Skip generation of mov instructions for undef PHI sources

There is no need to create the mov instructions for `undef` PHI sources.

Author: mmerecki

IGC/Compiler/CISACodeGen/EmitVISAPass.cpp

@@ -1383,6 +1383,8 @@ void EmitPass::MovPhiSources(llvm::BasicBlock *aBB) {
           if (isa<UndefValue>(Src)) {
             if (IGC_IS_FLAG_ENABLED(AssignZeroToUndefPhiNodes)) {
               Src = Constant::getNullValue(Src->getType());
+            } else if (IGC_IS_FLAG_ENABLED(DisableMovOfUndefPhiSources)) {
+              continue;
             }
           }
 

IGC/common/igc_flags.h

@@ -927,6 +927,8 @@ DECLARE_IGC_REGKEY(DWORD, MinimumValidAddress, 0,
                    true)
 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_REGKEY(bool, DisableMovOfUndefPhiSources, true, "Do not emit VISA mov instructions for undef PHI sources",
+                   false)
 DECLARE_IGC_REGKEY_ENUM(InjectPrintfFlag, 0, "Inject printf debugging flag", INJECT_PRINTF_OPTIONS, true)
 DECLARE_IGC_REGKEY(DWORD, AdHoc, 0,
                    "Unassigned debug key that can be used for experiments. Do not commit usages of this regkey", false)