Messages in this thread | | | Subject | Re: [PATCH v2 11/18] arm64: make mrs_s and msr_s macros work with LTO | From | Robin Murphy <> | Date | Thu, 16 Nov 2017 13:55:31 +0000 |
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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.
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