On 1/23/20 1:17 PM, Jeff Law wrote:
On Tue, 2020-01-21 at 15:00 -0700, Sandra Loosemore wrote:
In doing some nios2-elf testing, I ran into a bunch of failures in
constexpr-related tests in the C++ testsuite. This target defaults to
-fno-delete-null-pointer-checks at the request of Altera/Intel, in order
to support some of their BSPs where 0 is a legitimate memory address.
Some other bare-metal targets also default to
-fno-delete-null-pointer-checks.
This patch makes the dependence of these tests on
-fdelete-null-pointer-checks explicit. I've previously fixed some other
tests that failed on nios2-elf in the same way so this is borderline
obvious, but it's a little troubling to me that the correct semantics of
some of these testcases seems to depend on what we document in the
manual as an optimization option. :-S Maybe there is some other bug here?
Anyway, if nobody has any objections or better ideas, I will go ahead
and commit this in a few days.
It'd be nice to know why that flag matters for constexpr. But I've got
no problem with the change itself.
I haven't looked at all of the failing tests in detail, but the ones I
did peek at all seemed to be related to constant-folding pointer
comparisons against null.
The thing that nags at me is that e.g. in g++.dg/cpp0x/constexpr-odr1.C
we have:
template<int> struct A {
static const bool x;
static_assert(&x, ""); // odr-uses A<...>::x
};
int g;
template<int I>
const bool A<I>::x = (g = 42, false);
void f(A<0>) {} // A<0> must be complete, so is instantiated
int main()
{
if (g != 42)
__builtin_abort ();
}
Whether or not this is valid code depends on whether "&x != 0" can be
constant-folded. Without -fdelete-null-pointer-checks, g++ says:
/path/to/g++.dg/cpp0x/constexpr-odr1.C:
In instantiation of 'struct A<0>':
/path/to/g++.dg/cpp0x/constexpr-odr1.C:14:12:
required from here
/path/to/g++.dg/cpp0x/constexpr-odr1.C:6:17:
error: non-constant condition for static assertion
/path/to/g++.dg/cpp0x/constexpr-odr1.C:12:12:
error: '((& A<0>::x) != 0)' is not a constant expression
So is -fdelete-null-pointer-checks really an optimization option, or a
language semantics option?
BTW, here's Altera's original description of the over-eager optimization
problems they were trying to work around:
https://gcc.gnu.org/ml/gcc/2015-02/msg00163.html
-Sandra
