Make BSWAP16 nodes normalize upper 16 bits

src/coreclr/jit/codegen.h

@@ -1265,6 +1265,7 @@ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
     void genCodeForIndir(GenTreeIndir* tree);
     void genCodeForNegNot(GenTree* tree);
     void genCodeForBswap(GenTree* tree);
+    bool genCanOmitNormalizationForBswap16(GenTree* tree);
     void genCodeForLclVar(GenTreeLclVar* tree);
     void genCodeForLclFld(GenTreeLclFld* tree);
     void genCodeForStoreLclFld(GenTreeLclFld* tree);

src/coreclr/jit/codegenarm64.cpp

@@ -3071,13 +3071,18 @@ void CodeGen::genCodeForBswap(GenTree* tree)
     assert(operand->isUsedFromReg());
     regNumber operandReg = genConsumeReg(operand);
 
-    if (tree->OperIs(GT_BSWAP16))
+    if (tree->OperIs(GT_BSWAP))
     {
-        inst_RV_RV(INS_rev16, targetReg, operandReg, targetType);
+        inst_RV_RV(INS_rev, targetReg, operandReg, targetType);
     }
     else
     {
-        inst_RV_RV(INS_rev, targetReg, operandReg, targetType);
+        inst_RV_RV(INS_rev16, targetReg, operandReg, targetType);
+
+        if (!genCanOmitNormalizationForBswap16(tree))
+        {
+            GetEmitter()->emitIns_Mov(INS_uxth, EA_4BYTE, targetReg, targetReg, /* canSkip */ false);
+        }
     }
 
     genProduceReg(tree);

src/coreclr/jit/codegencommon.cpp

@@ -9591,3 +9591,37 @@ void CodeGen::genCodeForBitCast(GenTreeOp* treeNode)
     }
     genProduceReg(treeNode);
 }
+
+//----------------------------------------------------------------------
+// genCanOmitNormalizationForBswap16:
+//   Small peephole to check if a bswap16 node can omit normalization.
+//
+// Arguments:
+//   tree - The BSWAP16 node
+//
+// Remarks:
+//   BSWAP16 nodes are required to zero extend the upper 16 bits, but since the
+//   importer always inserts a normalizing cast (either sign or zero extending)
+//   we almost never need to actually do this.
+//
+bool CodeGen::genCanOmitNormalizationForBswap16(GenTree* tree)
+{
+    if (compiler->opts.OptimizationDisabled())
+    {
+        return false;
+    }
+
+    assert(tree->OperIs(GT_BSWAP16));
+    if ((tree->gtNext == nullptr) || !tree->gtNext->OperIs(GT_CAST))
+    {
+        return false;
+    }
+
+    GenTreeCast* cast = tree->gtNext->AsCast();
+    if (cast->gtOverflow() || (cast->CastOp() != tree))
+    {
+        return false;
+    }
+
+    return (cast->gtCastType == TYP_USHORT) || (cast->gtCastType == TYP_SHORT);
+}

src/coreclr/jit/codegenxarch.cpp

@@ -577,6 +577,11 @@ void CodeGen::genCodeForBswap(GenTree* tree)
     {
         // 16-bit byte swaps use "ror reg.16, 8"
         inst_RV_IV(INS_ror_N, targetReg, 8 /* val */, emitAttr::EA_2BYTE);
+
+        if (!genCanOmitNormalizationForBswap16(tree))
+        {
+            GetEmitter()->emitIns_Mov(INS_movzx, EA_2BYTE, targetReg, targetReg, /* canSkip */ false);
+        }
     }
 
     genProduceReg(tree);

src/coreclr/jit/gtlist.h

@@ -104,7 +104,7 @@ GTNODE(RUNTIMELOOKUP    , GenTreeRuntimeLookup, 0,GTK_UNOP|GTK_EXOP|DBK_NOTLIR)
 GTNODE(ARR_ADDR         , GenTreeArrAddr     ,0,GTK_UNOP|GTK_EXOP|DBK_NOTLIR)   // Wraps an array address expression
 
 GTNODE(BSWAP            , GenTreeOp          ,0,GTK_UNOP)               // Byte swap (32-bit or 64-bit)
-GTNODE(BSWAP16          , GenTreeOp          ,0,GTK_UNOP)               // Byte swap (16-bit)
+GTNODE(BSWAP16          , GenTreeOp          ,0,GTK_UNOP)               // Byte swap lower 16-bits and zero upper 16 bits
 
 //-----------------------------------------------------------------------------
 //  Binary operators (2 operands):

src/tests/JIT/Regression/JitBlue/Runtime_67723/Runtime_67723.cs

@@ -0,0 +1,14 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Buffers.Binary;
+
+public class Runtime_67223
+{
+    public static int Main()
+    {
+        short[] foo = { short.MinValue };
+        int test = BinaryPrimitives.ReverseEndianness(foo[0]);
+        return test == 0x80 ? 100 : -1;
+    }
+}

src/tests/JIT/Regression/JitBlue/Runtime_67723/Runtime_67723.csproj

@@ -0,0 +1,9 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+</Project>