Semaphore without probes: dummy notes

[gretay-js]

In the original version of probes (before PR #60), inlining of [%probe_is_enabled" <name>] into a another compilation unit can result in link error if there is no mention of the [%probe <name>] with the same <name> in the target-of-inlining compilation unit. For example:

$ cat t1.ml
let () = Printf.printf "%%probe_is_enabled = %b\n" (T2.probe_is_enabled ());

$ cat t2.ml
let [@inline always] probe_is_enabled () = [%probe_is_enabled "test2"]

$ ocamlopt.opt t2.ml t1.ml -o t.exe
t1.o: In function `camlT1__entry':
:(.text+0x7): undefined reference to `camlT1__semaphore_test2'
collect2: error: ld returned 1 exit status
File "caml_startup", line 1:
Error: Error during linking (exit code 1)
EXIT STATUS 2

This PR fixes it by emitting "dummy" probe sites. (PR#60 fixes this problem as well by creating a unique semaphore label per name.)

Additionally, this PR removes the source-level requirement on compilation unit that has [%probe_is_enabled <name>] to contain also [%probe <name>].

There is a small downside to removing this check, which can catch some spelling mistakes in probe names that would otherwise go unnoticed. This is not catching all such issues, and anyway most [%probe ] commands are expected to be generated automatically by a ppx and so won't have these issues.

The main motivation for supporting [%probe_is_enabled <name>] without any [%probe <name>] in the source of the program is to toggle tracing/logging code that is not inside a [%probe] at runtime from another process with low overhead, compared to RPC. A corresponding change in probes library will ensure that dummy probe sites are not updated. As a result, semaphore-only "probes" will be toggled without pausing the tracee. Systemtap will continue to work as before in the presence of dummy probes.

Dummy probes are implemented by adding special (unused) function with name ending in ___dummy_probes per compilation unit if needed, with a NOP instruction for each dummy probe site and without a cmp/call instruction to the probe wrapper.

It is a small change after refactoring of probe type (that is by itself an improvement). Therefore, this RP is best reviewed commit by commit.

It is necessary to advance the version of probe notes, because old tracer tool must not attempt to update dummy probe sites, which do not have an ocaml probe handler.

I'm not sure if this change needs bootstrap.

[gretay-js]

This design is based on discussion with @trishume.

[gretay-js]

I've rebased it and removed the bootstrap commit (it doesn't seem to be needed). This PR is now ready for review, but I haven't tested it with corresponding changes in probes_lib.

[gretay-js]

It turns out that #60 did not fix all the instances of the link error mentioned in the description.
So the change in this PR is needed even when %probe is in the same file as %probe_is_enable.

With flambda, the following example program fails to link:

$ cat t.ml
let _unused () = [%probe "foo" ()]
let is_enabled () = [%probe_is_enabled "foo"]

$ cat t.mli
val is_enabled : unit -> bool

$ cat s.ml
let () =   Printf.printf "%b\n" (T.is_enabled ())

$ ocamlopt.opt t.mli t.ml s.ml -o -t.exe
s.o: In function `camlS__entry':
:(.text+0x7): undefined reference to `caml_probes_semaphore_foo'
t.o: In function `camlT__is_enabled_18':
:(.text+0x3): undefined reference to `caml_probes_semaphore_foo'
collect2: error: ld returned 1 exit status
File "caml_startup", line 1:
Error: Error during linking (exit code 1)

Thanks to @trishume for the example.

[lpw25]

Approving the parts that touch typing