On 16/11/17 13:07, Yury Norov wrote:

On Thu, Nov 16, 2017 at 11:54:33AM +0000, Will Deacon wrote:

quoted

On Wed, Nov 15, 2017 at 01:34:21PM -0800, Sami Tolvanen wrote:

quoted

From: Alex Matveev <redacted>

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.