rustc_trans: make .note.rustc look more like debug info

Mark the global variable as const and private so the resulting section
is not flagged as writable and to avoid putting an unnecessary symbol in
the dynamic table of shared objects.

Unfortunately there doesn't seem to be a way to avoid the section being
marked SHF_ALLOC when declared as a variable in LLVM. Hack around that
by using objcopy to clear the flags on the section before the final
link.

This places the section at the end of the executable so it can be
stripped later without rearranging important code/data sections.

Author: tylerwhall

mk/cfg/arm-unknown-linux-gnueabi.mk

@@ -4,6 +4,7 @@ CC_arm-unknown-linux-gnueabi=gcc
 CXX_arm-unknown-linux-gnueabi=g++
 CPP_arm-unknown-linux-gnueabi=gcc -E
 AR_arm-unknown-linux-gnueabi=ar
+OBJCOPY_arm-unknown-linux-gnueabi=objcopy
 CFG_LIB_NAME_arm-unknown-linux-gnueabi=lib$(1).so
 CFG_STATIC_LIB_NAME_arm-unknown-linux-gnueabi=lib$(1).a
 CFG_LIB_GLOB_arm-unknown-linux-gnueabi=lib$(1)-*.so

mk/cfg/arm-unknown-linux-gnueabihf.mk

@@ -4,6 +4,7 @@ CC_arm-unknown-linux-gnueabihf=gcc
 CXX_arm-unknown-linux-gnueabihf=g++
 CPP_arm-unknown-linux-gnueabihf=gcc -E
 AR_arm-unknown-linux-gnueabihf=ar
+OBJCOPY_arm-unknown-linux-gnueabihf=objcopy
 CFG_LIB_NAME_arm-unknown-linux-gnueabihf=lib$(1).so
 CFG_STATIC_LIB_NAME_arm-unknown-linux-gnueabihf=lib$(1).a
 CFG_LIB_GLOB_arm-unknown-linux-gnueabihf=lib$(1)-*.so

mk/cfg/i686-unknown-linux-gnu.mk

@@ -3,6 +3,7 @@ CC_i686-unknown-linux-gnu=$(CC)
 CXX_i686-unknown-linux-gnu=$(CXX)
 CPP_i686-unknown-linux-gnu=$(CPP)
 AR_i686-unknown-linux-gnu=$(AR)
+OBJCOPY_i686-unknown-linux-gnu=$(OBJCOPY)
 CFG_LIB_NAME_i686-unknown-linux-gnu=lib$(1).so
 CFG_STATIC_LIB_NAME_i686-unknown-linux-gnu=lib$(1).a
 CFG_LIB_GLOB_i686-unknown-linux-gnu=lib$(1)-*.so

mk/cfg/x86_64-unknown-linux-gnu.mk

@@ -3,6 +3,7 @@ CC_x86_64-unknown-linux-gnu=$(CC)
 CXX_x86_64-unknown-linux-gnu=$(CXX)
 CPP_x86_64-unknown-linux-gnu=$(CPP)
 AR_x86_64-unknown-linux-gnu=$(AR)
+OBJCOPY_x86_64-unknown-linux-gnu=$(OBJCOPY)
 CFG_LIB_NAME_x86_64-unknown-linux-gnu=lib$(1).so
 CFG_STATIC_LIB_NAME_x86_64-unknown-linux-gnu=lib$(1).a
 CFG_LIB_GLOB_x86_64-unknown-linux-gnu=lib$(1)-*.so

mk/platform.mk

@@ -149,6 +149,7 @@ define CFG_MAKE_TOOLCHAIN
        CPP_$(1)=$(CROSS_PREFIX_$(1))$(CPP_$(1))
        AR_$(1)=$(CROSS_PREFIX_$(1))$(AR_$(1))
        RUSTC_CROSS_FLAGS_$(1)=-C linker=$$(call FIND_COMPILER,$$(CC_$(1))) \
+           -C objcopy=$$(call FIND_COMPILER,$$(OBJCOPY_$(1))) \
            -C ar=$$(call FIND_COMPILER,$$(AR_$(1))) $(RUSTC_CROSS_FLAGS_$(1))
 
        RUSTC_FLAGS_$(1)=$$(RUSTC_CROSS_FLAGS_$(1)) $(RUSTC_FLAGS_$(1))

src/librustc/session/config.rs

@@ -478,6 +478,8 @@ options! {CodegenOptions, CodegenSetter, basic_codegen_options,
          CG_OPTIONS, cg_type_desc, cgsetters,
     ar: Option<String> = (None, parse_opt_string,
         "tool to assemble archives with"),
+    objcopy: String = ("objcopy".to_string(), parse_string,
+        "objcopy"),
     linker: Option<String> = (None, parse_opt_string,
         "system linker to link outputs with"),
     link_args: Option<Vec<String>> = (None, parse_opt_list,

src/librustc_trans/back/link.rs

@@ -849,6 +849,33 @@ fn link_natively(sess: &Session, trans: &CrateTranslation, dylib: bool,
     }
 }
 
+fn fix_meta_section_attributes(sess: &Session, meta_name: &PathBuf) {
+    // First, fix up the note section attributes. We want the SHF_ALLOC and
+    // SHF_WRITE flags to be unset so the section will get placed near the
+    // end along with the debug info. This allows the section to be
+    // stripped later without renumbering important sections that contain
+    // code and data.
+    let mut o_cmd = Command::new(&sess.opts.cg.objcopy);
+    o_cmd.arg("--rename-section")
+         .arg(".note.rustc=.note.rustc,contents,noload,readonly")
+         .arg(&meta_name);
+    // Invoke objcopy
+    info!("{:?}", o_cmd);
+    match o_cmd.status() {
+        Ok(exitstatus) => {
+            if !exitstatus.success() {
+                sess.err(&format!("objcopy failed with exit code {:?}", exitstatus.code()));
+                sess.note(&format!("{:?}", &o_cmd));
+            }
+        },
+        Err(exitstatus) => {
+            sess.err(&format!("objcopy failed: {}", exitstatus));
+            sess.note(&format!("{:?}", &o_cmd));
+        }
+    }
+    sess.abort_if_errors();
+}
+
 fn link_args(cmd: &mut Command,
              sess: &Session,
              dylib: bool,
@@ -901,7 +928,10 @@ fn link_args(cmd: &mut Command,
     // executable. This metadata is in a separate object file from the main
     // object file, so we link that in here.
     if dylib {
-        cmd.arg(&obj_filename.with_extension("metadata.o"));
+        let meta_name = obj_filename.with_extension("metadata.o");
+
+        fix_meta_section_attributes(sess, &meta_name);
+        cmd.arg(&meta_name);
     }
 
     if t.options.is_like_osx {

src/librustc_trans/trans/base.rs

@@ -2520,6 +2520,9 @@ pub fn write_metadata(cx: &SharedCrateContext, krate: &ast::Crate) -> Vec<u8> {
     };
     unsafe {
         llvm::LLVMSetInitializer(llglobal, llconst);
+        llvm::LLVMSetGlobalConstant(llglobal, llvm::True);
+        llvm::LLVMSetUnnamedAddr(llglobal, llvm::True);
+        llvm::SetLinkage(llglobal, llvm::Linkage::PrivateLinkage);
         let name = loader::meta_section_name(cx.sess().target.target.options.is_like_osx);
         let name = CString::new(name).unwrap();
         llvm::LLVMSetSection(llglobal, name.as_ptr())