Fix asserts and GC holes with Unsafe.Subtract/ByteOffset (#99019)

Author: MichalPetryka

src/coreclr/jit/morph.cpp

@@ -9220,11 +9220,11 @@ GenTree* Compiler::fgMorphSmpOp(GenTree* tree, MorphAddrContext* mac, bool* optA
 
             // TODO #4104: there are a lot of other places where
             // this condition is not checked before transformations.
-            if (fgGlobalMorph)
+            noway_assert(op2);
+            if (fgGlobalMorph && !op2->TypeIs(TYP_BYREF))
             {
                 /* Check for "op1 - cns2" , we change it to "op1 + (-cns2)" */
 
-                noway_assert(op2);
                 if (op2->IsCnsIntOrI() && !op2->IsIconHandle())
                 {
                     // Negate the constant and change the node to be "+",
@@ -9242,7 +9242,7 @@ GenTree* Compiler::fgMorphSmpOp(GenTree* tree, MorphAddrContext* mac, bool* optA
                 noway_assert(op1);
                 if (op1->IsCnsIntOrI())
                 {
-                    noway_assert(varTypeIsIntOrI(tree));
+                    noway_assert(varTypeIsIntegralOrI(tree));
 
                     // The type of the new GT_NEG node cannot just be op2->TypeGet().
                     // Otherwise we may sign-extend incorrectly in cases where the GT_NEG

src/libraries/System.Runtime/tests/System.Runtime.CompilerServices.Unsafe.Tests/UnsafeTests.cs

@@ -462,6 +462,26 @@ public static void ByteOffsetStackByte4()
             Assert.Equal(new IntPtr(-3), Unsafe.ByteOffset(ref byte4.B3, ref byte4.B0));
         }
 
+        private static unsafe class StaticReadonlyHolder
+        {
+            public static readonly void* Pointer = (void*)RuntimeHelpers.AllocateTypeAssociatedMemory(typeof(StaticReadonlyHolder), 1);
+        }
+
+        [Fact]
+        public static unsafe void ByteOffsetConstantRef()
+        {
+            // https://github.com/dotnet/runtime/pull/99019
+            [MethodImpl(MethodImplOptions.NoInlining)]
+            static nint NullTest(ref byte origin) => Unsafe.ByteOffset(ref origin, ref Unsafe.NullRef<byte>());
+            Assert.Equal(0, NullTest(ref Unsafe.NullRef<byte>()));
+
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+            static ref byte GetStatic(ref byte x) => ref x;
+            [MethodImpl(MethodImplOptions.NoInlining)]
+            static nint StaticReadonlyTest(ref byte x) => Unsafe.ByteOffset(ref GetStatic(ref Unsafe.AsRef<byte>(StaticReadonlyHolder.Pointer)), ref x);
+            Assert.Equal(0, StaticReadonlyTest(ref Unsafe.AsRef<byte>(StaticReadonlyHolder.Pointer)));
+        }
+
         [Fact]
         public static unsafe void AsRef()
         {
@@ -597,7 +617,7 @@ public static void RefAddNuintByteOffset()
         }
 
         [Fact]
-        public static void RefSubtract()
+        public static unsafe void RefSubtract()
         {
             string[] a = new string[] { "abc", "def", "ghi", "jkl" };
 
@@ -609,6 +629,11 @@ public static void RefSubtract()
 
             ref string r3 = ref Unsafe.Subtract(ref r2, 3);
             Assert.Equal("abc", r3);
+
+            // https://github.com/dotnet/runtime/pull/99019
+            [MethodImpl(MethodImplOptions.NoInlining)]
+            static ref byte NullTest(nuint offset) => ref Unsafe.Subtract(ref Unsafe.NullRef<byte>(), offset);
+            Assert.True(Unsafe.IsNullRef(ref NullTest(0)));
         }
 
         [Fact]