[RyuJIT] Import String.Empty as CNS_STR

src/coreclr/jit/compiler.cpp

@@ -1393,6 +1393,8 @@ bool Compiler::s_dspMemStats = false;
 const bool Compiler::Options::compNoPInvokeInlineCB = false;
 #endif
 
+unsigned Compiler::s_emptyStringSconCPX = 0;
+
 /*****************************************************************************
  *
  *  One time initialization code

src/coreclr/jit/compiler.h

@@ -9491,6 +9491,8 @@ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
 
 //-------------------------- Global Compiler Data ------------------------------------
 
+    static unsigned s_emptyStringSconCPX;
+
 #ifdef DEBUG
 private:
     static LONG s_compMethodsCount; // to produce unique label names

src/coreclr/jit/importer.cpp

@@ -14842,9 +14842,16 @@ void Compiler::impImportBlockCode(BasicBlock* block)
                     {
                         assert(aflags & CORINFO_ACCESS_GET);
 
-                        LPVOID         pValue;
-                        InfoAccessType iat = info.compCompHnd->emptyStringLiteral(&pValue);
-                        op1                = gtNewStringLiteralNode(iat, pValue);
+                        if (s_emptyStringSconCPX != 0)
+                        {
+                            op1 = gtNewSconNode(s_emptyStringSconCPX, info.compCompHnd->getClassModule(impGetObjectClass()));
+                        }
+                        else
+                        {
+                            LPVOID         pValue;
+                            InfoAccessType iat = info.compCompHnd->emptyStringLiteral(&pValue);	
+                            op1                = gtNewStringLiteralNode(iat, pValue);
+                        }
                         goto FIELD_DONE;
                     }
                     break;

src/coreclr/jit/morph.cpp

@@ -9270,38 +9270,52 @@ GenTree* Compiler::fgMorphConst(GenTree* tree)
         return tree;
     }
 
+    CORINFO_MODULE_HANDLE scpHnd  = tree->AsStrCon()->gtScpHnd;
+    unsigned              sconCPX = tree->AsStrCon()->gtSconCPX;
+
     // TODO-CQ: Do this for compCurBB->isRunRarely(). Doing that currently will
     // guarantee slow performance for that block. Instead cache the return value
     // of CORINFO_HELP_STRCNS and go to cache first giving reasonable perf.
 
     if (compCurBB->bbJumpKind == BBJ_THROW)
     {
-        CorInfoHelpFunc helper = info.compCompHnd->getLazyStringLiteralHelper(tree->AsStrCon()->gtScpHnd);
+        CorInfoHelpFunc helper = info.compCompHnd->getLazyStringLiteralHelper(scpHnd);
         if (helper != CORINFO_HELP_UNDEF)
         {
             // For un-important blocks, we want to construct the string lazily
 
             GenTreeCall::Use* args;
             if (helper == CORINFO_HELP_STRCNS_CURRENT_MODULE)
             {
-                args = gtNewCallArgs(gtNewIconNode(RidFromToken(tree->AsStrCon()->gtSconCPX), TYP_INT));
+                args = gtNewCallArgs(gtNewIconNode(RidFromToken(sconCPX), TYP_INT));
             }
             else
             {
-                args = gtNewCallArgs(gtNewIconNode(RidFromToken(tree->AsStrCon()->gtSconCPX), TYP_INT),
-                                     gtNewIconEmbScpHndNode(tree->AsStrCon()->gtScpHnd));
+                args = gtNewCallArgs(gtNewIconNode(RidFromToken(sconCPX), TYP_INT),
+                                     gtNewIconEmbScpHndNode(scpHnd));
             }
 
             tree = gtNewHelperCallNode(helper, TYP_REF, args);
             return fgMorphTree(tree);
         }
     }
 
-    assert(tree->AsStrCon()->gtScpHnd == info.compScopeHnd || !IsUninitialized(tree->AsStrCon()->gtScpHnd));
+    assert(scpHnd == info.compScopeHnd || !IsUninitialized(scpHnd));
+
+    // Save SconCPX to s_emptyStringSconCPX if it's "" (empty string).
+    if ((s_emptyStringSconCPX == 0) &&
+        (scpHnd == info.compCompHnd->getClassModule(impGetObjectClass())))
+    {
+        int length = -1;
+        info.compCompHnd->getStringLiteral(scpHnd, sconCPX, &length);
+        if (length == 0)
+        {
+            s_emptyStringSconCPX = sconCPX;
+        }
+    }
 
     LPVOID         pValue;
-    InfoAccessType iat =
-        info.compCompHnd->constructStringLiteral(tree->AsStrCon()->gtScpHnd, tree->AsStrCon()->gtSconCPX, &pValue);
+    InfoAccessType iat = info.compCompHnd->constructStringLiteral(scpHnd, sconCPX, &pValue);
 
     tree = gtNewStringLiteralNode(iat, pValue);