[GLIBC][AARCH64]Rewrite elf_machine_load_address using _DYNAMIC symbol

[Renlin Li <renlin dot li at foss dot arm dot com>]

Hi all,

This patch rewrites aarch64 elf_machine_load_address to use special _DYNAMIC
symbol instead of _dl_start.

The static address of _DYNAMIC symbol is stored in the first GOT entry.
Here is the change which makes this solution work.
https://sourceware.org/ml/binutils/2013-06/msg00248.html

i386, x86_64 targets use the same method to do this as well.

The original implementation relies on a trick that R_AARCH64_ABS32 relocation
being resolved at link time and the static address fits in the 32bits.
However, in LP64, normally, the address is defined to be 64 bit.

Additionally, the original inline assembly is not optimized. It uses 4
instructions including a jump.

Optimally, the new implementation here is just two instructions:
ldr %1, _GLOBAL_OFFSET_TABLE_
adr %2, _DYNAMIC

The size of ld.so is around 130K, so it's save to use ldr, adr to get the address.
The address range for those two instruction is +/-1MB.

And by the way, this method is ILP32 safe as well.
aarch64 linux toolchain regression test OK. OK to commit?

Regards,
Renlin Li


ChangeLog:

2016-11-04  Renlin Li  <renlin.li@arm.com>

	* sysdeps/aarch64/dl-machine.h (elf_machine_load_address): Use
	_DYNAMIC symbol to calculate load address.

diff --git a/sysdeps/aarch64/dl-machine.h b/sysdeps/aarch64/dl-machine.h
index 217e179..b2f6618 100644
--- a/sysdeps/aarch64/dl-machine.h
+++ b/sysdeps/aarch64/dl-machine.h
@@ -49,25 +49,19 @@ elf_machine_load_address (void)
   /* To figure out the load address we use the definition that for any symbol:
      dynamic_addr(symbol) = static_addr(symbol) + load_addr
 
-     The choice of symbol is arbitrary. The static address we obtain
-     by constructing a non GOT reference to the symbol, the dynamic
-     address of the symbol we compute using adrp/add to compute the
-     symbol's address relative to the PC.
-     This depends on 32bit relocations being resolved at link time
-     and that the static address fits in the 32bits.  */
-
-  ElfW(Addr) static_addr;
-  ElfW(Addr) dynamic_addr;
+     _DYNAMIC symbol is used here as its static address is stored in
+     the special unrelocated first GOT entry.  */
 
+  ElfW(Addr) static_addr, dynamic_addr;
   asm ("					\n"
-"	adrp	%1, _dl_start;			\n"
-"	add	%1, %1, #:lo12:_dl_start	\n"
-"	ldr	%w0, 1f				\n"
-"	b	2f				\n"
-"1:						\n"
-"	.word	_dl_start			\n"
-"2:						\n"
-    : "=r" (static_addr),  "=r" (dynamic_addr));
+       "adr	%0, _DYNAMIC			\n"
+#ifdef __LP64__
+       "ldr	%1, _GLOBAL_OFFSET_TABLE_	\n"
+#else
+       "ldr	%w1, _GLOBAL_OFFSET_TABLE_	\n"
+#endif
+       : "=r" (dynamic_addr), "=r" (static_addr));
+
   return dynamic_addr - static_addr;
 }