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

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Date:   Mon, 16 Mar 2020 17:00:57 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Stefan Agner <stefan@...er.ch>
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

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?

>
> 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"")?


>
> 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.

> -
>  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
> --

-- 
Thanks,
~Nick Desaulniers