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[Bug dynamic-link/14538] Complicated x86-64 elf_machine_dynamic/elf_machine_load_address
- From: "hjl.tools at gmail dot com" <sourceware-bugzilla at sourceware dot org>
- To: glibc-bugs at sources dot redhat dot com
- Date: Sat, 01 Sep 2012 05:11:21 +0000
- Subject: [Bug dynamic-link/14538] Complicated x86-64 elf_machine_dynamic/elf_machine_load_address
- Auto-submitted: auto-generated
- References: <bug-14538-131@http.sourceware.org/bugzilla/>
http://sourceware.org/bugzilla/show_bug.cgi?id=14538
--- Comment #1 from H.J. Lu <hjl.tools at gmail dot com> 2012-09-01 05:11:21 UTC ---
The new version:
/* Return the link-time address of _DYNAMIC. Conveniently, this is the
first element of the GOT. This must be inlined in a function which
uses global data. */
static inline ElfW(Addr) __attribute__ ((unused))
elf_machine_dynamic (void)
{
/* This produces a PC32 reloc that resolves to zero at link time, so in
fact just loads from the GOT register directly. By doing it without
an asm we can let the compiler choose any register. */
extern const ElfW(Addr) _GLOBAL_OFFSET_TABLE_[] attribute_hidden;
return _GLOBAL_OFFSET_TABLE_[0];
}
/* Return the run-time load address of the shared object. */
static inline ElfW(Addr) __attribute__ ((unused))
elf_machine_load_address (void)
{
/* Compute the difference between the runtime address of _DYNAMIC as seen
by a PC32 reference, and the link-time address found in the special
unrelocated first GOT entry. */
extern ElfW(Dyn) bygotoff[] asm ("_DYNAMIC") attribute_hidden;
return (ElfW(Addr)) &bygotoff - elf_machine_dynamic ();
}
It generates:
movq _DYNAMIC(%rip), %rax
movq %rdx, start_time(%rip)
leaq _DYNAMIC(%rip), %rdx
movq %rdx, %r13
subq _GLOBAL_OFFSET_TABLE_(%rip), %r13
testq %rax, %rax
movq %rdx, 2472+_rtld_local(%rip)
movq %r13, 2456+_rtld_local(%rip)
je .L993
instead of
#APP
# 75 "../sysdeps/x86_64/dl-machine.h" 1
lea _dl_start(%rip), %r13
sub 1f(%rip), %r13
.section .data.rel.ro
1: .quad _dl_start
.previous
# 0 "" 2
#NO_APP
movq %rdx, start_time(%rip)
movq %r13, %rdx
addq _DYNAMIC@GOTPCREL(%rip), %rdx
movq %r13, 2456+_rtld_local(%rip)
movq (%rdx), %rax
movq %rdx, 2472+_rtld_local(%rip)
testq %rax, %rax
je .L994
The new one avoids one load and reduces one GOT entry.
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