Subject: Re: [PATCH v3 3/3] powerpc/64s: feature: Work around inline asm issues

From: Michael Ellerman <[email-masked]>
Date: Mon, 23 Nov 2020 16:44:56 +1100

Hi Bill,

Bill Wendling <[email-masked]> writes:
> The clang toolchain treats inline assembly a bit differently than
> straight assembly code. In particular, inline assembly doesn't have the
> complete context available to resolve expressions. This is intentional
> to avoid divergence in the resulting assembly code.
>
> We can work around this issue by borrowing a workaround done for ARM,
> i.e. not directly testing the labels themselves, but by moving the
> current output pointer by a value that should always be zero. If this
> value is not null, then we will trigger a backward move, which is
> explicitly forbidden.
>
> Signed-off-by: Bill Wendling <[email-masked]>
> ---
>  arch/powerpc/include/asm/feature-fixups.h | 17 +++++++++++++----
>  1 file changed, 13 insertions(+), 4 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/feature-fixups.h b/arch/powerpc/include/asm/feature-fixups.h
> index b0af97add751..f81036518edb 100644
> --- a/arch/powerpc/include/asm/feature-fixups.h
> +++ b/arch/powerpc/include/asm/feature-fixups.h
> @@ -36,6 +36,18 @@ label##2:						\
>  	.align 2;					\
>  label##3:
>  
> +/*
> + * If the .org directive fails, it means that the feature instructions
> + * are smaller than the alternate instructions. This used to be written
> + * as
> + *
> + * .ifgt (label##4b-label##3b) - (label##2b-label##1b)
> + *      .error "Feature section else case larger than body"
> + * .endif
> + *
> + * but clang's assembler complains about the expression being non-absolute
> + * when the code appears in an inline assembly statement.
> + */
>  #define MAKE_FTR_SECTION_ENTRY(msk, val, label, sect)		\
>  label##4:							\
>  	.popsection;						\
> @@ -48,12 +60,9 @@ label##5:							\
>  	FTR_ENTRY_OFFSET label##2b-label##5b;			\
>  	FTR_ENTRY_OFFSET label##3b-label##5b;			\
>  	FTR_ENTRY_OFFSET label##4b-label##5b;			\
> -	.ifgt (label##4b- label##3b)-(label##2b- label##1b);	\
> -	.error "Feature section else case larger than body";	\
> -	.endif;							\
> +	.org . - ((label##4b-label##3b) > (label##2b-label##1b)); \
>  	.popsection;

When I have an oversize alt section this doesn't seem to give me any
error using binutils?

If I hard code:

	.org . - (1);

It fails as expected.

But if I hard code:

	.org . - (1 > 0);

It builds?

cheers