[PATCH 7/7] Add some gcc/ld optimizations and magic

[Peter Hutterer]

On Sat, Aug 17, 2013 at 01:58:19PM +0200, David Herrmann wrote:
> There are several gcc/ld flags that optimize size and performance without
> requiring explicit code changes. In no particular order, this adds:
>  - gcc -pipe to avoid temporary files and use pipes during compilation
>  - gcc -fno-common avoids putting uninitialized global variables not
>    marked as "extern" into a common section. This catches compilation
>    errors if we didn't mark global variables explicitly as "extern".
>  - gcc -fno-strict-aliasing allows us to use unions for some binary magic.
>    Otherwise, -O2 might assume that two different types never point at the
>    same memory. We currently don't rely on this but it's common practice
>    so avoid any non-obvious runtime errors later.
>  - gcc -ffunction-sections and -fdata-sections put each function and
>    variable into a separate section. This enables ld's --gc-sections to
>    drop any unused sections (sections which aren't referenced from an
>    exported section). This is very useful to avoid putting dead code into
>    DSOs. We can now link any helper function into libevdev and the linker
>    removes all of them if they're unused.
>  - gcc -fstack-protector adds small stack-corruption protectors in
>    functions which have big buffers on the stack (>8bytes). If the
>    stack-protectors are corrupted, the process is aborted. This is highly
>    useful to debug stack-corruption issues which often are nearly
>    impossible to catch without this.
>  - ld --as-needed drops all linked libraries that are not actually
>    required by libevdev. So we can link to whatever we want and the linker
>    will drop everything which is not actually used.
>  - ld -z now, resolve symbols during linking, not during runtime.
>  - ld -z relro, add relocation-read-only section. This allows to put
>    read-only global variables and alike into a read-only section. This is
>    useful for variables that need a relocation and thus cannot be
>    explicitly put into a read-only section. This option tells the linker
>    to mark them read-only after relocations are done. (that's why -z now
>    makes sense in combination with this)
> 
> All of these options are common in other open-source projects, including
> systemd and weston. Don't ask me why they are not marked as default..

ACK to the changes but funny that: GCC_CFLAGS isn't actually used atm :)
and setting it properly produces all kinds of warnings, specifically the
override-init issue I just sent out the patch for. And once the actual
visibility stuff kicks in (which I had thought was already enabled) we need
that EXPORT macro. I'll wait for an updated version here from you before we
add the GCC_CFLAGS to AM_CPPFLAGS as intended.

> Signed-off-by: David Herrmann <dh.herrmann at gmail.com>
> ---
>  configure.ac         | 2 +-
>  libevdev/Makefile.am | 6 +++++-
>  2 files changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/configure.ac b/configure.ac
> index 889ff81..b1fc179 100644
> --- a/configure.ac
> +++ b/configure.ac
> @@ -41,7 +41,7 @@ fi
>  AM_CONDITIONAL(BUILD_TESTS, [test "x$HAVE_CHECK" = "xyes"])
>  
>  if test "x$GCC" = "xyes"; then
> -	GCC_CFLAGS="-Wall -Wextra -Wno-unused-parameter -g -Wstrict-prototypes -Wmissing-prototypes -fvisibility=hidden"
> +	GCC_CFLAGS="-Wall -Wextra -Wno-unused-parameter -g -Wstrict-prototypes -Wmissing-prototypes -fvisibility=hidden -pipe -fno-common -fno-strict-aliasing -ffunction-sections -fdata-sections -fstack-protector -fno-strict-aliasing -fdiagnostics-show-option -fno-common"

follow-up patch: -g shouldn't be in the default CFLAGS, that's a local setup
IMO.

Cheers,
   Peter

>  fi
>  AC_SUBST(GCC_CFLAGS)
>  
> diff --git a/libevdev/Makefile.am b/libevdev/Makefile.am
> index bd8e4fe..eedb468 100644
> --- a/libevdev/Makefile.am
> +++ b/libevdev/Makefile.am
> @@ -11,7 +11,11 @@ libevdev_la_SOURCES = \
>  libevdev_la_LDFLAGS = \
>  	-version-info $(LIBEVDEV_LT_VERSION) \
>  	-Wl,--version-script="$(srcdir)/libevdev.sym" \
> -	$(GCOV_LDFLAGS)
> +	$(GCOV_LDFLAGS) \
> +	-Wl,--as-needed \
> +	-Wl,--gc-sections \
> +	-Wl,-z,relro \
> +	-Wl,-z,now
>  
>  EXTRA_libevdev_la_DEPENDENCIES = $(srcdir)/libevdev.sym
>  
> -- 
> 1.8.3.4
>