Semantics of a common definition in an archive

[Fangrui Song]

I am playing with common definitions in an archive and have noticed a strange
property.

>a.s echo '.globl _start; _start: call foo'
>b.s echo '.globl foo; foo: .common var,4,4'
>c.s echo '.globl foo; foo: .data; .globl var; var:'
>d.s echo '.globl foo; foo: .common var,8,8'

gcc -c a.s b.s c.s d.s
ar rc b.a b.o
ar rc c.a c.o
ar rc d.a d.o


The archive index of c.a says c.o defines var.  It seems that ld is not
satisfied with the common definition in b.a(b.o) and checks whether c.a(c.o)
provides a regular definition. It does, so GNU ld pulls c.a(c.o) and errors
for the multiple definition of foo.

ld.bfd a.o b.a c.a
# ld.bfd: c.a(c.o): in function `foo':
# (.text+0x0): multiple definition of `foo'; b.a(b.o):(.text+0x0): first defined here
# ld.bfd: warning: alignment 1 of symbol `var' in c.a(c.o) is smaller than 4 in b.a(b.o)

Gold and LLD's semantics are different.

gold a.o b.a c.a   # succeeded. c.a(c.o) is not pulled.
ld.lld a.o b.a c.a # succeeded


ld does not pull d.a(d.o) because it provides a common definition, not better
than the common definition in b.a(b.o). Though the archive member is inspected,
align/size fields are not updated.

ld.bfd a.o b.a d.a
# good.
readelf -Ws a.out | grep var  # align=size=4; The common definition in d.a(d.o) is ignored
#      5: 0000000000402000     4 OBJECT  GLOBAL DEFAULT    2 var

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So, are the GNU ld behaviors described above all desired?  Apparently, when a
symbol in the archive index is currently common, ld does not treat it as "ground
truth". This behavior appears to be quite unusual. I think the documentation
should probably be improved to mention the desired behaviors.

(GCC 10 and clang 11 default to -fcommon for C. In Fortran, IIUC FORTRAN 77 uses
COMMON blocks. Fortran 90 does not recommend COMMON. These issues may become
less and less relevant over time.)