From: "H.J. Lu" <hjl.tools@gmail.com>
To: Binutils <binutils@sourceware.org>
Subject: Re: [PATCH] Support SHF_GNU_RETAIN ELF section flag
Date: Tue, 22 Sep 2020 16:58:23 -0700 [thread overview]
Message-ID: <CAMe9rOq9dBOXkMe8U-DvFy059++aPj0a5rOacZGLvcwrmiwLjg@mail.gmail.com> (raw)
In-Reply-To: <20200922202933.kgflmtnwzkdrmrvs@jozef-acer-manjaro>
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.
next prev parent reply other threads:[~2020-09-22 23:59 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-22 20:29 Jozef Lawrynowicz
2020-09-22 23:24 ` Hans-Peter Nilsson
2020-09-22 23:58 ` H.J. Lu [this message]
2020-09-23 1:09 ` Fangrui Song
2020-09-23 9:58 ` Jozef Lawrynowicz
2020-09-23 13:39 ` H.J. Lu
2020-09-23 13:51 ` Michael Matz
2020-09-23 16:52 ` Jozef Lawrynowicz
2020-09-23 17:13 ` H.J. Lu
2020-09-23 18:47 ` Jozef Lawrynowicz
2020-09-23 19:03 ` H.J. Lu
2020-09-23 20:04 ` Jozef Lawrynowicz
2020-09-23 20:17 ` H.J. Lu
2020-09-23 23:29 ` Fangrui Song
2020-09-24 11:39 ` Jozef Lawrynowicz
2020-09-24 19:06 ` Fangrui Song
2020-09-24 13:27 ` Michael Matz
2020-09-24 13:18 ` H.J. Lu
2020-09-24 13:49 ` Jozef Lawrynowicz
2020-09-24 13:59 ` H.J. Lu
2020-09-24 16:56 ` Jozef Lawrynowicz
2020-09-24 17:04 ` H.J. Lu
2020-09-24 17:18 ` Jozef Lawrynowicz
2020-09-24 17:37 ` H.J. Lu
2020-09-23 12:13 ` Jozef Lawrynowicz
2020-09-23 13:59 ` Alan Modra
2020-09-23 16:54 ` Jozef Lawrynowicz
2020-09-28 11:35 ` Pedro Alves
2020-09-28 12:28 ` Jozef Lawrynowicz
2020-09-28 14:46 ` Pedro Alves
2020-09-29 13:18 ` Michael Matz
2020-09-29 13:22 ` Jozef Lawrynowicz
2020-09-29 13:48 ` Pedro Alves
2020-09-29 13:55 ` Michael Matz
2020-09-29 14:04 ` Pedro Alves
2020-09-29 14:10 ` Michael Matz
2020-09-29 14:11 ` Pedro Alves
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=CAMe9rOq9dBOXkMe8U-DvFy059++aPj0a5rOacZGLvcwrmiwLjg@mail.gmail.com \
--to=hjl.tools@gmail.com \
--cc=binutils@sourceware.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).