[systemd-devel] [PATCH 2/2] macro: allow assert_se() assertions to also be optimized out when NDEBUG is set

[Djalal Harouni]

Hi Shawn,

On Thu, Mar 26, 2015 at 11:21:54PM -0700, Shawn Landden wrote:
> On Thu, Mar 26, 2015 at 5:47 PM, Djalal Harouni <tixxdz at opendz.org> wrote:
> > On Fri, Mar 27, 2015 at 12:30:53AM +0100, Tom Gundersen wrote:
> >> On Thu, Mar 26, 2015 at 9:19 AM, Lennart Poettering
> >> <lennart at poettering.net> wrote:
> >> > On Tue, 24.03.15 11:11, Shawn Landden (shawn at churchofgit.com) wrote:
> >> >
> >> >> Will result in slightly smaller binaries, and cuts out the branch, even if
> >> >> the expression is still executed.
> >> >
> >> > I am sorry, but the whole point of assert_se() is that it has side
> >> > effects. That's why it is called that way (the "se" suffix stands for
> >> > "side effects"), and is different from assert(). You are supposed to
> >> > use it whenever the code enclosed in it shall not be suppressed if
> >> > NDEBUG is defined. This patch of yours breaks that whole logic!
> >>
> >> Hm, am I reading the patch wrong? It looks good to me. It is simply
> >> dropping the branching and logging in the NDEBUG case, but the
> >> expression itself is still evaluated.
> > Yep but it seems that abort() will never be called,
> > log_assert_failed() => abort()
> >
> > And the logging mechanism is also one of those side effects. IMO unless
> > there are real valid reasons for any change to these macors... changing
> > anything here will just bring more bugs that we may never notice.
> >
> Those are the side effects of assert(), the side effects of the
> expression are still evaluated.
Yes but there are also the following points:
* assert() is simple, assert_se() is more complex and it may modify other
global sate. I don't think that we can break down side effects of
assert_se() into:
 1) side effects of assert()
 2) side effects of other expressions.

And then remove that part 1)


So given the fact that asser_se() do not depend on NDEBUG, did you
consider the following:

* You don't have control over what the expression do, it may just call
  abort() or exit() in case of fatal errors, so even if you remove the
  explicit abort() call, expressions may just call it.

* Some expressions were perhaps depending on the logging mechanism which
  is part of the side effect. Callers to assert_se() are perhaps using
  it for a reason, are you sure that you are not introducing a
  regression here ?! it will be difficult to debug the error if we don't
  have logs.

* Current expression may modify/interact with a global state which may
  cause a fatal error, and if the caller wants to know if that failed,
  then abort(), your patch just introduced a regression, without the
  explicit abort(), all callers now have to call abort().

* Did you grep for assert_se() uses in the source ?

I really do not think this is an improvement, we can't predict the
semantics of expression here, perhaps we can with simple assert() but
not with assert_se().

-- 
Djalal Harouni
http://opendz.org