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

[Fangrui Song]

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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>