[x265] PING [PATCH] aarch64: replace ldr pseudo-instruction with adrp+add

[Fangrui Song]

On Fri, Nov 18, 2022 at 3:12 PM Fangrui Song <maskray at google.com> wrote:
>
> On Tue, Oct 18, 2022 at 5:30 AM chen <chenm003 at 163.com> wrote:
> >
> > Hi Song,
> >
> > This is a long time history bug. the '-DPIC' just add to compile option when Nasm is enabled, the bug can simple fix with below patch (NO TEST since I haven't ARM environment).
> > After patch, the user may acivate these compile option with ENABLE_PIC=ON
> >
> > Could the x265 team please help verify patch?
> >
> > Regards,
> > Min Chen
> >
> > diff --git a/source/CMakeLists.txt b/source/CMakeLists.txt
> > index 13e4750de..80f8e59a9 100755
> > --- a/source/CMakeLists.txt
> > +++ b/source/CMakeLists.txt
> > @@ -266,6 +266,9 @@ if(GCC)
> >              add_definitions(-DHAVE_NEON)
> >          endif()
> >      endif()
> > +    if(ENABLE_PIC)
> > +        list(APPEND ARM_ARGS -DPIC)
> > +    endif()
> >      add_definitions(${ARM_ARGS})
> >      if(FPROFILE_GENERATE)
> >          if(INTEL_CXX)
> >
>
> Hi Min, I think my patch as-is is correct. Whether aarch64 builds
> wants -DPIC is a separate topic.
> Is a maintainer able to verify and install this patch? Thanks
>
> >  2022-10-18 14:37:10，"Fangrui Song" <maskray at google.com>
> >
> > On Thu, Oct 6, 2022 at 1:39 AM chen <chenm003 at 163.com> wrote:
> >>
> >> Hi Song,
> >>
> >>
> >> I means the current tree support these adrp+add mode with compile option -DPIC, so we need not patch the code.
> >>
> >>
> >> Regards,
> >> Min Chen
> >
> >
> > Hi Min, IIUC -DPIC is only defined for x86-64 (source/cmake/CMakeASM_NASMInformation.cmake).
> > AArch64 does not get -DPIC.
> >>
> >> 2022-10-06 06:42:33，"Fangrui Song" <maskray at google.com>
> >>
> >> Hi Min, sorry but I just saw your question. I do not understand the request.
> >> adrp+add is just strictly superior to ldr (which uses a constant pool and does not decrease code size) and avoids text relocations (which are prohibited in many systems).  `ldr             \rd, =\val+\offset` should just be removed. adrp+add also works on Mach-O systems as well.
> >>
> >>>
> >>> At 2022-09-24 15:21:35, "Fangrui Song" <maskray at google.com> wrote:
> >>> >Ping.  The breaks lld build and some binutils configurating defaulting
> >>> >to disallow text relocations.
> >>> >
> >>> >On 2022-08-29, Fangrui Song wrote:
> >>> >>On 2022-08-29, Fangrui Song wrote:
> >>> >>>On 2022-08-30, chen wrote:
> >>> >>>>Hi Song,
> >>> >>>>
> >>> >>>>
> >>> >>>>Thank you for your patch.
> >>> >>>>
> >>> >>>>
> >>> >>>>However, syntax of ':lo12:' depends on compiler, so more general LDR is better in here.
> >>> >>>>
> >>> >>>>
> >>> >>>>Regards,
> >>> >>>>Min Chen
> >>> >>>
> >>> >>>:lo12: is standard aarch64 assembly syntax.
> >>> >>>Which aarch64 compiler supported by x265 does not support :lo12:?
> >>> >>
> >>> >>Note that LDR has another problem that it produced an absolute relocation R_AARCH64_ABS64. It will trigger an error
> >>> >>when text relocations are disabled (default in ld.lld. See https://maskray.me/blog/2020-12-19-lld-and-gnu-linker-incompatibilities#:~:text=Text%20relocations)
> >>> >>
> >>> >>```
> >>> >>% : && /usr/bin/c++ -fPIC -O3 -DNDEBUG  -Wl,-Bsymbolic,-znoexecstack -shared ...
> >>> >>ld: error: relocation R_AARCH64_ABS64 cannot be used against local
> >>> >>symbol; recompile with -fPIC
> >>> >>>>>defined in sad-a.S.o
> >>> >>>>>>>> referenced by sad-a.S.o:(.text+0x9B00)
> >>> >>```
> >>> >>
> >>> >>adrp+add work fine.
> >>> >>
> >>> >>(I am a maintainer of lld/ELF.)
> >>> >>
> >>> >>>>At 2022-08-30 02:33:37, "Fangrui Song" <maskray at google.com> wrote:
> >>> >>>>>The ldr pseudo-instruction uses a literal pool, which is less efficient
> >>> >>>>>and does not decrease the code size.
> >>> >>>>>---
> >>> >>>>>source/common/aarch64/asm.S | 4 +---
> >>> >>>>>1 file changed, 1 insertion(+), 3 deletions(-)
> >>> >>>>>
> >>> >>>>>diff --git a/source/common/aarch64/asm.S b/source/common/aarch64/asm.S
> >>> >>>>>index 399c37cf2..2506f50aa 100644
> >>> >>>>>--- a/source/common/aarch64/asm.S
> >>> >>>>>+++ b/source/common/aarch64/asm.S
> >>> >>>>>@@ -130,11 +130,9 @@ ELF     .size   \name, . - \name
> >>> >>>>>       adrp            \rd, \val+(\offset)
> >>> >>>>>       add             \rd, \rd, :lo12:\val+(\offset)
> >>> >>>>> .endif
> >>> >>>>>-#elif defined(PIC)
> >>> >>>>>+#else
> >>> >>>>>       adrp            \rd, \val+(\offset)
> >>> >>>>>       add             \rd, \rd, :lo12:\val+(\offset)
> >>> >>>>>-#else
> >>> >>>>>-        ldr             \rd, =\val+\offset
> >>> >>>>>#endif
> >>> >>>>>.endm
> >>> >>>>>
> >>> >>>>>--
> >>> >>>>>2.37.2.672.g94769d06f0-goog
> >>> >>>>>
> >>> >>>>>_______________________________________________
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> >>> >>>>>https://mailman.videolan.org/listinfo/x265-devel
> >>> >>>
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> >>> >>>
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> >>>
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> >>
> >>
> >>
> >> --
> >> 宋方睿
> >>
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> >
> >
> >
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> >
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>
>
> --
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Ping. This fixes a copy relocation for aarch64.