On Mon, Mar 18, 2024 at 09:10:27PM -0400, Jason Merrill wrote:
> On 3/15/24 13:48, Marek Polacek wrote:
> > On Thu, Mar 14, 2024 at 03:39:04PM -0400, Jason Merrill wrote:
> > > On 3/8/24 12:02, Marek Polacek wrote:
> > > > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> > > >
> > > > -- >8 --
> > > > Consider
> > > >
> > > > constexpr int VAL = 1;
> > > > struct foo {
> > > > template <int B>
> > > > void bar(typename std::conditional<B==VAL, int, float>::type
> > > > arg) { }
> > > > };
> > > > template void foo::bar<1>(int arg);
> > > >
> > > > where we since r11-291 fail to emit the code for the explicit
> > > > instantiation. That's because cp_walk_subtrees/TYPENAME_TYPE now
> > > > walks TYPE_CONTEXT ('conditional' here) as well, and in a template
> > > > finds the B==VAL template argument. VAL is constexpr, which implies
> > > > const,
> > > > which in the global scope implies static.
> > > > constrain_visibility_for_template
> > > > then makes "struct conditional<(B == VAL), int, float>" non-TREE_PUBLIC.
> > > > Then symtab_node::needed_p checks TREE_PUBLIC, sees it's 0, and we don't
> > > > emit any code.
> > > >
> > > > I thought the fix would be some ODR-esque check to not consider
> > > > constexpr variables/fns that are used just for their value. But
> > > > it turned out to be tricky. For instance, we can't skip
> > > > determine_visibility in a template; we can't even skip it for value-dep
> > > > expressions. For example, no-linkage-expr1.C has
> > > >
> > > > using P = struct {}*;
> > > > template <int N>
> > > > void f(int(*)[((P)0, N)]) {}
> > > >
> > > > where ((P)0, N) is value-dep, but N is not relevant here: we have to
> > > > ferret out the anonymous type. When instantiating, it's already gone.
> > >
> > > Hmm, how is that different from the B == VAL case? In both cases we're
> > > naming an internal entity that gets folded away.
> > >
> > > I guess the difference is that B == VAL falls under the special allowance
> > > in
> > > https://eel.is/c++draft/basic.def.odr#14.5.1 because it's a constant used
> > > as
> > > a prvalue, and therefore is not odr-used under
> > > https://eel.is/c++draft/basic.def.odr#5.2
> > >
> > > So I would limit this change to decl_constant_var_p. Really we should
> > > also
> > > be checking that the lvalue-rvalue conversion is applied, but that's more
> > > complicated.
> >
> > Thanks. My previous version had it, but it didn't handle
> >
> > static constexpr int getval () { return 1; }
> >
> > template <int B>
> > void baz(typename conditional<B == getval (), int, float>::type arg) { }
> >
> > I'd say that "getval()" is one of "manifestly constant-evaluated
> > expressions that
> > are not value-dependent", so it should be treated the same as B == VAL.
>
> But it doesn't satisfy the 14.5 rule that corresponding names need to refer
> to the same entity; since getval names a function, it doesn't get the
> special exemption from that rule that VAL gets.
>
> So this should not be treated the same as B == VAL.
Thanks for the explanation.
> > I don't know if this is important to handle. Do you want me to poke
> > further or
> > should we just go with decl_constant_var_p and leave it at that for now?
>
> Just decl_constant_var_p.
Here it is:
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
-- >8 --
Consider
constexpr int VAL = 1;
struct foo {
template <int B>
void bar(typename std::conditional<B==VAL, int, float>::type arg) { }
};
template void foo::bar<1>(int arg);
where we since r11-291 fail to emit the code for the explicit
instantiation. That's because cp_walk_subtrees/TYPENAME_TYPE now
walks TYPE_CONTEXT ('conditional' here) as well, and in a template
finds the B==VAL template argument. VAL is constexpr, which implies const,
which in the global scope implies static. constrain_visibility_for_template
then makes "struct conditional<(B == VAL), int, float>" non-TREE_PUBLIC.
Then symtab_node::needed_p checks TREE_PUBLIC, sees it's 0, and we don't
emit any code.
I thought the fix would be some ODR-esque check to not consider
constexpr variables/fns that are used just for their value. But
it turned out to be tricky. For instance, we can't skip
determine_visibility in a template; we can't even skip it for value-dep
expressions. For example, no-linkage-expr1.C has
using P = struct {}*;
template <int N>
void f(int(*)[((P)0, N)]) {}
where ((P)0, N) is value-dep, but N is not relevant here: we have to
ferret out the anonymous type. When instantiating, it's already gone.
This patch uses decl_constant_var_p. This is to implement (an
approximation) [basic.def.odr]#14.5.1 and [basic.def.odr]#5.2.
PR c++/110323
gcc/cp/ChangeLog:
* decl2.cc (min_vis_expr_r) <case VAR_DECL>: Do nothing for
decl_constant_var_p VAR_DECLs.
gcc/testsuite/ChangeLog:
* g++.dg/template/explicit-instantiation6.C: New test.
* g++.dg/template/explicit-instantiation7.C: New test.
---
gcc/cp/decl2.cc | 6 ++-
.../g++.dg/template/explicit-instantiation6.C | 43 +++++++++++++++++++
.../g++.dg/template/explicit-instantiation7.C | 22 ++++++++++
3 files changed, 70 insertions(+), 1 deletion(-)
create mode 100644 gcc/testsuite/g++.dg/template/explicit-instantiation6.C
create mode 100644 gcc/testsuite/g++.dg/template/explicit-instantiation7.C
diff --git a/gcc/cp/decl2.cc b/gcc/cp/decl2.cc
index 2562d8aeff6..753e45e56ad 100644
--- a/gcc/cp/decl2.cc
+++ b/gcc/cp/decl2.cc
@@ -2718,7 +2718,11 @@ min_vis_expr_r (tree *tp, int */*walk_subtrees*/, void
*data)
/* Fall through. */
case VAR_DECL:
case FUNCTION_DECL:
- if (! TREE_PUBLIC (t))
+ if (decl_constant_var_p (t))
+ /* In a template, we can't trust VAR_DECLs, either. It's possible
+ they won't be ODR-used, and we could wrongly think the linkage
+ is internal (PR110323). */;
+ else if (! TREE_PUBLIC (t))
tpvis = VISIBILITY_ANON;
else
tpvis = DECL_VISIBILITY (t);
diff --git a/gcc/testsuite/g++.dg/template/explicit-instantiation6.C
b/gcc/testsuite/g++.dg/template/explicit-instantiation6.C
new file mode 100644
index 00000000000..8b77c9deb20
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/explicit-instantiation6.C
@@ -0,0 +1,43 @@
+// PR c++/110323
+// { dg-do compile { target c++14 } }
+
+template<bool B, class T, class F>
+struct conditional { using type = T; };
+
+template<class T, class F>
+struct conditional<false, T, F> { using type = F; };
+
+constexpr int VAL = 1;
+
+static constexpr int getval () { return 1; }
+
+template<typename>
+constexpr int TVAL = 1;
+
+static struct S {
+ constexpr operator bool() { return true; }
+} s;
+
+struct foo {
+ template <int B>
+ void bar(typename conditional<B == VAL, int, float>::type arg) { }
+
+ template <int B>
+ void qux(typename conditional<B == TVAL<int>, int, float>::type arg) { }
+
+ template <int B>
+ void sox(typename conditional<B == noexcept (VAL), int, float>::type arg)
{ }
+
+ template <int B>
+ void nim(typename conditional<B != sizeof (VAL), int, float>::type arg) { }
+};
+
+template void foo::bar<1>(int arg);
+template void foo::qux<1>(int arg);
+template void foo::sox<1>(int arg);
+template void foo::nim<1>(int arg);
+
+// { dg-final { scan-assembler
"_ZN3foo3barILi1EEEvN11conditionalIXeqT_L_ZL3VALEEifE4typeE" } }
+// { dg-final { scan-assembler
"_ZN3foo3quxILi1EEEvN11conditionalIXeqT_L_Z4TVALIiEEEifE4typeE" } }
+// { dg-final { scan-assembler
"_ZN3foo3soxILi1EEEvN11conditionalIXeqT_nxL_ZL3VALEEifE4typeE" } }
+// { dg-final { scan-assembler
"_ZN3foo3nimILi1EEEvN11conditionalIXneT_szL_ZL3VALEEifE4typeE" } }
diff --git a/gcc/testsuite/g++.dg/template/explicit-instantiation7.C
b/gcc/testsuite/g++.dg/template/explicit-instantiation7.C
new file mode 100644
index 00000000000..9a870e808fa
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/explicit-instantiation7.C
@@ -0,0 +1,22 @@
+// PR c++/110323
+// { dg-do compile { target c++11 } }
+
+using P = struct { }*;
+using N = struct A { }*;
+
+template<bool B, class T, class F>
+struct conditional { using type = T; };
+
+struct foo {
+ template <int B>
+ void bar(typename conditional<((P) 0, B), int, float>::type arg) { }
+
+ template <int B>
+ void baz(typename conditional<((N) 0, B), int, float>::type arg) { }
+};
+
+template void foo::bar<1>(int arg);
+template void foo::baz<1>(int arg);
+
+// { dg-final { scan-assembler-not
"_ZN3foo3barILi1EEEvN11conditionalIXcmcvP1XLi0EneT_Li0EEifE4typeE" } }
+// { dg-final { scan-assembler
"_ZN3foo3bazILi1EEEvN11conditionalIXcmcvP1ALi0EneT_Li0EEifE4typeE" } }
base-commit: bc91e3870e9c984c180b478a3449a9a2e56cd107
--
2.44.0
