[U-Boot] [PATCH] arm: prevent using movt/movw address loads
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(-)
diff --git a/arch/arm/config.mk b/arch/arm/config.mk index 540a119..2277c82 100644 --- a/arch/arm/config.mk +++ b/arch/arm/config.mk @@ -94,7 +94,11 @@ PLATFORM_RELFLAGS += -fno-optimize-sibling-calls endif endif
-# check that only R_ARM_RELATIVE relocations are generated ifneq ($(CONFIG_SPL_BUILD),y) -ALL-y += checkarmreloc +# Check that only R_ARM_RELATIVE relocations are generated. +ALL-y += checkarmreloc +# The movt / movw can hardcode 16 bit parts of the addresses in the +# instruction. Relocation is not supported for that case, so disable +# such usage by requiring word relocations. +PLATFORM_CPPFLAGS += $(call cc-option, -mword-relocations) endif
Hi, experts:
-# check that only R_ARM_RELATIVE relocations are generated ifneq ($(CONFIG_SPL_BUILD),y) -ALL-y += checkarmreloc +# Check that only R_ARM_RELATIVE relocations are generated. +ALL-y += checkarmreloc +# The movt / movw can hardcode 16 bit parts of the addresses in the +# instruction. Relocation is not supported for that case, so disable +# such usage by requiring word relocations. +PLATFORM_CPPFLAGS += $(call cc-option, -mword-relocations) endif
Jeroen's patch is very simple. So, is there any side-effect? If not, why not add it into 2013.10 release version? :)
Best wishes,
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? I am hesitant here because as Wolfgang points out, -O0 is usually the wrong way to debug a problem and I'll add we're well into the age where debuggers work just fine with optimized code. If there's some -O2 enabled gcc flag we want because of a measurable performance win, we should add it specifically to -Os.
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.
I am hesitant here because as Wolfgang points out, -O0 is usually the wrong way to debug a problem and I'll add we're well into the age where debuggers work just fine with optimized code.
mmm, I don't share your concern here. Not that I disagree with what Wolfgang said, but since it is unrelated to the patch itself. What I read was that Wolfgang tried to explain to a ML poster without a proper name that it might be even harder at times to find a bug at -O0, since it is a different binary and that it is not considered a bug. I assume the fast majority of U-boot developers know these to debug things..
If you really have that little trust in U-boot developers a more proper way would be to actually create a make rule checking cflags and point them to a nice debugging document. And I really hope you don't do that ;)
One thing I can think of in favour of -O0 is for educational purposes. You can run u-boot in qemu then without the, at times weird optimized jumps, to get an idea about basic code flow.
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.
Regards, Jeroen
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.
On Sat, Aug 24, 2013 at 5:55 AM, Jeroen Hofstee jeroen@myspectrum.nlwrote:
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
This is useful I think. I'm not sure that -O0 works very well anymore (at least I need to make a few tweaks to use it), but -O1 is useful in some cases to provide better debugging.
Acked-by: Simon Glass sjg@chromium.org
Hi Jeroen,
On Sat, 24 Aug 2013 13:55:38 +0200, Jeroen Hofstee jeroen@myspectrum.nl 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(-)
diff --git a/arch/arm/config.mk b/arch/arm/config.mk index 540a119..2277c82 100644 --- a/arch/arm/config.mk +++ b/arch/arm/config.mk @@ -94,7 +94,11 @@ PLATFORM_RELFLAGS += -fno-optimize-sibling-calls endif endif
-# check that only R_ARM_RELATIVE relocations are generated ifneq ($(CONFIG_SPL_BUILD),y) -ALL-y += checkarmreloc +# Check that only R_ARM_RELATIVE relocations are generated. +ALL-y += checkarmreloc +# The movt / movw can hardcode 16 bit parts of the addresses in the +# instruction. Relocation is not supported for that case, so disable +# such usage by requiring word relocations. +PLATFORM_CPPFLAGS += $(call cc-option, -mword-relocations) endif
Applied as a bugfix to u-boot-arm/master, thanks!
Amicalement,
participants (5)
-
Albert ARIBAUD -
Jeroen Hofstee -
Simon Glass -
TigerLiu＠viatech.com.cn -
Tom Rini