[Bug gas/25381] New: Support section flag 'o' (SHF_LINK_ORDER)

[i at maskray dot me]

https://sourceware.org/bugzilla/show_bug.cgi?id=25381

            Bug ID: 25381
           Summary: Support section flag 'o' (SHF_LINK_ORDER)
           Product: binutils
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: gas
          Assignee: unassigned at sourceware dot org
          Reporter: i at maskray dot me
  Target Milestone: ---

% cat a.c                                                       
void foo(){}
void bar(){}
% clang -fpatchable-function-entry=2 -ffunction-sections -S a.c # clang HEAD
https://reviews.llvm.org/D72222 https://reviews.llvm.org/D72222
% grep __patchable a.s
        .section      
__patchable_function_entries,"awo",@progbits,foo,unique,0
        .section      
__patchable_function_entries,"awo",@progbits,bar,unique,1
% clang -fpatchable-function-entry=2 -ffunction-sections -c a.c
% readelf -S a.o
...
  [Nr] Name              Type            Address          Off    Size   ES Flg
Lk Inf Al
  [ 0]                   NULL            0000000000000000 000000 000000 00    
0   0  0
  [ 1] .strtab           STRTAB          0000000000000000 000210 000087 00     
0   0  1
  [ 2] .text             PROGBITS        0000000000000000 000040 000000 00  AX 
0   0  4
  [ 3] .text.foo         PROGBITS        0000000000000000 000040 000008 00  AX 
0   0 16
  [ 4] __patchable_function_entries PROGBITS        0000000000000000 000048
000008 00 WAL  3   0  8
  [ 5] .rela__patchable_function_entries RELA            0000000000000000
0001b0 000018 18     14   4  8
  [ 6] .text.bar         PROGBITS        0000000000000000 000050 000008 00  AX 
0   0 16
  [ 7] __patchable_function_entries PROGBITS        0000000000000000 000058
000008 00 WAL  6   0  8
  [ 8] .rela__patchable_function_entries RELA            0000000000000000
0001c8 000018 18     14   7  8
...


The section flag 'o' takes an argument (`,foo` and `,bar` above), adds
SHF_LINK_ORDER to sh_flags, and sets sh_link to sh_shndx of the target symbol.

You may try `clang -fstack-size-section -ffunction-sections` with older clang
(clang >= 8).

https://sourceware.org/bugzilla/show_bug.cgi?id=25380 tracks the section
linkage feature (`,unique`)

This can be seen as a lightweight COMDAT. See
https://gcc.gnu.org/ml/gcc/2020-01/msg00067.html (__patchable_function_entries
is flawed) https://sourceware.org/ml/binutils/2019-11/msg00266.html
(SHF_LINK_ORDER and --gc-sections)

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