JIT: Import string.Empty as ""

src/coreclr/jit/gentree.cpp

@@ -5793,6 +5793,12 @@ GenTree* Compiler::gtNewStringLiteralNode(InfoAccessType iat, void* pValue)
 //
 GenTreeIntCon* Compiler::gtNewStringLiteralLength(GenTreeStrCon* node)
 {
+    if (node->IsStringEmptyField())
+    {
+        JITDUMP("Folded String.Empty.Length to 0\n");
+        return gtNewIconNode(0);
+    }
+
     int             length = -1;
     const char16_t* str    = info.compCompHnd->getStringLiteral(node->gtScpHnd, node->gtSconCPX, &length);
     if (length >= 0)
@@ -5802,11 +5808,11 @@ GenTreeIntCon* Compiler::gtNewStringLiteralLength(GenTreeStrCon* node)
         // str can be NULL for dynamic context
         if (str != nullptr)
         {
-            JITDUMP("String '\"%ws\".Length' is '%d'\n", str, length)
+            JITDUMP("Folded '\"%ws\".Length' to '%d'\n", str, length)
         }
         else
         {
-            JITDUMP("String 'CNS_STR.Length' is '%d'\n", length)
+            JITDUMP("Folded 'CNS_STR.Length' to '%d'\n", length)
         }
         return iconNode;
     }

src/coreclr/jit/gentree.h

@@ -3240,11 +3240,19 @@ struct GenTreeDblCon : public GenTree
 
 /* gtStrCon -- string  constant (GT_CNS_STR) */
 
+#define EMPTY_STRING_SCON (unsigned)-1
+
 struct GenTreeStrCon : public GenTree
 {
     unsigned              gtSconCPX;
     CORINFO_MODULE_HANDLE gtScpHnd;
 
+    // Returns true if this GT_CNS_STR was imported for String.Empty field
+    bool IsStringEmptyField()
+    {
+        return gtSconCPX == EMPTY_STRING_SCON && gtScpHnd == nullptr;
+    }
+
     // Because this node can come from an inlined method we need to
     // have the scope handle, since it will become a helper call.
     GenTreeStrCon(unsigned sconCPX, CORINFO_MODULE_HANDLE mod DEBUGARG(bool largeNode = false))

src/coreclr/jit/importer.cpp

@@ -15073,9 +15073,8 @@ void Compiler::impImportBlockCode(BasicBlock* block)
                     {
                         assert(aflags & CORINFO_ACCESS_GET);
 
-                        LPVOID         pValue;
-                        InfoAccessType iat = info.compCompHnd->emptyStringLiteral(&pValue);
-                        op1                = gtNewStringLiteralNode(iat, pValue);
+                        // Import String.Empty as "" (GT_CNS_STR with a fake SconCPX = 0)
+                        op1 = gtNewSconNode(EMPTY_STRING_SCON, nullptr);
                         goto FIELD_DONE;
                     }
                     break;

src/coreclr/jit/morph.cpp

@@ -5184,7 +5184,9 @@ GenTree* Compiler::fgMorphArrayIndex(GenTree* tree)
     noway_assert(elemTyp != TYP_STRUCT || elemStructType != nullptr);
 
     // Fold "cns_str"[cns_index] to ushort constant
-    if (opts.OptimizationEnabled() && asIndex->Arr()->OperIs(GT_CNS_STR) && asIndex->Index()->IsIntCnsFitsInI32())
+    // NOTE: don't do it for empty string, the operation will fail anyway
+    if (opts.OptimizationEnabled() && asIndex->Arr()->OperIs(GT_CNS_STR) &&
+        !asIndex->Arr()->AsStrCon()->IsStringEmptyField() && asIndex->Index()->IsIntCnsFitsInI32())
     {
         const int cnsIndex = static_cast<int>(asIndex->Index()->AsIntConCommon()->IconValue());
         if (cnsIndex >= 0)
@@ -9433,11 +9435,18 @@ GenTree* Compiler::fgMorphConst(GenTree* tree)
 
     tree->gtFlags &= ~(GTF_ALL_EFFECT | GTF_REVERSE_OPS);
 
-    if (tree->OperGet() != GT_CNS_STR)
+    if (!tree->OperIs(GT_CNS_STR))
     {
         return tree;
     }
 
+    if (tree->AsStrCon()->IsStringEmptyField())
+    {
+        LPVOID         pValue;
+        InfoAccessType iat = info.compCompHnd->emptyStringLiteral(&pValue);
+        return fgMorphTree(gtNewStringLiteralNode(iat, pValue));
+    }
+
     // 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.

src/coreclr/tools/Common/JitInterface/CorInfoImpl.cs

@@ -3278,7 +3278,9 @@ private bool canGetVarArgsHandle(CORINFO_SIG_INFO* pSig)
 
         private InfoAccessType emptyStringLiteral(ref void* ppValue)
         {
-            return constructStringLiteral(_methodScope, (mdToken)CorTokenType.mdtString, ref ppValue);
+            ISymbolNode stringObject = _compilation.NodeFactory.SerializedStringObject("");
+            ppValue = (void*)ObjectToHandle(stringObject);
+            return stringObject.RepresentsIndirectionCell ? InfoAccessType.IAT_PVALUE : InfoAccessType.IAT_VALUE;
         }
 
         private uint getFieldThreadLocalStoreID(CORINFO_FIELD_STRUCT_* field, ref void* ppIndirection)