Messages in this thread | | | Date | Tue, 25 Feb 2020 21:00:32 +0100 | From | Stefan Agner <> | Subject | Re: [PATCH] ARM: use assembly mnemonics for VFP register access |
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On 2020-02-25 20:45, Robin Murphy wrote: > On 2020-02-25 7:33 pm, Ard Biesheuvel wrote: >> On Tue, 25 Feb 2020 at 20:10, Nick Desaulniers <ndesaulniers@google.com> wrote: >>> >>> On Mon, Feb 24, 2020 at 9:22 PM Stefan Agner <stefan@agner.ch> wrote: >>>> >>>> Clang's integrated assembler does not allow to to use the mcr >>>> instruction to access floating point co-processor registers: >>>> arch/arm/vfp/vfpmodule.c:342:2: error: invalid operand for instruction >>>> fmxr(FPEXC, fpexc & ~(FPEXC_EX|FPEXC_DEX|FPEXC_FP2V|FPEXC_VV|FPEXC_TRAP_MASK)); >>>> ^ >>>> arch/arm/vfp/vfpinstr.h:79:6: note: expanded from macro 'fmxr' >>>> asm("mcr p10, 7, %0, " vfpreg(_vfp_) ", cr0, 0 @ fmxr " #_vfp_ ", %0" \ >>>> ^ >>>> <inline asm>:1:6: note: instantiated into assembly here >>>> mcr p10, 7, r0, cr8, cr0, 0 @ fmxr FPEXC, r0 >>>> ^ >>>> >>>> The GNU assembler supports the .fpu directive at least since 2.17 (when >>>> documentation has been added). Since Linux requires binutils 2.21 it is >>>> safe to use .fpu directive. Use the .fpu directive and mnemonics for VFP >>>> register access. >>>> >>>> This allows to build vfpmodule.c with Clang and its integrated assembler. >>>> >>>> Link: https://github.com/ClangBuiltLinux/linux/issues/905 >>>> Signed-off-by: Stefan Agner <stefan@agner.ch> >>>> --- >>>> arch/arm/vfp/vfpinstr.h | 12 ++++-------- >>>> 1 file changed, 4 insertions(+), 8 deletions(-) >>>> >>>> diff --git a/arch/arm/vfp/vfpinstr.h b/arch/arm/vfp/vfpinstr.h >>>> index 38dc154e39ff..799ccf065406 100644 >>>> --- a/arch/arm/vfp/vfpinstr.h >>>> +++ b/arch/arm/vfp/vfpinstr.h >>>> @@ -62,21 +62,17 @@ >>>> #define FPSCR_C (1 << 29) >>>> #define FPSCR_V (1 << 28) >>>> >>>> -/* >>>> - * Since we aren't building with -mfpu=vfp, we need to code >>>> - * these instructions using their MRC/MCR equivalents. >>>> - */ >>>> -#define vfpreg(_vfp_) #_vfp_ >>>> - >>>> #define fmrx(_vfp_) ({ \ >>>> u32 __v; \ >>>> - asm("mrc p10, 7, %0, " vfpreg(_vfp_) ", cr0, 0 @ fmrx %0, " #_vfp_ \ >>>> + asm(".fpu vfpv2\n" \ >>>> + "vmrs %0, " #_vfp_ \ >>>> : "=r" (__v) : : "cc"); \ >>>> __v; \ >>>> }) >>>> >>>> #define fmxr(_vfp_,_var_) \ >>>> - asm("mcr p10, 7, %0, " vfpreg(_vfp_) ", cr0, 0 @ fmxr " #_vfp_ ", %0" \ >>>> + asm(".fpu vfpv2\n" \ >>>> + "vmsr " #_vfp_ ", %0" \ >>>> : : "r" (_var_) : "cc") >>>> >>>> u32 vfp_single_cpdo(u32 inst, u32 fpscr); >>>> -- >>> >>> Hi Stefan, >>> Thanks for the patch. Reading through: >>> - FMRX, FMXR, and FMSTAT: >>> http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0068b/Bcfbdihi.html >>> - VMRS and VMSR: >>> http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0204h/Bcfbdihi.html >>> >>> Should a macro called `fmrx` that had a comment about `fmrx` be using >>> `vmrs` in place of `fmrx`? >>> >>> It looks like Clang treats them the same, but GCC keeps them separate: >>> https://godbolt.org/z/YKmSAs >>> Ah, this is only when streaming to assembly. Looks like they have the >>> same encoding, and produce the same disassembly. (Godbolt emits >>> assembly by default, and has the option to compile, then disassemble). >>> If I take my case from godbolt above: >>> >>> ➜ /tmp arm-linux-gnueabihf-gcc -O2 -c x.c >>> ➜ /tmp llvm-objdump -dr x.o >>> >>> x.o: file format elf32-arm-little >>> >>> >>> Disassembly of section .text: >>> >>> 00000000 bar: >>> 0: f1 ee 10 0a vmrs r0, fpscr >>> 4: 70 47 bx lr >>> 6: 00 bf nop >>> >>> 00000008 baz: >>> 8: f1 ee 10 0a vmrs r0, fpscr >>> c: 70 47 bx lr >>> e: 00 bf nop >>> >>> So indeed a similar encoding exists for the two different assembler >>> instructions. >> >> Does that hold for ARM (A32) instructions as well? > > It should do - they're all the same thing underneath. The UAL syntax > just renamed all the legacy VFP mnemonics from Fxxx to Vxxx form, > apart from a couple of things that were already deprecated. GAS still > accepts both regardless of ".syntax unified", and as a result GCC > never saw a reason to stop emitting the old mnemonics. >
Yes this is really only a mnemonic change when unified assembler language (UAL) got introduce, the ARM ARM has a list of mnemonic changes in the appendix.
Just do make sure I also did compare the disassembled object file of vfpmodule.c before and after this change.
I guess we could (should?) also change the macro name, but I guess that should be a separate commit anyway.
-- Stefan
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