Re: [PATCH 06/17] arm: Use -mstrict-align when the MMU is off

[Mark Kettenis <mark.kettenis@xs4all.nl>]

> From: Sam Edwards <cfsworks@gmail.com>
> Date: Sun, 23 Feb 2025 21:55:13 -0800

Hi Sam,

> On ARM, enabling the MMU is a prerequisite to enabling caching, which
> allows unaligned memory accesses by consolidating them into aligned
> accesses. When the MMU is disabled, however, all accesses must strictly
> adhere to the alignment rules.
> 
> Some compilers (e.g. Clang) contain optimizations that may break these
> rules on the assumption that misaligned accesses merely incur a
> performance penalty. We do not necessarily want to prevent the compiler
> from applying these optimizations where it believes there may be net
> gains, but we do need to tell it to respect the alignment requirements
> when the MMU is disabled.
> 
> Therefore, add the `-mstrict-align` compiler flag when SYS_ARM_MMU is
> not set.

The default for this option changed somewhat recently in clang for
non-Linux targets, which may explain why this wasn't an issue in the past.

One question though.  I believe there is some C code in U-Boot that
runs before the MMU gets turned on, even if SYS_ARM_MMU is set.  That
code should always be compiled with -mstrict-align.  And what about
SYS_ICACHE_OFF and SYS_DCACHE_OFF?

So I wonder if -mstrict-align should always be used for arm.  I doubt
the optimizations the compiler is doing in absence of that option
result in any measurable speedup.

> Signed-off-by: Sam Edwards <CFSworks@gmail.com>
> ---
>  arch/arm/config.mk | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/arm/config.mk b/arch/arm/config.mk
> index e0045e22271..98237b251f1 100644
> --- a/arch/arm/config.mk
> +++ b/arch/arm/config.mk
> @@ -33,6 +33,10 @@ PLATFORM_RELFLAGS	+= $(LLVM_RELFLAGS)
>  
>  PLATFORM_CPPFLAGS += -D__ARM__
>  
> +ifneq ($(CONFIG_SYS_ARM_MMU),y)
> +PLATFORM_RELFLAGS += -mstrict-align
> +endif
> +
>  ifdef CONFIG_ARM64
>  PLATFORM_ELFFLAGS += -B aarch64 -O elf64-littleaarch64
>  else
> -- 
> 2.45.2
> 
>