Counter intuitively, asserts hurt gcc static dataflow analysis.

[Marc Glisse]

On Wed, 9 May 2018, Jonathan Wakely wrote:

> On 4 May 2018 at 15:20, Florian Weimer wrote:
>> * John Carter:
>>
>>> But compile with ...
>>> gcc  -O3 -W -Wall -Wextra -o a a.c
>>> ...now results in NO warnings!
>>>
>>> ie. Although gcc _knows_ the assert  _will_ trigger at run time... it can't
>>> tell me at compile time anymore.
>>>
>>> ie. Counter intuitively, adding asserts and error checks to my code has
>>> made me less safe.
>>
>> In glibc, we could warn if the assert expression is constant and
>> false.  But I'm worried that this will produce lots and lots of false
>> positives after inlining, loop unrolling, and other optimizations.
>>
>> Has anyone tried something like this?
>
> I've been experimenting with something like that for assertions inside
> libstdc++. I want assertions that meet these properties:
>
> - enforced at compile time in constexpr evaluation (i.e. produce a
> compile-time error, not a runtime call to abort)
>
> - otherwise, issue a compile-time warning if the arguments are
> constant (using __builtin_constant_p)

This part doesn't work so well, especially with optimizations that 
duplicate code (clone function, thread path, etc). You would need enough 
optimization to see that the arguments are constant, but little enough 
that it still looks like the user's code, and that's a hard compromize 
between __builtin_constant_p and the new __builtin_early_constant_p.

> - otherwise, check at run-time.

-- 
Marc Glisse