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buildflags.md: Document LTO and how to disable it

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Florian Weimer 2021-11-15 15:44:56 +01:00
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buildflags.md
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@ -105,6 +105,17 @@ or:
BuildRequires: clang compiler-rt BuildRequires: clang compiler-rt
%endif %endif
### Disabling Link-Time Optimization
By default, builds use link-time optimization. In this build mode,
object code is generated at the time of the final link, by combining
information from all available translation units, and taking into
account which symbols are exported.
To disable this optimization, include this in the spec file:
%define _lto_cflags %{nil}
### Lazy binding ### Lazy binding
If your package depends on the semantics of lazy binding (e.g., it has If your package depends on the semantics of lazy binding (e.g., it has
@ -262,6 +273,11 @@ The general (architecture-independent) build flags are:
variables. (If the address of a variable is never taken, it is not variables. (If the address of a variable is never taken, it is not
possible that a buffer overflow is caused by incorrect pointer possible that a buffer overflow is caused by incorrect pointer
arithmetic involving a pointer to that variable.) arithmetic involving a pointer to that variable.)
* `-flto=auto`: Enable link-time optimization (LTO), using `make` job server
integration for parallel processing. (`gcc` toolchain only)
* `-ffat-lto-objects`: Generate EFL object files which contain both
object code and LTO intermediate representation. (`gcc` toolchain only)
* `-flto`: Enable link-time optimization. (`clang` toolchain only)
* `-grecord-gcc-switches`: Include select GCC command line switches in * `-grecord-gcc-switches`: Include select GCC command line switches in
the DWARF debugging information. This is useful for detecting the the DWARF debugging information. This is useful for detecting the
presence of certain build flags and general hardening coverage. presence of certain build flags and general hardening coverage.