Merge #318

318: Use LTO instead of removing the panic handler manually. Closes #309 r=jonas-schievink a=adamgreig



Co-authored-by: Adam Greig <[email protected]>

Author: bors[bot]

bin/thumbv6m-none-eabi-lto.a

@@ -11,7 +11,3 @@ harness = false
 
 [dependencies]
 ar = "0.8.0"
-
-[dependencies.object]
-version = "0.22.0"
-features = ["write"]

bin/thumbv6m-none-eabi.a

@@ -4,9 +4,6 @@
 //!
 //! Also see the docs in `asm.rs`.
 
-use object::read::{Object as _, ObjectSection as _};
-use object::write::{Object, Symbol, SymbolSection};
-use object::{ObjectSymbol, SymbolFlags};
 use std::collections::BTreeMap;
 use std::env::current_dir;
 use std::fs::{self, File};
@@ -25,90 +22,6 @@ fn rustc() -> Command {
     cmd
 }
 
-/// Patches an object file so that it doesn't contain a panic handler.
-///
-/// The panic handler defined in `asm/lib.rs` should never get linked to the final program.
-/// Unfortunately, Rust uses the same symbol for all panic handlers, and doesn't really like it if
-/// that ends up with multiple ones. It also demands that we define a panic handler for the inline
-/// assembly shim, even though none of that code should ever be able to panic. The result of this is
-/// that the supposedly unreachable panic handler does end up getting linked into the final program,
-/// unless it is built with optimizations enabled.
-///
-/// To fix that, we put the never-to-be-used panic handler into its own section via
-/// `#[link_section]`, and then use this function to delete that section.
-fn trim_panic_handler(obj_file: &str) {
-    let objdata = fs::read(&obj_file).unwrap();
-    let obj = object::File::parse(&objdata).unwrap();
-
-    let mut writer = Object::new(obj.format(), obj.architecture(), obj.endianness());
-    writer.flags = obj.flags(); // Preserve flags of input file
-
-    for (sec_index, section) in obj.sections().enumerate() {
-        assert_eq!(section.index().0, sec_index);
-
-        let name = section.name().unwrap();
-        if name.starts_with(".ARM")
-            || name.starts_with(".rel.ARM")
-            || name.contains("asm_panic_handler")
-            || name == ".strtab"
-            || name == ".symtab"
-        {
-            // We drop the ARM exception handling tables since they refer back to the panic handler
-            // symbol. They aren't used either way. We also drop `.strtab` and `.symtab` since they
-            // otherwise end up having the wrong section type. The object crate should rebuild any
-            // index tables when writing the file.
-            continue;
-        }
-
-        let segment = section
-            .segment_name()
-            .unwrap()
-            .map(|s| s.as_bytes())
-            .unwrap_or(&[]);
-        let sec_id = writer.add_section(segment.to_vec(), name.as_bytes().to_vec(), section.kind());
-
-        let align = if section.align() == 0 {
-            // Not sure why but `section.align()` can return 0.
-            1
-        } else {
-            section.align()
-        };
-        writer.append_section_data(sec_id, section.data().unwrap(), align);
-
-        // Import all symbols from the section.
-        for symbol in obj.symbols() {
-            if symbol.section_index() == Some(section.index()) {
-                writer.add_symbol(Symbol {
-                    name: symbol.name().unwrap_or("").as_bytes().to_vec(),
-                    value: symbol.address(),
-                    size: symbol.size(),
-                    kind: symbol.kind(),
-                    scope: symbol.scope(),
-                    weak: symbol.is_weak(),
-                    section: match symbol.section() {
-                        object::SymbolSection::Unknown => unimplemented!(),
-                        object::SymbolSection::None => SymbolSection::None,
-                        object::SymbolSection::Undefined => SymbolSection::Undefined,
-                        object::SymbolSection::Absolute => SymbolSection::Absolute,
-                        object::SymbolSection::Common => SymbolSection::Common,
-                        object::SymbolSection::Section(_) => SymbolSection::Section(sec_id),
-                    },
-                    flags: match symbol.flags() {
-                        SymbolFlags::None => SymbolFlags::None,
-                        SymbolFlags::Elf { st_info, st_other } => {
-                            SymbolFlags::Elf { st_info, st_other }
-                        }
-                        _ => unimplemented!(),
-                    },
-                });
-            }
-        }
-    }
-
-    let obj = writer.write().unwrap();
-    fs::write(&obj_file, obj).unwrap();
-}
-
 fn assemble_really(target: &str, cfgs: &[&str], plugin_lto: bool) {
     let mut cmd = rustc();
 
@@ -122,6 +35,10 @@ fn assemble_really(target: &str, cfgs: &[&str], plugin_lto: bool) {
     // We always optimize the assembly shims. There's not really any reason not to.
     cmd.arg("-O");
 
+    // We use LTO on the archive to ensure the (unused) panic handler is removed, preventing
+    // a linker error when the archives are linked into final crates with two panic handlers.
+    cmd.arg("-Clto=yes");
+
     // rustc will usually add frame pointers by default to aid with debugging, but that is a high
     // overhead for the tiny assembly routines.
     cmd.arg("-Cforce-frame-pointers=no");
@@ -158,11 +75,6 @@ fn assemble_really(target: &str, cfgs: &[&str], plugin_lto: bool) {
     let status = cmd.status().unwrap();
     assert!(status.success());
 
-    if !plugin_lto {
-        // Post-process the object file.
-        trim_panic_handler(&obj_file);
-    }
-
     // Archive `target.o` -> `bin/target.a`.
     let mut builder = ar::Builder::new(File::create(format!("bin/{}.a", file_stub)).unwrap());