 
     case Intrinsic::stackrestore:
-        emitLLVMStackRestore(inst);
+        // If stack is not initialized (no SP), we can assume there's no VLA.
+        // We can ignore llvm.stacksave and llvm.stackrestore intrinsics
+        if (m_currShader->hasSP())
+            emitLLVMStackRestore(inst);
         break;
 
     case Intrinsic::bswap:

IGC/Compiler/CISACodeGen/ShaderCodeGen.hpp

@@ -193,6 +193,10 @@ class CShader
     CVariable* GetARGV();
     CVariable* GetRETV();
     CVariable* GetPrivateBase();
+
+    bool hasSP() const { return m_SP != nullptr; }
+    bool hasFP() const { return m_FP != nullptr; }
+
     void InitializeStackVariables();
     void SaveStackState();
     void RestoreStackState();