RyuJIT: Don't emit null checks for constant strings (#37245)

* Fold "ldstr ==/!== null" (mikedn's PR)

* Formatting

* check IsUnsigned for GT_GT

* Address feedback

Author: EgorBo

src/coreclr/src/jit/flowgraph.cpp

@@ -7321,6 +7321,10 @@ bool Compiler::fgAddrCouldBeNull(GenTree* addr)
     {
         return false;
     }
+    else if (addr->OperIs(GT_CNS_STR))
+    {
+        return false;
+    }
     else if (addr->gtOper == GT_LCL_VAR)
     {
         unsigned varNum = addr->AsLclVarCommon()->GetLclNum();
@@ -23763,10 +23767,13 @@ Statement* Compiler::fgInlinePrependStatements(InlineInfo* inlineInfo)
     if (call->gtFlags & GTF_CALL_NULLCHECK && !inlineInfo->thisDereferencedFirst)
     {
         // Call impInlineFetchArg to "reserve" a temp for the "this" pointer.
-        nullcheck = gtNewNullCheck(impInlineFetchArg(0, inlArgInfo, lclVarInfo), block);
-
-        // The NULL-check statement will be inserted to the statement list after those statements
-        // that assign arguments to temps and before the actual body of the inlinee method.
+        GenTree* thisOp = impInlineFetchArg(0, inlArgInfo, lclVarInfo);
+        if (fgAddrCouldBeNull(thisOp))
+        {
+            nullcheck = gtNewNullCheck(impInlineFetchArg(0, inlArgInfo, lclVarInfo), block);
+            // The NULL-check statement will be inserted to the statement list after those statements
+            // that assign arguments to temps and before the actual body of the inlinee method.
+        }
     }
 
     /* Treat arguments that had to be assigned to temps */

src/coreclr/src/jit/gentree.cpp

@@ -14531,8 +14531,22 @@ GenTree* Compiler::gtFoldExprConst(GenTree* tree)
 
             /* String nodes are an RVA at this point */
 
-            if (op1->gtOper == GT_CNS_STR || op2->gtOper == GT_CNS_STR)
+            if (op1->OperIs(GT_CNS_STR) || op2->OperIs(GT_CNS_STR))
             {
+                // Fold "ldstr" ==/!= null
+                if (op2->IsIntegralConst(0))
+                {
+                    if (tree->OperIs(GT_EQ))
+                    {
+                        i1 = 0;
+                        goto FOLD_COND;
+                    }
+                    if (tree->OperIs(GT_NE) || (tree->OperIs(GT_GT) && tree->IsUnsigned()))
+                    {
+                        i1 = 1;
+                        goto FOLD_COND;
+                    }
+                }
                 return tree;
             }
 

src/coreclr/src/jit/morph.cpp

@@ -6110,8 +6110,8 @@ GenTree* Compiler::fgMorphField(GenTree* tree, MorphAddrContext* mac)
 
         bool addExplicitNullCheck = false;
 
-        // Implicit byref locals are never null.
-        if (!((objRef->gtOper == GT_LCL_VAR) && lvaIsImplicitByRefLocal(objRef->AsLclVarCommon()->GetLclNum())))
+        // Implicit byref locals and string literals are never null.
+        if (fgAddrCouldBeNull(objRef))
         {
             // If the objRef is a GT_ADDR node, it, itself, never requires null checking.  The expression
             // whose address is being taken is either a local or static variable, whose address is necessarily