On Thursday, 31 July 2014 at 21:25:25 UTC, Walter Bright wrote:
On 7/31/2014 1:33 PM, David Nadlinger wrote:
I've had the questionable pleasure of tracking down a couple
of related issues
in LLVM and the LDC codegen, so please take my word for it:
Requiring any
particular behavior such as halting in a case that can be
assumed to be
unreachable is at odds with how the term "unreachable" is used
in the wild – at
least in projects like GCC and LLVM.
For example:
int foo() {
while (...) {
...
}
assert(0);
}
the compiler needn't issue an error at the end "no return value
for foo()" because it can assume it never got there.
I'll rewrite that bit in the spec as it is clearly causing
confusion.
Don't rewrite it because you merely concede that it might be
confusing. Rewrite it because you admit it's contradictory. If
you just try to reword the spec without understanding how your
use of the terminology differs from the established meaning,
you'll probably come up with something that is confusing to the
rest of the world just as well.
Perhaps looking at the situation in terms of basic blocks and the
associated control flow graph will help:
As per your above post, assert(0) has nothing to do with making
any assumptions on the compiler side. It merely servers as a
terminator instruction of a BB, making it a leaf in the CFG. This
seems to be the definition you intend for the spec. Maybe add
something along the lines of "behaves like a function call that
never returns" as an explanation to make it easier to understand.
This is not what "unreachable" means. If assert(0) was
unreachable, then the compiler would be free to assume that no
CFG edges *into* the BB holding the instruction are ever taken
(and as a corollary, it could also decide not emit any code for
it). Thus, the term certainly shouldn't appear anywhere near
assert(0) in the spec, except to point out the difference.
Cheers,
David
