Use Link-Time Optimization (LTO) and Measure Effects

Whole-Program Visibility at Link

Link-Time Optimization (LTO) stores LLVM bitcode or GCC GIMPLE in .o files and runs optimization when linking. Cross-module inlining and devirtualization become possible. Chrome and Rust release builds rely on thin LTO for large link times.

Enabling LTO

Clang: -flto=thin at compile and link. GCC: -flto plus use gcc driver for the final link. Static libraries must be built with LTO objects too.

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• Link step memory spikes on huge projects; thin LTO mitigates
• Debug symbols need -g at LTO link for usable stacks
• Compare nm symbol counts before/after cross-module inline
• PGO profiles still apply with LTO

Working Through the Exercise

Keep the relevant documentation open while you implement. When your output disagrees with the reference, trace one failing case by hand before changing random lines.

• Trace first: walk one failing input step by step
• Read the manual: skim the official doc for the mechanism named in this task
• Compare baselines: save measurements from the prior step so speedups are honest

Why for this exercise

You will build a two-file project with and without LTO and measure call overhead across translation units. This exercise asks you to show one function inlined only in the LTO build.