[Bug ld/27259] New: ld: Support SHF_LINK_ORDER self-link

[i at maskray dot me]

https://sourceware.org/bugzilla/show_bug.cgi?id=27259

            Bug ID: 27259
           Summary: ld: Support SHF_LINK_ORDER self-link
           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: ---

The code below is a sketch of how Clang>=13 PGO intends to make metadata
sections subject to --gc-sections.
It uses a SHF_LINK_ORDER self-link trick. However, that currently causes an
infinite loop in GNU ld.

cat > a.s <<e
.hidden __start___llvm_prf_cnts
.hidden __stop___llvm_prf_cnts
.globl _start
_start:
  leaq __start___llvm_prf_cnts(%rip), %rdi
  leaq __stop___llvm_prf_cnts(%rip), %rsi

.section .text.foo,"ax",@progbits
.globl foo
foo:
  incq .L__profc_foo(%rip)

.section __llvm_prf_cnts,"aw",@progbits
.section __llvm_prf_data,"aw",@progbits

.section __llvm_prf_cnts,"awo",@progbits,.L__profc_foo,unique,1
###
#.section __llvm_prf_cnts,"aw",@progbits
.L__profc_foo:
.zero 8

.section __llvm_prf_data,"awo",@progbits,.L__profc_foo,unique,2
.L__profd_foo:
  .quad 6699318081062747564             # 0x5cf8c24cdb18bdac
  .quad 0                               # 0x0
  .quad .L__profc_foo
  .quad foo
  .quad 0
  .long 1                               # 0x1
  .zero 4
e

as a.s -o a.o
ld-new -shared a.o --print-gc-sections --gc-sections # infinite loop


If the alternative (indicated as ###) .section directive is used, there is no
infinite loop.
However, __llvm_prf_cnts & __llvm_prf_data cannot be garbage collected.

The reason is that the C identifier sections are considered as GC roots.
Adding SHF_LINK_ORDER is a trick to defeat the GC root semantics.

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