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[PATCH] Clearing st_value of PLT SHN_UNDEF symbols if pointer equality not needed for x86-64
- From: Jakub Jelinek <jakub at redhat dot com>
- To: binutils at sources dot redhat dot com
- Cc: jh at suse dot cz
- Date: Thu, 22 Apr 2004 15:31:16 +0200
- Subject: [PATCH] Clearing st_value of PLT SHN_UNDEF symbols if pointer equality not needed for x86-64
- Reply-to: Jakub Jelinek <jakub at redhat dot com>
Hi!
This is a x86-64 version of my 2003-11-22 patch which was i386 only.
I believe compiler generated code will never depend on pointer equality
if it uses R_X86_64_PC32 or R_X86_64_PLT32 relocations.
In hand written assembly one can certainly use:
leaq foo(%rip), %rax
or .long foo-.
and then compare pointers, but there is no reason to do that
(in -fno-pic code mov $foo, %eax is shorter, in -mcmodel=medium
movabsq $foo, %rax is generated and in -fpic code, given that
foo is known not to be in the same executable or shared library
as the instruction, a GOTPCREL relocation is created instead).
Ok to commit?
2004-04-22 Jakub Jelinek <jakub@redhat.com>
* elf64-x86-64.c (elf64_x86_64_copy_indirect_symbol): Copy also
ELF_LINK_POINTER_EQUALITY_NEEDED.
(elf64_x86_64_check_relocs): Set ELF_LINK_POINTER_EQUALITY_NEEDED
if r_type is not R_X86_64_PC32.
(elf64_x86_64_finish_dynamic_symbol): If
ELF_LINK_POINTER_EQUALITY_NEEDED is not set, clear st_value of
SHN_UNDEF symbols.
--- bfd/elf64-x86-64.c.jj 2004-04-22 16:40:52.000000000 +0200
+++ bfd/elf64-x86-64.c 2004-04-22 17:09:23.662915096 +0200
@@ -561,7 +561,8 @@ elf64_x86_64_copy_indirect_symbol (const
(ind->elf_link_hash_flags & (ELF_LINK_HASH_REF_DYNAMIC
| ELF_LINK_HASH_REF_REGULAR
| ELF_LINK_HASH_REF_REGULAR_NONWEAK
- | ELF_LINK_HASH_NEEDS_PLT));
+ | ELF_LINK_HASH_NEEDS_PLT
+ | ELF_LINK_POINTER_EQUALITY_NEEDED));
else
_bfd_elf_link_hash_copy_indirect (bed, dir, ind);
}
@@ -812,6 +813,8 @@ elf64_x86_64_check_relocs (bfd *abfd, st
/* We may need a .plt entry if the function this reloc
refers to is in a shared lib. */
h->plt.refcount += 1;
+ if (r_type != R_X86_64_PC32)
+ h->elf_link_hash_flags |= ELF_LINK_POINTER_EQUALITY_NEEDED;
}
/* If we are creating a shared library, and this is a reloc
@@ -2519,11 +2522,16 @@ elf64_x86_64_finish_dynamic_symbol (bfd
if ((h->elf_link_hash_flags & ELF_LINK_HASH_DEF_REGULAR) == 0)
{
/* Mark the symbol as undefined, rather than as defined in
- the .plt section. Leave the value alone. This is a clue
+ the .plt section. Leave the value if there were any
+ relocations where pointer equality matters (this is a clue
for the dynamic linker, to make function pointer
comparisons work between an application and shared
- library. */
+ library), otherwise set it to zero. If a function is only
+ called from a binary, there is no need to slow down
+ shared libraries because of that. */
sym->st_shndx = SHN_UNDEF;
+ if ((h->elf_link_hash_flags & ELF_LINK_POINTER_EQUALITY_NEEDED) == 0)
+ sym->st_value = 0;
}
}
Jakub