Allow marking data objects as used for the linker

Author: bjorn3

cranelift/jit/src/backend.rs

@@ -611,6 +611,7 @@ impl Module for JITModule {
             data_relocs: _,
             custom_segment_section: _,
             align,
+            used: _,
         } = data;
 
         // Make sure to allocate at least 1 byte. Allocating 0 bytes is UB. Previously a dummy

cranelift/module/src/data_context.rs

@@ -1,4 +1,4 @@
-//! Defines `DataContext`.
+//! Defines `DataDescription`.
 
 use cranelift_codegen::binemit::{Addend, CodeOffset, Reloc};
 use cranelift_codegen::entity::PrimaryMap;
@@ -62,9 +62,12 @@ pub struct DataDescription {
     pub data_relocs: Vec<(CodeOffset, ir::GlobalValue, Addend)>,
     /// Object file section
     pub custom_segment_section: Option<(String, String)>,
-    /// Alignment in bytes. `None` means that the default alignment of the respective module should
-    /// be used.
+    /// Alignment in bytes. `None` means that the default alignment of the
+    /// respective module should be used.
     pub align: Option<u64>,
+    /// Whether or not to request the linker to preserve this data object even
+    /// if not referenced.
+    pub used: bool,
 }
 
 impl DataDescription {
@@ -78,6 +81,7 @@ impl DataDescription {
             data_relocs: vec![],
             custom_segment_section: None,
             align: None,
+            used: false,
         }
     }
 
@@ -90,6 +94,7 @@ impl DataDescription {
         self.data_relocs.clear();
         self.custom_segment_section = None;
         self.align = None;
+        self.used = false;
     }
 
     /// Define a zero-initialized object with the given size.
@@ -117,6 +122,12 @@ impl DataDescription {
         self.align = Some(align);
     }
 
+    /// Set whether or not the linker should preserve this data object even if
+    /// not referenced.
+    pub fn set_used(&mut self, used: bool) {
+        self.used = used;
+    }
+
     /// Declare an external function import.
     ///
     /// Users of the `Module` API generally should call

cranelift/module/src/module.rs

@@ -979,7 +979,7 @@ pub trait Module {
         relocs: &[ModuleReloc],
     ) -> ModuleResult<()>;
 
-    /// Define a data object, producing the data contents from the given `DataContext`.
+    /// Define a data object, producing the data contents from the given `DataDescription`.
     fn define_data(&mut self, data_id: DataId, data: &DataDescription) -> ModuleResult<()>;
 }
 

cranelift/object/src/backend.rs

@@ -15,8 +15,7 @@ use object::write::{
     Object, Relocation, SectionId, StandardSection, Symbol, SymbolId, SymbolSection,
 };
 use object::{
-    RelocationEncoding, RelocationFlags, RelocationKind, SectionKind, SymbolFlags, SymbolKind,
-    SymbolScope,
+    RelocationEncoding, RelocationFlags, RelocationKind, SectionFlags, SectionKind, SymbolFlags, SymbolKind, SymbolScope
 };
 use std::collections::HashMap;
 use std::collections::hash_map::Entry;
@@ -394,6 +393,7 @@ impl Module for ObjectModule {
             data_relocs: _,
             ref custom_segment_section,
             align,
+            used,
         } = data;
 
         let pointer_reloc = match self.isa.triple().pointer_width().unwrap() {
@@ -422,7 +422,7 @@ impl Module for ObjectModule {
             } else {
                 StandardSection::ReadOnlyDataWithRel
             };
-            if self.per_data_object_section {
+            if self.per_data_object_section || used {
                 // FIXME pass empty symbol name once add_subsection produces `.text` as section name
                 // instead of `.text.` when passed an empty symbol name. (object#748) Until then
                 // pass `subsection` to produce `.text.subsection` as section name to reduce
@@ -451,6 +451,34 @@ impl Module for ObjectModule {
             )
         };
 
+        if used {
+            match self.object.format() {
+                object::BinaryFormat::Elf => {
+                    let section = self.object.section_mut(section);
+                    match &mut section.flags {
+                        SectionFlags::None => {
+                            section.flags = SectionFlags::Elf {
+                                sh_flags: object::elf::SHF_GNU_RETAIN.into(),
+                            }
+                        }
+                        SectionFlags::Elf { sh_flags } => {
+                            *sh_flags |= u64::from(object::elf::SHF_GNU_RETAIN)
+                        }
+                        _ => unreachable!(),
+                    }
+                }
+                object::BinaryFormat::Coff => {},
+                object::BinaryFormat::MachO => {
+                    let symbol = self.object.symbol_mut(symbol);
+                    assert!(matches!(symbol.flags, SymbolFlags::None));
+                    symbol.flags = SymbolFlags::MachO {
+                        n_desc: object::macho::N_NO_DEAD_STRIP,
+                    }
+                }
+                _ => unreachable!(),
+            }
+        }
+
         let align = std::cmp::max(align.unwrap_or(1), self.isa.symbol_alignment());
         let offset = match *init {
             Init::Uninitialized => {