[PATCH] D51262: Implement P0553 and P0556

[Marshall Clow via Phabricator via cfe-commits]

mclow.lists marked an inline comment as done.
mclow.lists added inline comments.


================
Comment at: libcxx/include/bit:378
+    	const unsigned __retVal = 1u << (__n + __extra);
+    	return (_Tp) (__retVal >> __extra);
+    }
----------------
mclow.lists wrote:
> mclow.lists wrote:
> > Quuxplusone wrote:
> > > mclow.lists wrote:
> > > > mclow.lists wrote:
> > > > > Quuxplusone wrote:
> > > > > > Why so complicated? Is there a unit test that demonstrates why you can't just use `return _Tp{1} << __n;` in this case as well?
> > > > > > 
> > > > > > Also, is this a kosher use of `sizeof(X) * 8` as a stand-in for `numeric_limits<X>::digits`?
> > > > > > 
> > > > > > Also, speaking of unit tests, I don't see any unit tests for e.g. `std::ceil2(256)` or `std::ceil2(65536)`. Shouldn't there be some?
> > > > > Yes. I want to generate some UB here, so that this is not a "core constant expression" as per P1355.
> > > > Yes, the `ceil2.fail.cpp` test will not fail (for short types) if I just return `_Tp{1} << __n;` - because of integer promotion.
> > > > 
> > > Yikes. Sounds like there's some "library maintainer"ship going on here, then. Naively, I would have thought that the Working Draft had left the behavior of such constructs undefined in order to make the library vendor's life **easier** and the code **more efficient**. But you're saying that you need to pessimize the code here in order to make it "sufficiently undefined" so that its behavior at compile time will be well-defined and produce a diagnostic instead of being undefined and producing garbage?
> > > 
> > > Maybe WG21 needs to invent a "really actually undefined behavior" that does not have unwanted interactions with constexpr, so that library writers can go back to generating fast clean code!
> > > 
> > > Rant aside, why don't you just do `_LIBCPP_ASSERT(__n < numeric_limits<_Tp>::digits);` and leave it at that?  An assertion failure isn't a compile-time constant, is it?
> > > Also, is this a kosher use of sizeof(X) * 8 as a stand-in for numeric_limits<X>::digits?
> > 
> > Good catch.
> that's exactly what P1355 says.
There are three things that I'd like this code to do in the case of bad inputs:
* Produce a diagnostic if called at constexpr time (required by P1355)
* Cause UBSAN to catch it at runtime
* Cause a libc++ assertion to go off if they are enabled.

Note that these are all more or less independent.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D51262/new/

https://reviews.llvm.org/D51262