Intrinsify string.FastAllocateString

src/coreclr/System.Private.CoreLib/src/System/String.CoreCLR.cs

@@ -11,11 +11,13 @@ namespace System
 {
     public partial class String
     {
+        [Intrinsic]
         [MethodImpl(MethodImplOptions.InternalCall)]
         [RequiresUnsafe]
         internal static extern unsafe string FastAllocateString(MethodTable *pMT, nint length);
 
         [DebuggerHidden]
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
         internal static unsafe string FastAllocateString(nint length)
         {
             return FastAllocateString(TypeHandle.TypeHandleOf<string>().AsMethodTable(), length);

src/coreclr/jit/gentree.cpp

@@ -2086,6 +2086,10 @@ GenTree* Compiler::getArrayLengthFromAllocation(GenTree* tree)
 
             assert((arrayLength == nullptr) || ((optMethodFlags & OMF_HAS_NEWARRAY) != 0));
         }
+        else if (call->IsSpecialIntrinsic(this, NI_System_String_FastAllocateString))
+        {
+            arrayLength = call->gtArgs.GetUserArgByIndex(1)->GetNode();
+        }
     }
 
     if (arrayLength != nullptr)

src/coreclr/jit/importercalls.cpp

@@ -4029,6 +4029,7 @@ GenTree* Compiler::impIntrinsic(CORINFO_CLASS_HANDLE    clsHnd,
             }
 
             case NI_System_ArgumentNullException_ThrowIfNull:
+            case NI_System_String_FastAllocateString:
                 isSpecial = true;
                 break;
 
@@ -10727,6 +10728,10 @@ NamedIntrinsic Compiler::lookupNamedIntrinsic(CORINFO_METHOD_HANDLE method)
                         {
                             result = NI_System_String_Equals;
                         }
+                        else if (strcmp(methodName, "FastAllocateString") == 0)
+                        {
+                            result = NI_System_String_FastAllocateString;
+                        }
                         else if (strcmp(methodName, "get_Chars") == 0)
                         {
                             result = NI_System_String_get_Chars;

src/coreclr/jit/morph.cpp

@@ -4377,8 +4377,14 @@ GenTree* Compiler::fgMorphPotentialTailCall(GenTreeCall* call)
 
     if (call->IsSpecialIntrinsic())
     {
-        failTailCall("Might turn into an intrinsic");
-        return nullptr;
+        // FastAllocateString is marked as a special intrinsic only to give the JIT
+        // hints about its return value (non-null, length tracked by VN). It is not
+        // re-expanded, so it is safe to tail call.
+        if (!call->IsSpecialIntrinsic(this, NI_System_String_FastAllocateString))
+        {
+            failTailCall("Might turn into an intrinsic");
+            return nullptr;
+        }
     }
 
 #ifdef TARGET_ARM

src/coreclr/jit/namedintrinsiclist.h

@@ -139,6 +139,7 @@ enum NamedIntrinsic : unsigned short
     NI_System_Runtime_InteropService_MemoryMarshal_GetArrayDataReference,
 
     NI_System_String_Equals,
+    NI_System_String_FastAllocateString,
     NI_System_String_get_Chars,
     NI_System_String_get_Length,
     NI_System_String_op_Implicit,

src/coreclr/jit/valuenum.cpp

@@ -2564,8 +2564,10 @@ ValueNum ValueNumStore::VNForFunc(var_types typ, VNFunc func, ValueNum arg0VN)
             }
 
             // Case 4: ARR_LENGTH(new T[(long)size]) -> size
+            //         ARR_LENGTH(String.FastAllocateString(pMT, (long)size)) -> size
             VNFuncApp newArrFuncApp;
-            if (GetVNFunc(arg0VN, &newArrFuncApp) && (newArrFuncApp.m_func == VNF_JitNewArr))
+            if (GetVNFunc(arg0VN, &newArrFuncApp) &&
+                ((newArrFuncApp.m_func == VNF_JitNewArr) || (newArrFuncApp.m_func == VNF_StrFastAllocate)))
             {
                 ValueNum actualSizeVN = VNIgnoreIntToLongCast(newArrFuncApp.m_args[1]);
                 if (TypeOfVN(actualSizeVN) == TYP_INT)
@@ -14112,6 +14114,7 @@ void Compiler::fgValueNumberHelperCallFunc(GenTreeCall* call, VNFunc vnf, ValueN
         }
         break;
 
+        case VNF_StrFastAllocate:
         case VNF_JitNewArr:
         case VNF_JitNewLclArr:
         {
@@ -14344,6 +14347,13 @@ bool Compiler::fgValueNumberSpecialIntrinsic(GenTreeCall* call)
 
     switch (lookupNamedIntrinsic(call->gtCallMethHnd))
     {
+        case NI_System_String_FastAllocateString:
+        {
+            assert(call->gtArgs.CountUserArgs() == 2);
+            fgValueNumberHelperCallFunc(call, VNF_StrFastAllocate, ValueNumStore::VNPForEmptyExcSet());
+            return true;
+        }
+
         case NI_System_Type_GetTypeFromHandle:
         {
             // Optimize Type.GetTypeFromHandle(TypeHandleToRuntimeTypeHandle(clsHandle)) to a frozen handle.

src/coreclr/jit/valuenumfuncs.h

@@ -162,6 +162,7 @@ ValueNumFuncDef(JitNewMdArr, 4, false, true)
 ValueNumFuncDef(JitReadyToRunNew, 2, false, true)
 ValueNumFuncDef(JitReadyToRunNewArr, 3, false, true)
 ValueNumFuncDef(JitReadyToRunNewLclArr, 3, false, true)
+ValueNumFuncDef(StrFastAllocate, 3, false, true) // Args: 0: MethodTable, 1: length, 2: unique VN.
 ValueNumFuncDef(Box, 3, false, true)
 ValueNumFuncDef(BoxNullable, 3, false, false)