[PATCH] ARC: build: Turn off -Wmaybe-uninitialized for ARC gcc 4.8

[PATCH] ARC: build: Turn off -Wmaybe-uninitialized for ARC gcc 4.8

[Vineet Gupta]

linux-next has been reporting gazillion warnings for ARC build and
I finally decided to take a bite:

http://kisskb.ellerman.id.au/kisskb/buildresult/12638735/

Most of the them are due to -Wmaybe-uninitialized

| ../kernel/sysctl.c: In function '__do_proc_doulongvec_minmax':
| ../kernel/sysctl.c:1928:12: warning: 'p' may be used uninitialized in this function [-Wmaybe-uninitialized]
|   ret = tmp - *buf;
|            ^
| ../kernel/sysctl.c:2342:29: note: 'p' was declared here
|  char *kbuf = NULL, *p;
|                     ^
| ...
| ...

Cursory look at code seemed fine and a definite gcc false positive in say
kernel/sysctl.c

Mystery was why only for ARC (and not with ARM linaro toolchain based
off same gcc 4.8). Turns out that -O3 (default for ARC) triggers these
and if I enable -O3 for ARM kernel build, I see the same splat.

I doubt if gcc folks are going to fix warnings in gcc 4.8 (specially when this
was the much advertised front page feature @
https://gcc.gnu.org/gcc-4.8/porting_to.html

Better to to silent these for ARC, atleast for gcc 4.8 when we know that
these warnings are likely benign.

When we switch to later gcc, these will come back on and at that time we
could fixing gcc and/or relevant kernel code.

Cc: Claudiu Zissulescu <Claudiu.Zissulescu@synopsys.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Michal Marek <mmarek@suse.cz>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: linux-kbuild@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
---
 arch/arc/Makefile | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/arc/Makefile b/arch/arc/Makefile
index fed12f39d8ce..aeb101e8e674 100644
--- a/arch/arc/Makefile
+++ b/arch/arc/Makefile
@@ -48,9 +48,14 @@ endif
 upto_gcc44    :=  $(call cc-ifversion, -le, 0404, y)
 atleast_gcc44 :=  $(call cc-ifversion, -ge, 0404, y)
 atleast_gcc48 :=  $(call cc-ifversion, -ge, 0408, y)
+is_gcc48      :=  $(call cc-ifversion, -eq, 0408, y)
 
 cflags-$(atleast_gcc44)			+= -fsection-anchors
 
+# gcc 4.8 spits out false positives for default -O3
+# disable these for 4.8 and revisit when we upgrade to newer ver
+cflags-$(is_gcc48)			+= $(call cc-disable-warning,maybe-uninitialized,)
+
 cflags-$(CONFIG_ARC_HAS_LLSC)		+= -mlock
 cflags-$(CONFIG_ARC_HAS_SWAPE)		+= -mswape
 
-- 
2.5.0

Re: [PATCH] ARC: build: Turn off -Wmaybe-uninitialized for ARC gcc 4.8

[Arnd Bergmann]

On Friday 18 March 2016 14:16:23 Vineet Gupta wrote:
> diff --git a/arch/arc/Makefile b/arch/arc/Makefile
> index fed12f39d8ce..aeb101e8e674 100644
> --- a/arch/arc/Makefile
> +++ b/arch/arc/Makefile
> @@ -48,9 +48,14 @@ endif
>  upto_gcc44    :=  $(call cc-ifversion, -le, 0404, y)
>  atleast_gcc44 :=  $(call cc-ifversion, -ge, 0404, y)
>  atleast_gcc48 :=  $(call cc-ifversion, -ge, 0408, y)
> +is_gcc48      :=  $(call cc-ifversion, -eq, 0408, y)
>  
>  cflags-$(atleast_gcc44)                        += -fsection-anchors
>  
> +# gcc 4.8 spits out false positives for default -O3
> +# disable these for 4.8 and revisit when we upgrade to newer ver
> +cflags-$(is_gcc48)                     += $(call cc-disable-warning,maybe-uninitialized,)
> +
>  cflags-$(CONFIG_ARC_HAS_LLSC)          += -mlock
>  cflags-$(CONFIG_ARC_HAS_SWAPE)         += -mswape

Is this any better with gcc-4.9 or gcc-5? Maybe it's better to add the flag to
the line that adds -O3 for consistency. We do the same thing for -Os in the
global Makefile, as that triggers a similar load of warnings.

	Arnd

Re: [PATCH] ARC: build: Turn off -Wmaybe-uninitialized for ARC gcc 4.8

[Vineet Gupta]

On Friday 18 March 2016 03:22 PM, Arnd Bergmann wrote:
> On Friday 18 March 2016 14:16:23 Vineet Gupta wrote:
>> diff --git a/arch/arc/Makefile b/arch/arc/Makefile
>> index fed12f39d8ce..aeb101e8e674 100644
>> --- a/arch/arc/Makefile
>> +++ b/arch/arc/Makefile
>> @@ -48,9 +48,14 @@ endif
>>  upto_gcc44    :=  $(call cc-ifversion, -le, 0404, y)
>>  atleast_gcc44 :=  $(call cc-ifversion, -ge, 0404, y)
>>  atleast_gcc48 :=  $(call cc-ifversion, -ge, 0408, y)
>> +is_gcc48      :=  $(call cc-ifversion, -eq, 0408, y)
>>  
>>  cflags-$(atleast_gcc44)                        += -fsection-anchors
>>  
>> +# gcc 4.8 spits out false positives for default -O3
>> +# disable these for 4.8 and revisit when we upgrade to newer ver
>> +cflags-$(is_gcc48)                     += $(call cc-disable-warning,maybe-uninitialized,)
>> +
>>  cflags-$(CONFIG_ARC_HAS_LLSC)          += -mlock
>>  cflags-$(CONFIG_ARC_HAS_SWAPE)         += -mswape
> 
> Is this any better with gcc-4.9 or gcc-5? 

I don't think there's a production ARC toolchain with gcc 4.9 which we can use yet
- Claudiu is still in the the middle of upstreaming the new ARC HS port bits so
things are still in flight there. These tools are off of github !

Maybe it's better to add the flag to
> the line that adds -O3 for consistency. We do the same thing for -Os in the
> global Makefile, as that triggers a similar load of warnings.

Sure, but I prefer this to be only for gcc 4.8 as this warning seems to be
healthy in small doses :-) At least it keeps the door open for future discussion
with gcc guys !

The following nested construct actually works - does that look OK to you ?

ARCH_CFLAGS += -O3 $(call cc-ifversion, -lt, 0408, $(call
cc-disable-warning,maybe-uninitialized,))

Thx,
-Vineet
> 
> 	Arnd
> --
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> 

Re: [PATCH] ARC: build: Turn off -Wmaybe-uninitialized for ARC gcc 4.8

[Arnd Bergmann]

On Friday 18 March 2016 15:50:11 Vineet Gupta wrote:
> Sure, but I prefer this to be only for gcc 4.8 as this warning seems to be
> healthy in small doses  At least it keeps the door open for future discussion
> with gcc guys !

FWIW, testing on ARM with gcc-6.0 -O3, I also get tons of maybe-uninitialized
warnings. It's unlikely that this is architecture specific or fixed in newer
compiler versions.

> The following nested construct actually works - does that look OK to you ?
> 
> ARCH_CFLAGS += -O3 $(call cc-ifversion, -lt, 0408, $(call cc-disable-warning,maybe-uninitialized,))

Yes, that seems ok.

I don't really understand why -O3 is needed though, maybe it's better to
assume that it won't be needed in future gcc versions and do

ARCH_CFLAGS += $(call cc-ifversion, -lt, 0408, -O3 $(call cc-disable-warning,maybe-uninitialized,))

	Arnd

Re: [PATCH] ARC: build: Turn off -Wmaybe-uninitialized for ARC gcc 4.8

[Vineet Gupta]

On Friday 18 March 2016 03:59 PM, Arnd Bergmann wrote:
> On Friday 18 March 2016 15:50:11 Vineet Gupta wrote:
>> Sure, but I prefer this to be only for gcc 4.8 as this warning seems to be
>> healthy in small doses  At least it keeps the door open for future discussion
>> with gcc guys !
> 
> FWIW, testing on ARM with gcc-6.0 -O3, I also get tons of maybe-uninitialized
> warnings. It's unlikely that this is architecture specific or fixed in newer
> compiler versions.

So we disable this for good just like -Os.
What a shame - seemed like a reasonable safety net for programming errors.

>> The following nested construct actually works - does that look OK to you ?
>>
>> ARCH_CFLAGS += -O3 $(call cc-ifversion, -lt, 0408, $(call cc-disable-warning,maybe-uninitialized,))
> 
> Yes, that seems ok.

There was typo actually -lt needed to be -eq

> I don't really understand why -O3 is needed though, maybe it's better to
> assume that it won't be needed in future gcc versions and do

Not sure what you mean, -O3 for triggering the warnings or -O3 in ARC makefile at all.
Assuming it's latter, this is how its been forever and was added consciously as
performance seemed better with -O3 than the default -O2.


> 
> ARCH_CFLAGS += $(call cc-ifversion, -lt, 0408, -O3 $(call cc-disable-warning,maybe-uninitialized,))
> 
> 	Arnd
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kbuild" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> 

Re: [PATCH] ARC: build: Turn off -Wmaybe-uninitialized for ARC gcc 4.8

[Arnd Bergmann]

On Friday 18 March 2016 16:13:28 Vineet Gupta wrote:
> On Friday 18 March 2016 03:59 PM, Arnd Bergmann wrote:
> > On Friday 18 March 2016 15:50:11 Vineet Gupta wrote:
> >> Sure, but I prefer this to be only for gcc 4.8 as this warning seems to be
> >> healthy in small doses  At least it keeps the door open for future discussion
> >> with gcc guys !
> > 
> > FWIW, testing on ARM with gcc-6.0 -O3, I also get tons of maybe-uninitialized
> > warnings. It's unlikely that this is architecture specific or fixed in newer
> > compiler versions.
> 
> So we disable this for good just like -Os.
> What a shame - seemed like a reasonable safety net for programming errors.

Yes, it's an immensely useful warning, if you manage to avoid the
false positives.

> >> The following nested construct actually works - does that look OK to you ?
> >>
> >> ARCH_CFLAGS += -O3 $(call cc-ifversion, -lt, 0408, $(call cc-disable-warning,maybe-uninitialized,))
> > 
> > Yes, that seems ok.
> 
> There was typo actually -lt needed to be -eq
> 
> > I don't really understand why -O3 is needed though, maybe it's better to
> > assume that it won't be needed in future gcc versions and do
> 
> Not sure what you mean, -O3 for triggering the warnings or -O3 in ARC makefile at all.
> Assuming it's latter, this is how its been forever and was added consciously as
> performance seemed better with -O3 than the default -O2.

I think it's dangerous to use -O3 in one architecture when nothing else
uses it. If you don't have a strong reason to use -O3, maybe just drop that
use the default -O2 -Wmaybe-uninitialized like everyone else does.

On a related note, I have submitted a patch that turns CONFIG_CC_OPTIMIZE_FOR_SIZE
into a choice statement, so we actually get the -Wmaybe-uninitialized warnings
in an allyesconfig or allmodconfig build. It would be trivial to extend that
to give the choice between -Os, -O2 and -O3, and then pick -O3 in a defconfig,
over the -O2 default.

	Arnd

Re: [PATCH] ARC: build: Turn off -Wmaybe-uninitialized for ARC gcc 4.8

[Vineet Gupta]

On Friday 18 March 2016 05:43 PM, Arnd Bergmann wrote:
> I think it's dangerous to use -O3 in one architecture when nothing else
> uses it. If you don't have a strong reason to use -O3, maybe just drop that
> use the default -O2 -Wmaybe-uninitialized like everyone else does.

I know what u mean. In fact top level makefile change has bitten us atleast once.
However ARC gcc tends to generate better code (likely others do too) at -O3 and
increased inlining etc reflects in several benchmarks. Maybe its due to the
micro-architecture and smaller hardware call return stack - I'm not sure. So I'm
ready to pay that maintenance price. And it has been like this for 5+ years and I
fear that switching to -O2 might unravel something unwarranted for.

OTOH, I'd rather prefer -O3 for all arches but that ain't gonna happen for obvious
reasons :-)

> On a related note, I have submitted a patch that turns CONFIG_CC_OPTIMIZE_FOR_SIZE
> into a choice statement, so we actually get the -Wmaybe-uninitialized warnings
> in an allyesconfig or allmodconfig build. It would be trivial to extend that
> to give the choice between -Os, -O2 and -O3, and then pick -O3 in a defconfig,
> over the -O2 default.

Is it posted already. I couldn't find it with quick googling.

Thx,
-Vineet

Re: [PATCH] ARC: build: Turn off -Wmaybe-uninitialized for ARC gcc 4.8

[Arnd Bergmann]

On Friday 18 March 2016 18:31:53 Vineet Gupta wrote:
> 
> > On a related note, I have submitted a patch that turns CONFIG_CC_OPTIMIZE_FOR_SIZE
> > into a choice statement, so we actually get the -Wmaybe-uninitialized warnings
> > in an allyesconfig or allmodconfig build. It would be trivial to extend that
> > to give the choice between -Os, -O2 and -O3, and then pick -O3 in a defconfig,
> > over the -O2 default.
> 
> Is it posted already. I couldn't find it with quick googling.
> 
> 
https://lkml.org/lkml/2016/2/12/315

	Arnd