On Thu, 16 Dec 2021, Patrick Palka wrote:
> On Wed, 15 Dec 2021, Jason Merrill wrote:
>
> > On 12/15/21 12:49, Patrick Palka wrote:
> > > This patch makes us remember the function selected by overload
> > > resolution during ahead of time processing of a non-dependent call
> > > expression, so that we avoid repeating most of the work of overload
> > > resolution at instantiation time. This mirrors what we already do for
> > > non-dependent operator expressions via build_min_non_dep_op_overload.
> > >
> > > Some caveats:
> > >
> > > * When processing ahead of time a non-dependent call to a member
> > > function template inside a class template (as in
> > > g++.dg/template/deduce4.C), we end up generating an "inverted" partial
> > > instantiation such as S<T>::foo<int, int>(), the kinds of which we're
> > > apparently not prepared to fully instantiate (e.g. tsubst_baselink
> > > mishandles it). So this patch disables this optimization for such
> > > functions and adds a FIXME.
> >
> > I wonder if it would be worthwhile to build a TEMPLATE_ID_EXPR to remember
> > the
> > deduced template args, even if we are failing to remember the actual
> > function?
>
> Hmm, that transformation could have observable effects, since overload
> resolution for f<int>(0) might end up instantiating more things than for
> f(0) due to the explicit-args substitution step:
>
> template<class T> struct A { using type = typename T::type; };
>
> template<class T> void f(T);
> template<class T, class U = typename T::type> typename A<T>::type f(T);
>
> Here overload resolution for f(0) succeeds and selects the first
> overload but for f<int>(0) induces a hard error. Also I worry that such
> a transformation might affect declaration matching in weird ways due
> to conflating f(0) with f<int>(0).
>
> We could at least though prune the overload set to the corresponding
> selected function template rather than the "inside-out" specialization;
> I'll try to implement that.
Here's a bootstrapped and regtested patch which implements that:
-- >8 --
This patch makes us remember the function selected by overload resolution
during ahead of time processing of a non-dependent call expression, so
that we avoid repeating most of the work of overload resolution for the
call at instantiation time. Note that we already do this for
non-dependent operator expressions via build_min_non_dep_op_overload.
Some caveats:
* When processing ahead of time a non-dependent call to a member
function template inside a common class template (as in
g++.dg/template/deduce4.C), we end up generating an "inverted" partial
instantiation such as S<T>::foo<int, int>(), the likes of which we're
apparently not prepared to fully instantiate (e.g. tsubst_baselink
mishandles it). So this patch makes us prune to the selected
template instead of the specialization in this case.
* This change triggered a latent FUNCTION_DECL pretty printing issue
in cpp0x/error2.C -- since we now resolve the call to foo<0> ahead
of time, the error now looks like:
error: expansion pattern ‘foo()()=0’ contains no parameter pack
where the FUNCTION_DECL for foo<0> is clearly misprinted. But this
pretty-printing issue could be reproduced without this patch if
we define foo as a non-template function. Since this testcase was
added to verify pretty printing of TEMPLATE_ID_EXPR, I work around
this test failure by making the call to foo type-dependent and thus
immune to this ahead of time pruning.
* We now reject parts of cpp0x/fntmp-equiv1.C because we notice that
the call d(f, b) in
template <unsigned long f, unsigned b, typename> e<d(f, b)> d();
is always non-constexpr because the selected d isn't. I tried fixing
this by making it constexpr, but then the call to d from main becomes
ambiguous. So I settled with removing this part of the testcase.
gcc/cp/ChangeLog:
* call.c (build_new_method_call): For a non-dependent call
expression inside a template, returning a templated tree
whose overload set contains just the selected function.
* semantics.c (finish_call_expr): Likewise.
gcc/testsuite/ChangeLog:
* g++.dg/cpp0x/error2.C: Make the call to foo type-dependent in
order to avoid latent pretty-printing issue for FUNCTION_DECL
inside MODOP_EXPR.
* g++.dg/cpp0x/fntmp-equiv1.C: Remove ill-formed parts of
testcase that we now diagnose.
* g++.dg/template/non-dependent16.C: New test.
* g++.dg/template/non-dependent16a.C: New test.
* g++.dg/template/non-dependent17.C: New test.
---
gcc/cp/call.c | 27 ++++++++++++++
gcc/cp/semantics.c | 15 ++++++++
gcc/testsuite/g++.dg/cpp0x/error2.C | 4 +-
gcc/testsuite/g++.dg/cpp0x/fntmp-equiv1.C | 4 --
.../g++.dg/template/non-dependent16.C | 37 +++++++++++++++++++
.../g++.dg/template/non-dependent16a.C | 36 ++++++++++++++++++
.../g++.dg/template/non-dependent17.C | 21 +++++++++++
7 files changed, 138 insertions(+), 6 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/template/non-dependent16.C
create mode 100644 gcc/testsuite/g++.dg/template/non-dependent16a.C
create mode 100644 gcc/testsuite/g++.dg/template/non-dependent17.C
diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index 495dcdd77b3..1fbfc580a1e 100644
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -11163,6 +11163,33 @@ build_new_method_call (tree instance, tree fns,
vec<tree, va_gc> **args,
}
if (INDIRECT_REF_P (call))
call = TREE_OPERAND (call, 0);
+
+ /* Prune all but the selected function from the original overload
+ set so that we can avoid some duplicate work at instantiation time. */
+ if (really_overloaded_fn (fns))
+ {
+ if (DECL_TEMPLATE_INFO (fn)
+ && DECL_MEMBER_TEMPLATE_P (DECL_TI_TEMPLATE (fn))
+ && dependent_type_p (DECL_CONTEXT (fn)))
+ {
+ /* FIXME: We're not prepared to fully instantiate "inside-out"
+ partial instantiations such as A<T>::f<int>(). So instead
+ use the selected template, not the specialization. */
+
+ if (OVL_SINGLE_P (fns))
+ /* If the original overload set consists of a single function
+ template, this isn't beneficial. */
+ goto skip_prune;
+
+ fn = ovl_make (DECL_TI_TEMPLATE (fn));
+ if (template_only)
+ fn = lookup_template_function (fn, explicit_targs);
+ }
+ orig_fns = copy_node (orig_fns);
+ BASELINK_FUNCTIONS (orig_fns) = fn;
+ }
+
+skip_prune:
call = (build_min_non_dep_call_vec
(call,
build_min (COMPONENT_REF, TREE_TYPE (CALL_EXPR_FN (call)),
diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
index 6603066c620..6ffd82cb0ec 100644
--- a/gcc/cp/semantics.c
+++ b/gcc/cp/semantics.c
@@ -2893,6 +2893,21 @@ finish_call_expr (tree fn, vec<tree, va_gc> **args, bool
disallow_virtual,
{
if (INDIRECT_REF_P (result))
result = TREE_OPERAND (result, 0);
+
+ /* Prune all but the selected function from the original overload
+ set so that we can avoid some duplicate work at instantiation time. */
+ if (TREE_CODE (result) == CALL_EXPR
+ && really_overloaded_fn (orig_fn))
+ {
+ orig_fn = CALL_EXPR_FN (result);
+ if (TREE_CODE (orig_fn) == COMPONENT_REF)
+ {
+ /* The non-dependent result of build_new_method_call. */
+ orig_fn = TREE_OPERAND (orig_fn, 1);
+ gcc_assert (BASELINK_P (orig_fn));
+ }
+ }
+
result = build_call_vec (TREE_TYPE (result), orig_fn, orig_args);
SET_EXPR_LOCATION (result, input_location);
KOENIG_LOOKUP_P (result) = koenig_p;
diff --git a/gcc/testsuite/g++.dg/cpp0x/error2.C
b/gcc/testsuite/g++.dg/cpp0x/error2.C
index e6af294c180..eb966362ccb 100644
--- a/gcc/testsuite/g++.dg/cpp0x/error2.C
+++ b/gcc/testsuite/g++.dg/cpp0x/error2.C
@@ -3,7 +3,7 @@
template<int> int foo();
-template<typename F> void bar(F f)
+template<typename F, int N> void bar(F f)
{
- f((foo<0>()=0)...); // { dg-error "pattern '\\(foo\\<0\\>\\)\\(\\)=0'" }
+ f((foo<N>()=0)...); // { dg-error "pattern '\\(foo\\<N\\>\\)\\(\\)=0'" }
}
diff --git a/gcc/testsuite/g++.dg/cpp0x/fntmp-equiv1.C
b/gcc/testsuite/g++.dg/cpp0x/fntmp-equiv1.C
index 833ae6fc85c..60ebad8d1d3 100644
--- a/gcc/testsuite/g++.dg/cpp0x/fntmp-equiv1.C
+++ b/gcc/testsuite/g++.dg/cpp0x/fntmp-equiv1.C
@@ -1,10 +1,7 @@
// PR c++/86946, DR 1321
// { dg-do compile { target c++11 } }
-int d(int, int);
template <long> class e {};
-template <unsigned long f, unsigned b, typename> e<sizeof(d(f, b))> d();
-template <unsigned long f, unsigned b, typename> e<d(f, b)> d();
template <class T, class U> constexpr T d2(T, U) { return 42; }
template <unsigned long f, unsigned b, typename> e<d2(f, b)> d2();
@@ -17,7 +14,6 @@ template <unsigned long f, unsigned b, typename>
e<sizeof(d3(f, b))> d3();
int main()
{
- d<1,2,int>();
d2<1,2,int>();
d3<1,2,int>();
}
diff --git a/gcc/testsuite/g++.dg/template/non-dependent16.C
b/gcc/testsuite/g++.dg/template/non-dependent16.C
new file mode 100644
index 00000000000..ee8ef902529
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/non-dependent16.C
@@ -0,0 +1,37 @@
+// This test verifies that after resolving a non-dependent call expression
+// ahead of time, we prune all but the selected candidate from the overload
+// set. Without this optimization, overload resolution for the final call to
+// f<void>() would be exponential in the size of the overload set.
+
+// { dg-do compile { target c++11 } }
+
+template<class T> void f();
+template<class T> auto f() -> decltype(f<void>(), 1, *T());
+template<class T> auto f() -> decltype(f<void>(), 2, *T());
+template<class T> auto f() -> decltype(f<void>(), 3, *T());
+template<class T> auto f() -> decltype(f<void>(), 4, *T());
+template<class T> auto f() -> decltype(f<void>(), 5, *T());
+template<class T> auto f() -> decltype(f<void>(), 6, *T());
+template<class T> auto f() -> decltype(f<void>(), 7, *T());
+template<class T> auto f() -> decltype(f<void>(), 8, *T());
+template<class T> auto f() -> decltype(f<void>(), 9, *T());
+template<class T> auto f() -> decltype(f<void>(), 10, *T());
+template<class T> auto f() -> decltype(f<void>(), 11, *T());
+template<class T> auto f() -> decltype(f<void>(), 12, *T());
+template<class T> auto f() -> decltype(f<void>(), 13, *T());
+template<class T> auto f() -> decltype(f<void>(), 14, *T());
+template<class T> auto f() -> decltype(f<void>(), 15, *T());
+template<class T> auto f() -> decltype(f<void>(), 16, *T());
+template<class T> auto f() -> decltype(f<void>(), 17, *T());
+template<class T> auto f() -> decltype(f<void>(), 18, *T());
+template<class T> auto f() -> decltype(f<void>(), 19, *T());
+template<class T> auto f() -> decltype(f<void>(), 20, *T());
+template<class T> auto f() -> decltype(f<void>(), 21, *T());
+template<class T> auto f() -> decltype(f<void>(), 22, *T());
+template<class T> auto f() -> decltype(f<void>(), 23, *T());
+template<class T> auto f() -> decltype(f<void>(), 24, *T());
+template<class T> auto f() -> decltype(f<void>(), 25, *T());
+
+int main() {
+ f<void>();
+}
diff --git a/gcc/testsuite/g++.dg/template/non-dependent16a.C
b/gcc/testsuite/g++.dg/template/non-dependent16a.C
new file mode 100644
index 00000000000..0e04d646c0b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/non-dependent16a.C
@@ -0,0 +1,36 @@
+// Like non-dependent16.C, but using member functions.
+
+// { dg-do compile { target c++11 } }
+
+struct A {
+ template<class T> static void f();
+ template<class T> static auto f() -> decltype(f<void>(), 1, *T());
+ template<class T> static auto f() -> decltype(f<void>(), 2, *T());
+ template<class T> static auto f() -> decltype(f<void>(), 3, *T());
+ template<class T> static auto f() -> decltype(f<void>(), 4, *T());
+ template<class T> static auto f() -> decltype(f<void>(), 5, *T());
+ template<class T> static auto f() -> decltype(f<void>(), 6, *T());
+ template<class T> static auto f() -> decltype(f<void>(), 7, *T());
+ template<class T> static auto f() -> decltype(f<void>(), 8, *T());
+ template<class T> static auto f() -> decltype(f<void>(), 9, *T());
+ template<class T> static auto f() -> decltype(f<void>(), 10, *T());
+ template<class T> static auto f() -> decltype(f<void>(), 11, *T());
+ template<class T> static auto f() -> decltype(f<void>(), 12, *T());
+ template<class T> static auto f() -> decltype(f<void>(), 13, *T());
+ template<class T> static auto f() -> decltype(f<void>(), 14, *T());
+ template<class T> static auto f() -> decltype(f<void>(), 15, *T());
+ template<class T> static auto f() -> decltype(f<void>(), 16, *T());
+ template<class T> static auto f() -> decltype(f<void>(), 17, *T());
+ template<class T> static auto f() -> decltype(f<void>(), 18, *T());
+ template<class T> static auto f() -> decltype(f<void>(), 19, *T());
+ template<class T> static auto f() -> decltype(f<void>(), 20, *T());
+ template<class T> static auto f() -> decltype(f<void>(), 21, *T());
+ template<class T> static auto f() -> decltype(f<void>(), 22, *T());
+ template<class T> static auto f() -> decltype(f<void>(), 23, *T());
+ template<class T> static auto f() -> decltype(f<void>(), 24, *T());
+ template<class T> static auto f() -> decltype(f<void>(), 25, *T());
+};
+
+int main() {
+ A::f<void>();
+}
diff --git a/gcc/testsuite/g++.dg/template/non-dependent17.C
b/gcc/testsuite/g++.dg/template/non-dependent17.C
new file mode 100644
index 00000000000..bc664999e84
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/non-dependent17.C
@@ -0,0 +1,21 @@
+// A variant of deduce4.C with multiple overloads of foo. Verify we don't
+// crash when after ahead-of-time pruning of the overload set for the
+// non-dependent call to foo.
+// { dg-do compile }
+
+template <typename T>
+struct S {
+ template <typename U, typename V>
+ static void foo(V) { }
+ template <typename U>
+ static void foo(...) { }
+
+ void bar () { foo<int>(10); }
+};
+
+void
+test ()
+{
+ S<int> s;
+ s.bar ();
+}
--
2.34.1.297.g69a9c10c95
