ARM Compiler toolchain Using the Linker - ARM Information Center

5.7 About link-time code generation
5.7.1 Considerations when using LTCG
5.7.2 Example
Using linker optimizations
Link-time code generation (LTCG) enables cross source-file optimization by delaying code
generation until the link stage. This can significantly reduce code size. To enable LTCG, compile
your source with -c --ltcg to create objects in an intermediate format. You must then link these
object files with --ltcg to instruct the linker to perform code generation.
Both armcc and armlink have a --ltcg command-line option to enable LTCG.
Note
You can also use LTCG with Profiler-guided optimizations.
Be aware of the following when using LTCG:
• If no input objects are compiled with --ltcg, the final object is the same as that produced
without LTCG.
• Debug information is not preserved in any object files compiled with --ltcg. However,
debug information is still preserved for any object files that are not compiled with --ltcg.
• You cannot use specific object file names in scatter files because LTCG causes a
temporary object file to be created and used for linking. If you are using a scatter file, then
you must match files using the wildcard *. Otherwise no match is made on the temporary
file.
• If there is more than one entry point in the input object files provided to the linker, the
linker clears all entry points. Therefore, you must specify an entry point on the linker
command-line with:
--entry=symbol
Any input section containing an entry point is not removed. For example, where an object
is built without --ltcg.
Note
The linker option --entry=object(symbol) is not supported when using LTCG.
The following example shows a typical use of LTCG:
1. Create ELF object files one.o and two.o with --ltcg:
armcc -c --ltcg one.c -o one.o
armcc -c --ltcg two.c -o two.o
The compiler generates intermediate code into the object files.
2. Link them using the command:
armlink --ltcg one.o two.o -o ltcg_image.axf
The linker:
1. Combines all the immediate code together, losing the link to original object file names.
2. Performs code generation for the intermediate code.
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