[Bug ld/25022] New: Make /DISCARD/ discard SHF_LINK_ORDER sections

[i at maskray dot me]

https://sourceware.org/bugzilla/show_bug.cgi?id=25022

            Bug ID: 25022
           Summary: Make /DISCARD/ discard SHF_LINK_ORDER sections
           Product: binutils
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: ld
          Assignee: unassigned at sourceware dot org
          Reporter: i at maskray dot me
  Target Milestone: ---

This depends on https://sourceware.org/bugzilla/show_bug.cgi?id=24526
(Generalize GC support for SHF_LINK_ORDER)

# a.script
SECTIONS { /DISCARD/ : { *(.foo) } }

# a.s
.section .foo,"a"
.quad 0

.section .bar,"ao",@progbits,.foo  # "o" is llvm-mc specific syntax that sets
SHF_LINK_ORDER and sh_link
.quad 0

.section .zed,"ao",@progbits,.foo
.quad 0

.section .moo,"ao",@progbits,.zed
.quad 0


GNU ld issues an error

% llvm-mc -filetype=obj -triple=x86_64 a.s -o a.o
% ld.bfd -T a.script a.o
ld.bfd: a.out: sh_link of section `.bar' points to discarded section `.foo' of
`a.o'
ld.bfd: final link failed: bad value

In lld, .bar is considered a dependent section of .foo
When /DISCARD/ discards .foo, it discards .bar as well. Transitively, .zed and
.moo are discarded. This rule can be seen as a generalization of SHT_RELA or
SHT_ARM_EXIDX.

% ld.lld -T a.script a.o
ld.lld: warning: cannot find entry symbol _start; defaulting to 0x0

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