Only expose VO bit side metadata base address on 64bits target

Cargo.toml

@@ -189,7 +189,8 @@ no_mmap_annotation = []
 # However, in rare cases, a binding may want to directly access and manipulate VO bit. For example, a binding cannot cooperate
 # with MMTk to identify each object for setting the VO bit. This is usually due to the limitation of the VM.
 # In such cases, a binding may use this feature and manipulate the VO bit to their needs. The binding's manipulation on VO bit
-# may violate MMTk's semantics, and may result in undefined behaviors for VO bit related APIs.
+# may violate MMTk's semantics, and may result in undefined behaviors for VO bit related APIs. This feature should
+# only be used if you understand how VO bit works internally. Use at your own risk.
 vo_bit_access = []
 
 # Do not modify the following line - ci-common.sh matches it

src/util/metadata/side_metadata/constants.rs

@@ -30,6 +30,7 @@ pub(crate) const GLOBAL_SIDE_METADATA_BASE_OFFSET: SideMetadataOffset =
     SideMetadataOffset::addr(GLOBAL_SIDE_METADATA_BASE_ADDRESS);
 
 /// Base address of VO bit, public to VM bindings which may need to use this.
+#[cfg(target_pointer_width = "64")]
 pub const VO_BIT_SIDE_METADATA_ADDR: Address =
     crate::util::metadata::vo_bit::VO_BIT_SIDE_METADATA_ADDR;
 

src/util/metadata/vo_bit/mod.rs

@@ -58,6 +58,7 @@ use crate::vm::VMBinding;
 pub(crate) const VO_BIT_SIDE_METADATA_SPEC: SideMetadataSpec =
     crate::util::metadata::side_metadata::spec_defs::VO_BIT;
 
+#[cfg(target_pointer_width = "64")]
 pub const VO_BIT_SIDE_METADATA_ADDR: Address = VO_BIT_SIDE_METADATA_SPEC.get_absolute_offset();
 
 /// Atomically set the VO bit for an object.