Assigning null values for undef phi nodes

This change is to give an option to assigns a null value to such a phi node which has an undefined value during emitting vISA.

Author: krystian-andrzejewski

IGC/Compiler/CISACodeGen/EmitVISAPass.cpp

@@ -1599,7 +1599,14 @@ void EmitPass::MovPhiSources(llvm::BasicBlock* aBB)
 
                     if (isa<UndefValue>(Src))
                     {
-                        continue;
+                        if (IGC_IS_FLAG_ENABLED(AssignZeroToUndefPhiNodes))
+                        {
+                            Src = Constant::getNullValue(Src->getType());
+                        }
+                        else
+                        {
+                            continue;
+                        }
                     }
 
                     Value* dstRootV = m_deSSA ? m_deSSA->getRootValue(PN) : PN;

IGC/common/igc_flags.h

@@ -468,7 +468,8 @@ DECLARE_IGC_REGKEY(bool, UseVMaskPredicateForLoads, true, "Use VMask as predicat
 DECLARE_IGC_REGKEY(bool, UseVMaskPredicateForIndirectMove, true, "Use VMask as predicate for subspan usage (indirect mov only)", true)
 DECLARE_IGC_REGKEY(bool, StackOverflowDetection,        false, "Inserts checks for stack overflow when stack calls are used.", true)
 DECLARE_IGC_REGKEY(bool, BufferBoundsChecking,          false, "Setting this to 1 (true) enables buffer bounds checking", false)
-DECLARE_IGC_REGKEY(DWORD, MinimumValidAddress,          0,     "If it's greater than 0, it enables minimal valid address checking where the threshold is the given value (in hex).", false)
+DECLARE_IGC_REGKEY(DWORD, MinimumValidAddress, 0, "If it's greater than 0, it enables minimal valid address checking where the threshold is the given value (in hex).", false)
+DECLARE_IGC_REGKEY(bool, AssignZeroToUndefPhiNodes, false, "Assigns a null value to such a phi node which has an undefined value during emitting vISA", false)
 
 DECLARE_IGC_GROUP("IGC Features")
 DECLARE_IGC_REGKEY(bool, EnableOCLSIMD16,               true,  "Enable OCL SIMD16 mode", true)