2.4.3. Link time optimization

Violating the One Definition Rule (ODR) produces a -Wodr warning

Violations of the ODR resulting in undefined behavior produce a -Wodr warning. This usually points to a bug in your program. The -Wodr warning is enabled by default.

LTO causes increased memory consumption

The compiler consumes more memory when it processes the translation units the program consists of. On systems with limited memory, disable LTO or lower the parallelism level when building your program.

GCC removes seemingly unused functions

GCC might remove functions it considers unused because the compiler is unable to recognize which symbols an asm() statement references. A compilation error might occur as a result. To prevent this, add __attribute__((used)) to the symbols you use in your program.

Compiling with the -fPIC option causes errors

Because GCC does not parse the contents of asm() statements, compiling your code with the -fPIC command-line option can cause errors. To prevent this, use the -fno-lto option when compiling your translation unit. More information is available at LTO FAQ {mdash}; Symbol usage from assembly language.

Implementing symbol versioning by using the .symver directive in an asm() statement is not compatible with LTO. However, it is possible to implement symbol versioning using the symver attribute. For example:

__attribute__ ((_symver_ ("<symbol>@VERS_1"))) void <symbol>_v1 (void) { }