Make BSWAP16 nodes normalize upper 16 bits (#67903)

Currently the JIT's constant folding (gtFoldExprConst and VNs
EvalOpSpecialized) assumes that BSWAP16 zero extends into the upper 16
bits. This was not the case, and in fact the behavior of BSWAP16
depended on platform.

Normally this would not be a problem since we always insert normalizing
casts when creating BSWAP16 nodes, however VN was smart enough to remove
this cast in some cases (see the test).

Change the semantics of BSWAP16 nodes to zero extend into the upper 16
bits to match constant folding, and add a small peephole to avoid
inserting this normalization in the common case where it is not
necessary.

Fixes #67723
Subsumes #67726

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

@@ -3083,13 +3083,18 @@ 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

@@ -9553,3 +9553,37 @@ 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>