Re: 回复: [PATCH v4 04/13] LoongArch: ABI Implementation

[Fangrui Song <maskray@google.com>]

On 2022-05-24, caiyinyu@loongson.cn wrote:
>
>
>从 Windows 版邮件发送
>
>发件人: Fangrui Song
>发送时间: 2022年5月9日 16:24
>收件人: caiyinyu
>抄送: libc-alpha@sourceware.org; xuchenghua@loongson.cn; joseph_myers@mentor.com
>主题: Re: [PATCH v4 04/13] LoongArch: ABI Implementation
>
>>+/* Runtime address of .got */
>>+#define _GLOBAL_OFFSET_TABLE_ \
>>+  ({ \
>>+    ElfW (Addr) * r; \
>>+    asm("la.pcrel %0, _GLOBAL_OFFSET_TABLE_" : "=r"(r)); \
>>+    r; \
>>+  })
>>+
>>+/* Return the link-time address of _DYNAMIC.  */
>>+static inline ElfW (Addr) elf_machine_dynamic (void)
>>+{
>>+  return _GLOBAL_OFFSET_TABLE_[0];
>>+}
>
>_GLOBAL_OFFSET_TABLE_[0] == link_time_dynamic is not recommended.
>See
>https://maskray.me/blog/2021-09-05-build-glibc-with-lld#global_offset_table_0
>aarch64/riscv/x86_64/etc have been updated to use the preferred
>approach.
>
>Fixed.
>
>
>diff --git a/sysdeps/loongarch/dl-machine.h b/sysdeps/loongarch/dl-machine.h
>index f902410213..361f7555f9 100644
>--- a/sysdeps/loongarch/dl-machine.h
>+++ b/sysdeps/loongarch/dl-machine.h
>@@ -70,28 +70,18 @@ elf_machine_matches_host (const ElfW (Ehdr) * ehdr)
>   return 1;
> }
>
>-/* Runtime address of .got */
>-#define _GLOBAL_OFFSET_TABLE_ \
>-  ({ \
>-    ElfW (Addr) * r; \
>-    asm("la.pcrel %0, _GLOBAL_OFFSET_TABLE_" : "=r"(r)); \
>-    r; \
>-  })
>-
>-/* Return the link-time address of _DYNAMIC.  */
>-static inline ElfW (Addr) elf_machine_dynamic (void)
>+/* Return the run-time load address of the shared object.  */
>+static inline ElfW (Addr) elf_machine_load_address (void)
> {
>-  return _GLOBAL_OFFSET_TABLE_[0];
>+  extern const ElfW(Ehdr) __ehdr_start attribute_hidden;
>+  return (ElfW(Addr)) &__ehdr_start;
> }
>
>-/* Return the run-time load address of the shared object.  */
>-static inline ElfW (Addr) elf_machine_load_address (void)
>+/* Return the link-time address of _DYNAMIC.  */
>+static inline ElfW (Addr) elf_machine_dynamic (void)
> {
>-  ElfW (Addr) got_linktime_addr;
>-  asm("la.got %0, _GLOBAL_OFFSET_TABLE_"
>-      /* Link-time address in GOT entry before runtime relocation */
>-      : "=r"(got_linktime_addr));
>-  return (ElfW (Addr)) _GLOBAL_OFFSET_TABLE_ - got_linktime_addr;
>+  extern ElfW(Dyn) _DYNAMIC[] attribute_hidden;
>+  return (ElfW(Addr)) _DYNAMIC - elf_machine_load_address ();
> }

Thank you. The updated elf_machine_load_address and elf_machine_dynamic
look good.