linux-kernel - Re: [PATCH] ARM: warn if pre-UAL assembler syntax is used

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Date:   Tue, 17 Mar 2020 08:55:45 +0100
From:   Stefan Agner <stefan@...er.ch>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Russell King <linux@...linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Manoj Gupta <manojgupta@...gle.com>,
        Jian Cai <jiancai@...gle.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] ARM: warn if pre-UAL assembler syntax is used

On 2020-03-17 01:00, Nick Desaulniers wrote:
> Revert "ARM: 8846/1: warn if divided syntax assembler is used"On Thu,
> Feb 27, 2020 at 4:19 PM Stefan Agner <stefan@...er.ch> wrote:
>>
>> Remove the -mno-warn-deprecated assembler flag for GCC versions newer
>> than 5.1 to make sure the GNU assembler warns in case non-unified
>> syntax is used.
> 
> Hi Stefan, sorry for the late reply from me; digging out my backlog.
> Do you happen to have a godbolt link perhaps that demonstrates this?
> It sounds like GCC itself is emitting pre-UAL?

Yes, that is what Russell observed and caused the revert:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=b752bb405a13

I do not have a godbolt link at hand, I just built the complete kernel
using some GCC toolchains I had locally available and noticed that the
problem persists up to and including GCC 5.0. I did not track down what
exactly is causing GCC to emit pre-UAL.

> 
>>
>> This also prevents a warning when building with Clang and enabling
>> its integrated assembler:
>> clang-10: error: unsupported argument '-mno-warn-deprecated' to option 'Wa,'
>>
>> This is a second attempt of commit e8c24bbda7d5 ("ARM: 8846/1: warn if
>> divided syntax assembler is used").
> 
> Would it be helpful to also make note of
> commit b752bb405a13 ("Revert "ARM: 8846/1: warn if divided syntax
> assembler is used"")?

Sure, I can do that.

> 
> 
>>
>> Signed-off-by: Stefan Agner <stefan@...er.ch>
>> ---
>>  arch/arm/Makefile | 14 +++++++++-----
>>  1 file changed, 9 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/arm/Makefile b/arch/arm/Makefile
>> index db857d07114f..a6c8c9f39185 100644
>> --- a/arch/arm/Makefile
>> +++ b/arch/arm/Makefile
>> @@ -119,21 +119,25 @@ ifeq ($(CONFIG_CC_IS_CLANG),y)
>>  CFLAGS_ABI     += -meabi gnu
>>  endif
>>
>> -# Accept old syntax despite ".syntax unified"
>> -AFLAGS_NOWARN  :=$(call as-option,-Wa$(comma)-mno-warn-deprecated,-Wa$(comma)-W)
> 
> This existing code is quite bad for Clang, which doesn't support
> `-Wa,-mno-warn-deprecated`, so this falls back to `-Wa,-W`, which
> disables all warnings from the assembler, which we definitely do not
> want.  That alone is worth putting in the GCC guard.  But I would like
> more info about GCC above before signing off.

FWIW, I submitted this to the patch tracker already, but I don't think
it got merged already.

--
Stefan

> 
>> -
>>  ifeq ($(CONFIG_THUMB2_KERNEL),y)
>> -CFLAGS_ISA     :=-mthumb -Wa,-mimplicit-it=always $(AFLAGS_NOWARN)
>> +CFLAGS_ISA     :=-mthumb -Wa,-mimplicit-it=always
>>  AFLAGS_ISA     :=$(CFLAGS_ISA) -Wa$(comma)-mthumb
>>  # Work around buggy relocation from gas if requested:
>>  ifeq ($(CONFIG_THUMB2_AVOID_R_ARM_THM_JUMP11),y)
>>  KBUILD_CFLAGS_MODULE   +=-fno-optimize-sibling-calls
>>  endif
>>  else
>> -CFLAGS_ISA     :=$(call cc-option,-marm,) $(AFLAGS_NOWARN)
>> +CFLAGS_ISA     :=$(call cc-option,-marm,)
>>  AFLAGS_ISA     :=$(CFLAGS_ISA)
>>  endif
>>
>> +ifeq ($(CONFIG_CC_IS_GCC),y)
>> +ifeq ($(call cc-ifversion, -lt, 0501, y), y)
>> +# GCC <5.1 emits pre-UAL code and causes assembler warnings, suppress them
>> +CFLAGS_ISA     +=$(call as-option,-Wa$(comma)-mno-warn-deprecated,-Wa$(comma)-W)
>> +endif
>> +endif
>> +
>>  # Need -Uarm for gcc < 3.x
>>  KBUILD_CFLAGS  +=$(CFLAGS_ABI) $(CFLAGS_ISA) $(arch-y) $(tune-y) $(call cc-option,-mshort-load-bytes,$(call cc-option,-malignment-traps,)) -msoft-float -Uarm
>>  KBUILD_AFLAGS  +=$(CFLAGS_ABI) $(AFLAGS_ISA) $(arch-y) $(tune-y) -include asm/unified.h -msoft-float
>> --