Messages in this thread | | | Date | Sun, 19 Apr 2020 23:20:50 +0200 | From | Stefan Agner <> | Subject | Re: [PATCH v2 0/3] ARM: make use of UAL VFP mnemonics when possible |
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On 2020-04-19 16:12, Russell King - ARM Linux admin wrote: > On Sun, Apr 19, 2020 at 02:35:48PM +0200, Stefan Agner wrote: >> To build the kernel with Clang's integrated assembler the VFP code needs >> to make use of the unified assembler language (UAL) VFP mnemonics. >> >> At first I tried to get rid of the co-processor instructions to access >> the floating point unit along with the macros completely. However, due >> to missing FPINST/FPINST2 argument support in older binutils versions we >> have to keep them around. Once we drop support for binutils 2.24 and >> older, the move to UAL VFP mnemonics will be straight forward with this >> changes applied. >> >> Tested using Clang with integrated assembler as well as external >> (binutils assembler), various gcc/binutils version down to 4.7/2.23. >> Disassembled and compared the object files in arch/arm/vfp/ to make >> sure this changes leads to the same code. Besides different inlining >> behavior I was not able to spot a difference. >> >> In v2 the check for FPINST argument support is now made in Kconfig. > > Given what I said in the other thread, Clang really _should_ allow > the MCR/MRC et.al. instructions to access the VFP registers. There > is no reason to specifically block them.
I agree, and I am working on changing this.
There have been discussions about co-processor register access a while back in the LLVM/Clang community [1]. Peter Smith pointed this out in the ClangBuiltLinux issue tracker [2], which also has some more context. I did submit a patch [3] to convert use of cp10/cp11 in ARMv7 contexts to a warning. However it got stale, I'll have to revisit.
There is actually another case where this issue blocks Clang's integrated assembler: In arch/arm/kernel/perf_event_v7.c, function venum_read_pmresr mcr/mrc is used to access the performance monitor registers for Qualcomm's Krait/Scorpion PMU, and in this case there is no mnemonic available.
> > As we have seen with FPA, having that ability when iWMMXT comes along > is very useful. In any case: > > 1. The ARM ARM (DDI0406) states that "These instructions are MRC and MCR > instructions for coprocessors 10 and 11." in section A7.8. > > 2. The ARM ARM (DDI0406) describes the MRC and MCR instructions as > being able to access _any_ co-processor.
These are good arguments I can use in case my patch stirs up a discussion, thanks for the hints!
> > So, Clang deciding that it's going to block access to coprocessor 10 > and 11 because some version of the architecture _also_ defines these > as VFP instructions is really not on, and Clang needs to be fixed > irrespective of these patches - and I want to know that *is* going to > get fixed before I take these patches into the kernel.
I'll try. We'll see.
[1] https://bugs.llvm.org/show_bug.cgi?id=20025 [2] https://github.com/ClangBuiltLinux/linux/issues/306 [3] https://reviews.llvm.org/D59733
-- Stefan
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