GCC LTO objects and system ar archive interoperability

The Short Answer

No, you cannot reliably use the host system's ar to archive -flto object files if you intend to maintain LTO functionality. You must use gcc-ar (and gcc-ranlib where applicable) to ensure that the LTO symbol tables are correctly indexed. Without these wrappers, the linker (ld) will likely fail to resolve symbols within the archive or will treat the objects as standard binary blobs, effectively disabling cross-translation unit (TU) optimizations.

Technical Explanation

When you compile with -flto, GCC does not produce standard machine code in the object file; instead, it embeds GIMPLE intermediate representation. A standard ar tool is unaware of this format and cannot index the internal LTO symbols into the archive's symbol table.

Impact on Linking and Optimization

• Symbol Resolution: If ar is used, the archive index may be empty or incomplete. When ld searches the archive, it may conclude the required symbols are missing, leading to "undefined reference" errors.
• Optimization Loss: Even if the linker manages to find the objects, cross-TU inlining and interprocedural optimizations only occur if the GCC linker plugin can parse the bytecode. If the archive was not indexed for LTO, the plugin may skip these objects, resulting in a binary that is functionally correct but lacks the performance gains of LTO.
• The Role of the Plugin: ld cannot process LTO bytecode natively. It requires the GCC LTO plugin to transform the GIMPLE IR into machine code during the final link stage. gcc-ar ensures the archive is structured so the plugin can efficiently locate these IR sections.

Required Workflow

To ensure LTO interoperability across archives, replace your standard binutils calls with the GCC wrappers:

1. Archiving: Use gcc-ar rc libname.a obj1.o obj2.o instead of ar.
2. Indexing: Use gcc-ranlib libname.a if your toolchain does not automatically index during the gcc-ar phase.
3. Linking: Always link using the GCC driver (gcc or g++) with the -flto flag, rather than calling ld directly. This ensures the LTO plugin is loaded and the correct optimization passes are triggered.

Verification

To verify if your archive is correctly indexed for LTO, you can use nm. LTO objects typically contain special markers or a lack of standard text sections that gcc-ar handles specifically. A more practical test is to compare the final binary size and execution time of a build using ar versus gcc-ar; a significant increase in size or decrease in performance usually indicates LTO was silently disabled.

Missing Diagnostic: To provide a more specific version-based recommendation, please specify the GCC major version and the target architecture (e.g., x86_64, ARM) you are using.