On Fri, Sep 20, 2013 at 07:15:29PM +0200, Jeroen Hofstee wrote:
Hello Tom,
On 09/19/2013 11:16 PM, Tom Rini wrote:
On Sat, Aug 24, 2013 at 01:55:38PM +0200, Jeroen Hofstee wrote:
The movt/movw instruction can be used to hardcode an memory location in the instruction itself. The linker starts complaining about this if the compiler decides to do so: "relocation R_ARM_MOVW_ABS_NC against `a local symbol' can not be used" and it is not support by U-boot as well. Prevent their use by requiring word relocations. This allows u-boot to be build at other optimalization levels then -Os.
Signed-off-by: Jeroen Hofstee jeroen@myspectrum.nl Cc: TigerLiu@viatech.com.cn Cc: Albert ARIBAUD albert.u.boot@aribaud.net
arch/arm/config.mk | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-)
Is this also something we need for llvm?
You guessed that right, for clang actually (llvm has already been taught to not emit movw/movt pairs, when requested not to do so). So with the -mword-relocations || present I can teach clang to tell llvm not to do it.
I am not aware of any reason why gcc could not decide to do the same in future releases. A pointer comparison e.g. is of exactly the same size (afaik). In this case U-boot will no longer compile without mentioned flag.
OK.
[snip]
If there's some -O2 enabled gcc flag we want because of a measurable performance win, we should add it specifically to -Os.
First of all the default -Os is unchanged and I have no intention to change it. -O2 won't build without the patch last time I checked ;)
Anyway, I like the flag since it helps to not special case clang and it guarantees builds with gcc at all optimisation levels, now and in the future. I don't care if it goes in this release or the next one.
Right, I'm OK picking this patch up then, on the grounds of making clang/llvm work now, and potentially keeping a future gcc happy.