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

From: Robin Murphy
Date: Thu Nov 16 2017 - 08:55:52 EST

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@xxxxxxxxx>

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.



My unfinished branch:
Andi Kleen tree:
Sami Tolvanen's recent work for clang:

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

The same (?) problem is observed with clang, and people there
considered it as feature, not a bug.

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.