[AARCH64] Rewrite elf_machine_load_address using _DYNAMIC symbol

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 (part of binutils 2.24):
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.

Here is the C version one which should be portable in all cases.

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

2 files changed, 10 insertions, 34 deletions

diff --git a/ChangeLog b/ChangeLog
index a34405285f..eae1159cf3 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+2017-10-18  Renlin Li  <renlin.li@arm.com>
+
+	* sysdeps/aarch64/dl-machine.h (elf_machine_load_address): Use
+	_DYNAMIC symbol to calculate load address.
+
 2017-10-18  Paul A. Clarke  <pc@us.ibm.com>
 
 	* sysdeps/powerpc/fpu/fenv_private.h (_FPU_MASK_TRAPS_RN):
diff --git a/sysdeps/aarch64/dl-machine.h b/sysdeps/aarch64/dl-machine.h
index b1245476dc..e7656129f5 100644
--- a/sysdeps/aarch64/dl-machine.h
+++ b/sysdeps/aarch64/dl-machine.h
@@ -51,40 +51,11 @@ 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 32/16bit relocations being resolved at link time
-     and that the static address fits in the 32/16 bits.  */
-
-  ElfW(Addr) static_addr;
-  ElfW(Addr) dynamic_addr;
-
-  asm ("					\n"
-"	adrp	%1, _dl_start;			\n"
-#ifdef __LP64__
-"	add	%1, %1, #:lo12:_dl_start	\n"
-#else
-"	add	%w1, %w1, #:lo12:_dl_start	\n"
-#endif
-"	ldr	%w0, 1f				\n"
-"	b	2f				\n"
-"1:						\n"
-#ifdef __LP64__
-"	.word	_dl_start			\n"
-#else
-# ifdef __AARCH64EB__
-"	.short  0                               \n"
-# endif
-"	.short  _dl_start                       \n"
-# ifndef __AARCH64EB__
-"	.short  0                               \n"
-# endif
-#endif
-"2:						\n"
-    : "=r" (static_addr),  "=r" (dynamic_addr));
-  return dynamic_addr - static_addr;
+    _DYNAMIC sysmbol is used here as its link-time address stored in
+    the special unrelocated first GOT entry.  */
+
+    extern ElfW(Dyn) _DYNAMIC[] attribute_hidden;
+    return (ElfW(Addr)) &_DYNAMIC - elf_machine_dynamic ();
 }
 
 /* Set up the loaded object described by L so its unrelocated PLT