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
