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

[chen]

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




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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>>>>>x265-devel at videolan.org
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>>>
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