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[PATCH] Support SHF_GNU_RETAIN ELF section flag

H.J. Lu hjl.tools@gmail.com
Tue Sep 22 23:58:23 GMT 2020 
On Tue, Sep 22, 2020 at 1:30 PM Jozef Lawrynowicz
<jozef.l@mittosystems.com> wrote:
>
> The attached patch adds support for the new SHF_GNU_RETAIN ELF section
> flag, which was discussed on the GNU gABI mailing list here:
> https://sourceware.org/pipermail/gnu-gabi/2020q3/000429.html
>
> The flag is GNU-specific so uses a bit in the SHF_MASKOS mask.
> Its precise definition is as follows:
>
> =======================================================================
> Section Attribute Flags
> +-------------------------------------+
> | Name           | Value              |
> +-------------------------------------+
> | SHF_GNU_RETAIN | 0x200000 (1 << 21) |
> +-------------------------------------+
>
> SHF_GNU_RETAIN
>   The link editor should not garbage collect the section if it is
>   unused.
>
> =======================================================================
>
> The overall intention for this new flag is to enable a new "retain"
> attribute to be applied to declarations of functions and data in the
> source code. This attribute can be used to ensure the definition
> associated with the declaration is present in the linked output file,
> even if linker garbage collection would normally remove the containing
> section because it is unused.
>
> The new ".retain" assembler directive can be used to apply
> SHF_GNU_RETAIN to a section. GCC will emit this directive when
> assembling definitions of functions and data that have had the "retain"
> attribute applied.
>
> Note that there is *not* a direct mapping of SHF_GNU_RETAIN to the BFD
> section flag SEC_KEEP. SEC_KEEP would prevent the user being able to
> explicitly remove an SHF_GNU_RETAIN section by placing it in /DISCARD/,
> which could be necessary in some situations.
>
> I successfully regtested the patch for the Binutils, GAS and LD
> testsuites for the following CPUs, applying the "-elf" suffix when
> configuring:
>   aarch64 alpha arc arm avr bfin bpf cr16 cris crx csky d10v d30v dlx
>   epiphany fr30 frv ft32 h8300 hppa i386 ia64 ia64-vms ip2k iq2000 lm32
>   m32c m32r m68hc11 m68hc12 m68k mcore mep metag microblaze mips mmix
>   moxie msp430 mt nds32 nfp nios2 or1k pj ppc pru riscv rl78 rx s12z
>   s390 score sh sparc spu tic6x tilegx tilepro v850 vax visium wasm32
>   x86-64 x86 xc16x xgate xstormy16 xtensa z80
>
> The new tests are passing for all targets except mmix-elf. This target
> has a lot of LD failures, particularly --gc-sections doesn't appear to
> have any effect. I don't know anything about the target, but I wonder if
> it should be added to the hard-coded list of targets that doesn't
> support --gc-sections.  I have therefore XFAIL'd the new LD tests for
> this target.
>
> I also regtested for i386-pe, to ensure there was no spill-over of the
> new functionality into any non-ELF areas.
>
> Ok to apply?
>
> Thanks,
> Jozef

+  /* A GNU extension for preventing linker garbage collection of sections.  */
+  {"retain", obj_elf_retain, 0},
+

Why is this needed?  Isn't

@@ -857,6 +861,9 @@ obj_elf_parse_section_letters (char *str, size_t len,
  case 'd':
   *gnu_attr |= SHF_GNU_MBIND;
   break;
+ case 'R':
+  *gnu_attr |= SHF_GNU_RETAIN;
+  break;
  case '?':
   *is_clone = TRUE;
   break;

enough?  The .section directive is used for all other SHF_XXX bits.   I don't
think we need a separate directive for SHF_GNU_RETAIN?


-- 
H.J.
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