From: Michael Ellerman <mpe@ellerman.id.au>
To: Bill Wendling <morbo@google.com>, linuxppc-dev@lists.ozlabs.org
Cc: Nick Desaulniers <ndesaulniers@google.com>,
Bill Wendling <morbo@google.com>
Subject: Re: [PATCH v3 3/3] powerpc/64s: feature: Work around inline asm issues
Date: Mon, 23 Nov 2020 16:44:56 +1100 [thread overview]
Message-ID: <87d0041vaf.fsf@mpe.ellerman.id.au> (raw)
In-Reply-To: <20201120224034.191382-4-morbo@google.com>
Hi Bill,
Bill Wendling <morbo@google.com> 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 <morbo@google.com>
> ---
> 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
next prev parent reply other threads:[~2020-11-23 5:46 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-17 0:47 [PATCH 0/2] Fixes for clang/lld Bill Wendling
2020-10-17 0:47 ` [PATCH 1/2] powerpc/wrapper: Add "-z notext" flag to disable diagnostic Bill Wendling
2020-10-17 0:47 ` [PATCH 2/2] powerpc/boot: Use clang when CC is clang Bill Wendling
2020-11-18 22:35 ` [PATCH 0/3] PPC: fixes for clang support Bill Wendling
2020-11-20 22:40 ` [PATCH v3 " Bill Wendling
2020-11-20 22:40 ` [PATCH 1/3] powerpc/wrapper: add "-z notext" flag to disable diagnostic Bill Wendling
2020-11-20 22:40 ` [PATCH v3 2/3] powerpc/boot: Use clang when CC is clang Bill Wendling
2020-11-20 22:40 ` [PATCH v3 3/3] powerpc/64s: feature: Work around inline asm issues Bill Wendling
2020-11-23 5:44 ` Michael Ellerman [this message]
2020-11-23 6:34 ` Segher Boessenkool
2020-11-23 19:43 ` Bill Wendling
2020-11-23 19:53 ` Bill Wendling
2020-11-23 19:56 ` Segher Boessenkool
2020-11-23 20:01 ` Bill Wendling
2020-11-23 20:08 ` Segher Boessenkool
2020-11-23 20:17 ` Bill Wendling
2020-11-24 3:43 ` Michael Ellerman
2020-11-25 5:13 ` Bill Wendling
2020-11-27 1:03 ` Michael Ellerman
2020-11-27 1:10 ` Bill Wendling
2020-11-27 1:59 ` Bill Wendling
2020-11-18 22:35 ` [PATCH 1/3] powerpc/wrapper: add "-z notext" flag to disable diagnostic Bill Wendling
2020-11-18 22:35 ` [PATCH 2/3] powerpc/boot: Use clang when CC is clang Bill Wendling
2020-11-18 22:35 ` [PATCH 3/3] powerpc/64s: feature: work around inline asm issues Bill Wendling
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