JIT: ARR_LENGTH(new T[CNS]) --> CNS (#85496)

Author: EgorBo

src/coreclr/jit/rangecheck.cpp

@@ -70,7 +70,8 @@ RangeCheck::OverflowMap* RangeCheck::GetOverflowMap()
 int RangeCheck::GetArrLength(ValueNum vn)
 {
     ValueNum arrRefVN = m_pCompiler->vnStore->GetArrForLenVn(vn);
-    return m_pCompiler->vnStore->GetNewArrSize(arrRefVN);
+    int      size;
+    return m_pCompiler->vnStore->TryGetNewArrSize(arrRefVN, &size) ? size : 0;
 }
 
 //------------------------------------------------------------------------

src/coreclr/jit/valuenum.cpp

@@ -2433,6 +2433,14 @@ ValueNum ValueNumStore::VNForFunc(var_types typ, VNFunc func, ValueNum arg0VN)
                     }
                 }
             }
+
+            // Case 3: ARR_LENGTH(new T[cns])
+            // TODO: Add support for MD arrays
+            int knownSize;
+            if ((resultVN == NoVN) && TryGetNewArrSize(addressVN, &knownSize))
+            {
+                resultVN = VNForIntCon(knownSize);
+            }
         }
 
         // Try to perform constant-folding.
@@ -6491,18 +6499,24 @@ bool ValueNumStore::IsVNNewArr(ValueNum vn, VNFuncApp* funcApp)
 
 // TODO-MDArray: support array dimension length of a specific dimension for JitNewMdArr, with a GetNewMDArrSize()
 // function.
-int ValueNumStore::GetNewArrSize(ValueNum vn)
+bool ValueNumStore::TryGetNewArrSize(ValueNum vn, int* size)
 {
     VNFuncApp funcApp;
     if (IsVNNewArr(vn, &funcApp))
     {
         ValueNum arg1VN = funcApp.m_args[1];
-        if (IsVNConstant(arg1VN) && TypeOfVN(arg1VN) == TYP_INT)
+        if (IsVNConstant(arg1VN))
         {
-            return ConstantValue<int>(arg1VN);
+            ssize_t val = CoercedConstantValue<ssize_t>(arg1VN);
+            if ((size_t)val <= INT_MAX)
+            {
+                *size = (int)val;
+                return true;
+            }
         }
     }
-    return 0;
+    *size = 0;
+    return false;
 }
 
 bool ValueNumStore::IsVNArrLen(ValueNum vn)

src/coreclr/jit/valuenum.h

@@ -946,8 +946,8 @@ class ValueNumStore
     // Check if "vn" is "new [] (type handle, size)"
     bool IsVNNewArr(ValueNum vn, VNFuncApp* funcApp);
 
-    // Check if "vn" IsVNNewArr and return <= 0 if arr size cannot be determined, else array size.
-    int GetNewArrSize(ValueNum vn);
+    // Check if "vn" IsVNNewArr and return false if arr size cannot be determined.
+    bool TryGetNewArrSize(ValueNum vn, int* size);
 
     // Check if "vn" is "a.Length" or "a.GetLength(n)"
     bool IsVNArrLen(ValueNum vn);