[PATCH v4 3/8] arm64/fpsimdmacros: Allow the macro "for" to be used in more cases

[Will Deacon]

On Fri, Aug 28, 2020 at 07:11:50PM +0100, Mark Brown wrote:
> From: Julien Grall <julien.grall at arm.com>
> 
> The current version of the macro "for" is not able to work when the
> counter is used to generate registers using mnemonics. This is because
> gas is not able to evaluate the expression generated if used in
> register's name (i.e x\n).
> 
> Gas offers a way to evaluate macro arguments by using % in front of
> them under the alternate macro mode.

altmacro mode doesn't appear to be very widely used in the kernel at all,
so I'm a bit nervous about this.

> The implementation of "for" is updated to use the alternate macro mode
> and %, so we can use the macro in more cases. As the alternate macro
> mode may have side-effects, this is disabled when expanding the body.
> 
> While it is enough to prefix the argument of the macro "__for_body"
> with %, the arguments of "__for" are also prefixed to get a more
> bearable value in case of compilation error.
> 
> Suggested-by: Dave Martin <dave.martin at arm.com>
> Signed-off-by: Julien Grall <julien.grall at arm.com>
> Reviewed-by: Dave Martin <Dave.Martin at arm.com>
> Signed-off-by: Mark Brown <broonie at kernel.org>
> ---
>  arch/arm64/include/asm/fpsimdmacros.h | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/fpsimdmacros.h b/arch/arm64/include/asm/fpsimdmacros.h
> index 636e9d9c7929..75293f111a6b 100644
> --- a/arch/arm64/include/asm/fpsimdmacros.h
> +++ b/arch/arm64/include/asm/fpsimdmacros.h
> @@ -166,19 +166,23 @@
>  
>  .macro __for from:req, to:req
>  	.if (\from) == (\to)
> -		_for__body \from
> +		_for__body %\from
>  	.else
> -		__for \from, (\from) + ((\to) - (\from)) / 2
> -		__for (\from) + ((\to) - (\from)) / 2 + 1, \to
> +		__for %\from, %((\from) + ((\to) - (\from)) / 2)
> +		__for %((\from) + ((\to) - (\from)) / 2 + 1), %\to
>  	.endif
>  .endm
>  
>  .macro _for var:req, from:req, to:req, insn:vararg
>  	.macro _for__body \var:req
> +		.noaltmacro
>  		\insn
> +		.altmacro

Why do we need to disable alt macro mode here?

>  	.endm
>  
> +	.altmacro
>  	__for \from, \to
> +	.noaltmacro

Why do we enable it here, rather than in the __for macro itself?

Will