[Bug ld/26256] New: Support mixed SHF_LINK_ORDER & non-SHF_LINK_ORDER components in an output section

[i at maskray dot me]

https://sourceware.org/bugzilla/show_bug.cgi?id=26256

            Bug ID: 26256
           Summary: Support mixed SHF_LINK_ORDER & non-SHF_LINK_ORDER
                    components in an output section
           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: ---

See https://groups.google.com/forum/#!topic/generic-abi/hgx_m1aXqUo
"SHF_LINK_ORDER's original semantics make upgrade difficult"

Mixed SHF_LINK_ORDER and non-SHF_LINK_ORDER components are not allowed. This
makes it infeasible to add SHF_LINK_ORDER to an existing metadata section if
backward compatibility with older object files are concerned.

It would be nice if the two different semantics (ordering requirement & garbage
collection) were not overloaded on one section flag, however, it is probably
difficult to obtain a generic flag at this point
(https://groups.google.com/forum/#!topic/generic-abi/hgx_m1aXqUo
"SHF_LINK_ORDER's original semantics make upgrade difficult").

On the other hand, defining an order is not going to be troublesome, and will
help metadata sections.

cat > a.s <<e
.section .text.bar,"a",@progbits
.byte 2
.section .text.foo,"a",@progbits
.byte 1
.section .rodata.foo,"ao",@progbits,.text.foo
.byte 1
## If the two .rodata.bar sections are in the same input section description,
## 03 (sh_link!=0) will be ordered before 02 (sh_link=0).
.section .rodata.bar,"a",@progbits
.byte 2
.section .rodata.bar,"ao",@progbits,.text.bar
.byte 3
e
as-new a.s -o a.o
ld-new a.o  # .rodata has both ordered [`.rodata.foo' in a.o] and unordered
[`.rodata.bar' in a.o] sections


I am going to change LLD to perform ordering within an input section
description.

For example, .rodata : {*(.rodata.foo) *(.rodata.bar)}, has two
InputSectionDescription's. If there is at least one SHF_LINK_ORDER and at least
one non-SHF_LINK_ORDER in .rodata.foo, they are ordered within *(.rodata.foo):
we arbitrarily place SHF_LINK_ORDER components before non-SHF_LINK_ORDER
components (like Solaris ld).

*(.rodata.bar) is ordered similarly, but the two input section descriptions
don't interact.

Section content of .rodata for a few different linker scripts:

# RUN: echo 'SECTIONS { .rodata : {BYTE(0) *(.rodata*) BYTE(4)} \               
# RUN:  .text : {*(.text.foo) *(.text.bar)} }' > %t1.lds                        
# RUN: ld.lld -T %t1.lds %t.o -o %t1

0003010204

# RUN: echo 'SECTIONS { .rodata : {*(.rodata.foo) *(.rodata.bar)} }' > %t2.lds  
# RUN: ld.lld -T %t2.lds %t.o -o %t2

010302

## Non-contiguous SHF_LINK_ORDER sections, separated by a symbol assignment.    
# RUN: echo 'SECTIONS { .rodata : {*(.rodata.foo) a = .; *(.rodata.bar)} }' >
%t5.lds                                                                 
# RUN: ld.lld -T %t5.lds %t.o -o %t5                                       
# RUN: llvm-readelf -S -x .rodata %t5 | FileCheck --check-prefix=CHECK2 %s

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