[llvm-bugs] [Bug 47777] New: Merge equally named output sections in linker scripts

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=47777

            Bug ID: 47777
           Summary: Merge equally named output sections in linker scripts
           Product: lld
           Version: unspecified
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: ELF
          Assignee: unassignedbugs at nondot.org
          Reporter: konstantin.schwarz at hightec-rt.com
                CC: llvm-bugs at lists.llvm.org, smithp352 at googlemail.com

This is rather a feature request than a bug report.
Consider the following assembler input:
$ cat foo.s

.text
.globl _start
_start:
  nop


and the following linker script:
$ cat linker.ld

SECTIONS {

  .text : {
    *(.text*)
  }

  .myTable : {
    LONG(0);
  }

/* Subcomponent #include's go here..., e.g. */
  .myTable : {
    LONG(1);
  }


/* End marker of the table */
  .myTable : {
    LONG(-1);
  }
}


Linking with GNU ld (GNU ld (GNU Binutils for Ubuntu) 2.30) (ld -static -T
link.ld foo.o) results in the following section headers:

Section Headers:
  [Nr] Name              Type             Address           Offset
       Size              EntSize          Flags  Link  Info  Align
  [ 0]                   NULL             0000000000000000  00000000
       0000000000000000  0000000000000000           0     0     0
  [ 1] .text             PROGBITS         0000000000000000  00200000
       0000000000000001  0000000000000000  AX       0     0     4
  [ 2] .myTable          PROGBITS         0000000000000001  00200001
       000000000000000c  0000000000000000  WA       0     0     1
  [ 3] .symtab           SYMTAB           0000000000000000  00200010
       0000000000000060  0000000000000018           4     3     8
  [ 4] .strtab           STRTAB           0000000000000000  00200070
       0000000000000008  0000000000000000           0     0     1
  [ 5] .shstrtab         STRTAB           0000000000000000  00200078
       000000000000002a  0000000000000000           0     0     1


Linking with ld.lld gives

Section Headers:
  [Nr] Name              Type             Address           Offset
       Size              EntSize          Flags  Link  Info  Align
  [ 0]                   NULL             0000000000000000  00000000
       0000000000000000  0000000000000000           0     0     0
  [ 1] .text             PROGBITS         0000000000000000  00001000
       0000000000000001  0000000000000000  AX       0     0     4
  [ 2] .myTable          PROGBITS         0000000000000001  00001001
       0000000000000004  0000000000000000  AX       0     0     1
  [ 3] .myTable          PROGBITS         0000000000000005  00001005
       0000000000000004  0000000000000000  AX       0     0     1
  [ 4] .myTable          PROGBITS         0000000000000009  00001009
       0000000000000004  0000000000000000  AX       0     0     1
  [ 5] .comment          PROGBITS         0000000000000000  0000100d
       0000000000000076  0000000000000001  MS       0     0     1
  [ 6] .symtab           SYMTAB           0000000000000000  00001088
       0000000000000030  0000000000000018           8     1     8
  [ 7] .shstrtab         STRTAB           0000000000000000  000010b8
       0000000000000033  0000000000000000           0     0     1
  [ 8] .strtab           STRTAB           0000000000000000  000010eb
       0000000000000008  0000000000000000           0     0     1


As can be seen, GNU ld merges all equally named output sections into one single
output section of the same name.

Our users make use of this "feature" to create different kinds of tables in
linker scripts:
An application typically consists of many different components, potentially
developed by different vendors.
Each component can provide its own linker script snippet, #include'd in the
final application's "main" linker script.
The main linker script contains the start and end markers of the tables, and
each subcomponent can amend to the table
by providing an equally named output section. The linker merges all output
sections into one section, enabling the creation of
contiguous tables:

$ objdump -D ld-built.elf:

Disassembly of section .myTable:

0000000000000001 <.myTable>:
   1:   00 00                   add    %al,(%rax)
   3:   00 00                   add    %al,(%rax)
   5:   01 00                   add    %eax,(%rax)
   7:   00 00                   add    %al,(%rax)
   9:   ff                      (bad)  
   a:   ff                      (bad)  
   b:   ff                      (bad)  
   c:   ff                      .byte 0xff



$ objdump -D lld-built.elf

Disassembly of section .myTable:

0000000000000001 <.myTable>:
   1:   00 00                   add    %al,(%rax)
        ...

Disassembly of section .myTable:

0000000000000005 <.myTable>:
   5:   01 00                   add    %eax,(%rax)
        ...

Disassembly of section .myTable:

0000000000000009 <.myTable>:
   9:   ff                      (bad)  
   a:   ff                      (bad)  
   b:   ff                      (bad)  
   c:   ff                      .byte 0xff


Since I could not find any documentation or reference for this behavior in the
GNU ld manuals, I assume this is some kind of implementation
defined behavior. GNU gold behaves similarly to lld in this case.
Would it still be feasible to support this in lld, or can this be achieved my
other means in the linker script?

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