On 7/31/2014 12:40 AM, David Bregman wrote:
On Wednesday, 30 July 2014 at 22:01:23 UTC, Walter Bright wrote:
I'd like to sum up my position and intent on all this.
1. I can discern no useful, practical difference between the notions of assume
and assert.
People have explained the difference repeatedly, a ridiculous number of times
now. Could you please take a minute to understand it this time instead of
flippantly dismissing it again?
assert does a runtime check, assume does not
assume affects code generation/optimization, assert does not
assert is for debugging, assume is not
assume is for optimization, assert is not
In terms of what they practically do, they have *nothing* in common, their
functions are entirely orthogonal.
They are inextricably entangled. Consider:
if (x == 0) abort(); // essentially what assert(x) does
... at this point, the optimizer knows, beyond doubt, that x!=0 ...
if (x) // optimizer can remove this check
...
which has the behavior of assume as you listed above, yet it is assert. We can
pretend assert doesn't affect code, like we can pretend to have massless points
in physics class, but in reality points have a mass and assert most definitely
affects code generation, and does in every compiler I've checked, and it affects
it in just the way the assume does.
Still think there is no practical difference?
Yes.
2. The compiler can make use of assert expressions to improve optimization,
even in -release mode.
This will introduce a lot of undefined behavior, including making @safe code
with asserts unsafe. I really think this needs to be acknowledged. As far as I
can tell from the other thread, it still hasn't been.
I did acknowledge it for the array bounds case.
Note that your assume() will have the same effect, and worse, there will be no
option to have the compiler insert a check, because then it would be an assert()
and you might as well just use assert().
This is why I see the distinction as being pointless.
