Flag EnableByValStructArgPromotion can enable/disable struct argument promotion

Flag is enabled by default, can be turned off for debugging and performance measurement purposes.

Author: dlei6g

IGC/AdaptorCommon/LegalizeFunctionSignatures.cpp

@@ -139,6 +139,9 @@ inline Type* LegalizedIntVectorType(Module& M, Type* ty)
 // Returns true for small structures that only contain primitive types
 inline bool isPromotableStructType(Module& M, Type* ty, bool isReturnValue = false)
 {
+    if (IGC_IS_FLAG_DISABLED(EnableByValStructArgPromotion))
+        return false;
+
     // We can separate promoting argument and return value sizes.
     // Return value is limited to 64-bits due to vISA stackcall conventions.
     // Argument is not limited to 64-bits, but can be adjusted to minimize spill.

IGC/common/igc_flags.h

@@ -376,6 +376,7 @@ DECLARE_IGC_REGKEY(bool, EnableLTO,                     true,  "Enable link time
 DECLARE_IGC_REGKEY(bool, EnableLTODebug,                false, "Enable debug information for LTO", true)
 DECLARE_IGC_REGKEY(DWORD, FunctionControl,              0,     "Control function inlining/subroutine/stackcall. See value defs in igc_flags.hpp.", true)
 DECLARE_IGC_REGKEY(bool, EnableStackCallFuncCall,       false, "If enabled, the default function call mode will be set to stack call. Otherwise, subroutine call is used.", false)
+DECLARE_IGC_REGKEY(bool, EnableByValStructArgPromotion, true, "If enabled, byval/sret struct arguments are promoted to pass-by-value if possible.", true)
 DECLARE_IGC_REGKEY(bool, ForceInlineStackCallWithImplArg, true, "If enabled, stack calls that uses implicit args will be force inlined.", true)
 DECLARE_IGC_REGKEY(DWORD, FunctionCloningThreshold,    0,
     "Limits how many times functions can be cloned when called from multiple function groups." \