[Bug translator/11695] DWIM promotion of ++ to <<<

["mjw at redhat dot com" <sourceware-bugzilla at sourceware dot org>]

------- Additional Comments From mjw at redhat dot com  2010-06-14 09:26 -------
(In reply to comment #2)
> So this transformation is only worthwhile when updating the value is more common
> than reading it.

The example that triggered this was like that:

$ stap -e 'global modules;
  probe timer.profile { if (target() == pid()) modules[umodname(uaddr())]++ }
  probe end { foreach ([m] in modules- limit 5)
              { printf("%4d:%s\n", modules[m], m) } }' -x `pidof firefox`

There are two advantages to using aggregates. It is "cheaper" in this use case.
And you don't necessarily need to write the end probe since systemtap detects
that it is only written to, so will generate the end probe printing it itself.
(That is bug #6954 for global arrays).

The disadvantage is that is it slightly obscure (you don't actually want the
whole aggregate, only the @count). And that the generated printing of the global
is somewhat messy (you get the whole aggregate printed out).

It might always be a win for global arrays that are never read.

-- 


http://sourceware.org/bugzilla/show_bug.cgi?id=11695

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