JIT: Optimize redundant sign extensions in indexers

src/coreclr/jit/compiler.h

@@ -3983,7 +3983,7 @@ class Compiler
 
     static void impBashVarAddrsToI(GenTree* tree1, GenTree* tree2 = nullptr);
 
-    GenTree* impImplicitIorI4Cast(GenTree* tree, var_types dstTyp);
+    GenTree* impImplicitIorI4Cast(GenTree* tree, var_types dstTyp, bool isUnsigned = false);
 
     GenTree* impImplicitR4orR8Cast(GenTree* tree, var_types dstTyp);
 

src/coreclr/jit/importer.cpp

@@ -3171,15 +3171,25 @@ void Compiler::impBashVarAddrsToI(GenTree* tree1, GenTree* tree2)
     }
 }
 
-/*****************************************************************************
- *  TYP_INT and TYP_I_IMPL can be used almost interchangeably, but we want
- *  to make that an explicit cast in our trees, so any implicit casts that
- *  exist in the IL (at least on 64-bit where TYP_I_IMPL != TYP_INT) are
- *  turned into explicit casts here.
- *  We also allow an implicit conversion of a ldnull into a TYP_I_IMPL(0)
- */
+//------------------------------------------------------------------------
+// impImplicitIorI4Cast: Adds an explicit cast to TYP_INT or TYP_I_IMPL
+//
+// Arguments:
+//    tree       - node to cast
+//    dstTyp     - cast type
+//    isUnsigned - is node known to be unsigned?
+//
+// Return Value:
+//    A new tree surrounded with a cast op if needed
+//
+// Notes:
+//    TYP_INT and TYP_I_IMPL can be used almost interchangeably, but we want
+//    to make that an explicit cast in our trees, so any implicit casts that
+//    exist in the IL(at least on 64 - bit where TYP_I_IMPL != TYP_INT) are
+//    turned into explicit casts here.
+//    We also allow an implicit conversion of a ldnull into a TYP_I_IMPL(0)
 
-GenTree* Compiler::impImplicitIorI4Cast(GenTree* tree, var_types dstTyp)
+GenTree* Compiler::impImplicitIorI4Cast(GenTree* tree, var_types dstTyp, bool isUnsigned)
 {
     var_types currType   = genActualType(tree->gtType);
     var_types wantedType = genActualType(dstTyp);
@@ -3198,12 +3208,12 @@ GenTree* Compiler::impImplicitIorI4Cast(GenTree* tree, var_types dstTyp)
         else if (varTypeIsI(wantedType) && (currType == TYP_INT))
         {
             // Note that this allows TYP_INT to be cast to a TYP_I_IMPL when wantedType is a TYP_BYREF or TYP_REF
-            tree = gtNewCastNode(TYP_I_IMPL, tree, false, TYP_I_IMPL);
+            tree = gtNewCastNode(TYP_I_IMPL, tree, isUnsigned, TYP_I_IMPL);
         }
         else if ((wantedType == TYP_INT) && varTypeIsI(currType))
         {
             // Note that this allows TYP_BYREF or TYP_REF to be cast to a TYP_INT
-            tree = gtNewCastNode(TYP_INT, tree, false, TYP_INT);
+            tree = gtNewCastNode(TYP_INT, tree, isUnsigned, TYP_INT);
         }
 #endif // TARGET_64BIT
     }
@@ -4152,7 +4162,7 @@ GenTree* Compiler::impIntrinsic(GenTree*                newobjThis,
                 GenTreeBoundsChk(GT_ARR_BOUNDS_CHECK, TYP_VOID, index, length, SCK_RNGCHK_FAIL);
 
             // Element access
-            GenTree*             indexIntPtr = impImplicitIorI4Cast(indexClone, TYP_I_IMPL);
+            GenTree*             indexIntPtr = impImplicitIorI4Cast(indexClone, TYP_I_IMPL, /*isUnsigned:*/ true);
             GenTree*             sizeofNode  = gtNewIconNode(elemSize);
             GenTree*             mulNode     = gtNewOperNode(GT_MUL, TYP_I_IMPL, indexIntPtr, sizeofNode);
             CORINFO_FIELD_HANDLE ptrHnd      = info.compCompHnd->getFieldInClass(clsHnd, 0);

src/coreclr/jit/morph.cpp

@@ -5626,7 +5626,9 @@ GenTree* Compiler::fgMorphArrayIndex(GenTree* tree)
         }
         else
         {
-            index = gtNewCastNode(TYP_I_IMPL, index, false, TYP_I_IMPL);
+            // Mark cast as unsigned since index is never negative
+            // at this point (handled by GT_ARR_BOUNDS_CHECK)
+            index = gtNewCastNode(TYP_I_IMPL, index, true, TYP_I_IMPL);
         }
     }
 #endif // TARGET_64BIT