Re: [PATCH v2 11/18] arm64: make mrs_s and msr_s macros work with LTO

Robin Murphy
2017-11-16T13:55:41Z

On 16/11/17 13:07, Yury Norov wrote:
> On Thu, Nov 16, 2017 at 11:54:33AM +0000, Will Deacon wrote:
>> On Wed, Nov 15, 2017 at 01:34:21PM -0800, Sami Tolvanen wrote:
>>> From: Alex Matveev <alxmtvv@gmail.com>
>>>
>>> Use UNDEFINE_MRS_S and UNDEFINE_MSR_S to define corresponding macros
>>> in-place and workaround gcc and clang limitations on redefining macros
>>> across different assembler blocks.
>>
>> What limitations? Can you elaborate please? Is this a fix?
>
> Hi Will,
>
> Regarding GCC.
>
> When it joins preprocessed source files into single asm file,
> mrs_s/msr_s becomes either not declared or declared multiple times.
>
> ./ccuFb68h.s:33120: Error: Macro `mrs_s' was already defined
> ./ccuFb68h.s:33124: Error: Macro `msr_s' was already defined
>
> I'm not sure that GCC works correctly in this case, and I sent the
> email to Linaro toolchain group to clarify it. See below.
>
> Yury
>
> [...]
>
> Links:
> My unfinished branch:
> https://github.com/norov/linux/tree/lto
> Andi Kleen tree:
> https://github.com/andikleen/linux-misc/tree/lto-411-1
> Sami Tolvanen's recent work for clang:
> https://lkml.org/lkml/2017/11/3/606
>
> Question we have for now:
> There's mrs_s/msr_s macro that doesn't work with LTO - linker
> complains very loudly that macro is either not declared, or declared
> multiple times. (To reproduce - try to build my kernel branch w/o last
> patch).
>
> The same (?) problem is observed with clang, and people there
> considered it as feature, not a bug.
>
> https://bugs.llvm.org/show_bug.cgi?id=19749
>
> We have the fix for both clang and gcc, but it looks hacky. Maybe it
> worth to fix mrs/msr issue on toolchain side?

Given that this whole mrs_s infrastructure is a workaround for older
assemblers which don't support the "S<op0>_<op1>_<Cn>_<Cm>_<op2>" syntax
for arbitrary unnamed system registers (which IIRC was a fairly late
addition to the architecture), the only way it could be "fixed" on the
toolchain side is by removing all those older toolchains from existence.
Good luck with that ;)

In *theory*, it might be possible to do something similar to what we do
with CONFIG_BROKEN_GAS_INST to detect offending assemblers and only
define and use these macros when necessary (hopefully Clang and other
LTO-capable toolchains do accept the proper syntax), but I've no idea
how invasive or difficult that might turn out to be.

Robin.