[Bug ld/31158] ld: Should --gc-sections respect RHS of a symbol assignment?

[i at maskray dot me]

https://sourceware.org/bugzilla/show_bug.cgi?id=31158

--- Comment #2 from Fangrui Song <i at maskray dot me> ---
Interesting. BFD's behavior depends on whether the assigned symbol is
referenced.
Let's enhance the test.

cat > a.s <<e
.globl _start, y1, y2
_start: .dc.a x1
.section .rodata.y1, "a"; y1: .quad 0x1122334455667788
.section .rodata.y2, "a"; y2: .quad 0x1122334455667788
e

echo 'x1 = y1; x2 = y2;' > a.t
as a.s -o a.o
ld.lld --gc-sections a.o a.t -o a.lld
ld.bfd --gc-sections a.o a.t -o a.gc.bfd
ld.bfd a.o a.t -o a.nogc.bfd
ld.gold --gc-sections a.o a.t -o a.gold

% readelf -Ws a.nogc.bfd | grep '[xy]'
Symbol table '.symtab' contains 9 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     1: 0000000000402008     0 NOTYPE  GLOBAL DEFAULT    2 y2
     2: 0000000000402008     0 NOTYPE  GLOBAL DEFAULT    2 x2
     5: 0000000000402000     0 NOTYPE  GLOBAL DEFAULT    2 x1
     8: 0000000000402000     0 NOTYPE  GLOBAL DEFAULT    2 y1
% readelf -Ws a.gc.bfd | grep '[xy]'
Symbol table '.symtab' contains 8 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     1: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  ABS x2
     4: 0000000000402000     0 NOTYPE  GLOBAL DEFAULT    2 x1
     7: 0000000000402000     0 NOTYPE  GLOBAL DEFAULT    2 y1

x2 is non-zero in a GC link and zero in a no-GC link.
There appears unusual: a live symbol's value can be very different when GC is
enabled.
Or, is this a clever trick to check whether a section is GCed?

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