Original bug ID: 7150
Reporter: @yallop
Assigned to: @chambart
Status: assigned (set by @chambart on 2017-06-02T10:13:45Z)
Resolution: reopened
Priority: normal
Severity: minor
Target version: 4.03.1+dev
Category: compiler driver
Related to: #5441
Monitored by: @Drup @diml @ygrek @hcarty

Bug description

Here's a demonstration of unhelpful reordering behaviour involving options passed through by ocamlopt to the linker.

First, let's compile a couple of modules:

$ ocamlopt -c a.ml b.ml

Next, we'll build a library from one of the modules with some linker options, passed via -cclib:

$ ocamlopt -a -o liba.cmxa a.cmx -cclib -L/usr/adir/lib -cclib -L/usr/adir2/lib

Finally, we'll build an executable from the library and the other module, passing some more linker options:

$ ocamlopt -verbose -o b.native b.cmx -cclib -L/usr/lib/bdir/ -cclib -v liba.cmxa -cclib -L/usr/bdir2/lib 

Here's the linker invocation from the last step:

gcc -o 'b.native'   '-L/home/jeremy/.opam/4.02.3/lib/ocaml'  '/tmp/camlstartup7c51ee.o' '/home/jeremy/.opam/4.02.3/lib/ocaml/std_exit.o' 'liba.a' 'b.o' '/home/jeremy/.opam/4.02.3/lib/ocaml/stdlib.a' '-L/usr/adir/lib' '-L/usr/adir2/lib' '-L/usr/lib/bdir/' '-v' '-L/usr/bdir2/lib' '/home/jeremy/.opam/4.02.3/lib/ocaml/libasmrun.a' -lm  -ldl

In particular, note the order of the linker options

'-L/usr/adir/lib' '-L/usr/adir2/lib' '-L/usr/lib/bdir/' '-v' '-L/usr/bdir2/lib'

Although liba.cmxa appeared between -L/usr/lib/bdir and -L/usr/bdir2/lib in the ocamlopt command line, in the gcc command line the options embedded in liba.cmxa have all been moved to the beginning of the line.

Moving the options in this way is a problem because the order of -L options is significant. Moving cmxa-embedded options to the beginning of the line results in those options taking precedence over whatever's specified on the command-line. On my system, for example, the lwt-unix.cmxa library embeds -L/usr/lib, which then appears at the beginning of the library search path. As a result system libraries are chosen before user-installed libraries, which is not usually the desired behaviour.