On Fri, Aug 23, 2019 at 02:20:53PM -0700, Jason Merrill wrote:
> On 8/19/19 11:28 AM, Marek Polacek wrote:
> > On Fri, Aug 16, 2019 at 06:20:20PM -0700, Jason Merrill wrote:
> > > On 8/16/19 2:29 PM, Marek Polacek wrote:
> > > > This patch is an attempt to fix the annoying -Wunused-but-set-*
> > > > warnings that
> > > > tend to occur with constexpr if. When we have something like
> > > >
> > > > template < typename T >
> > > > int f(T v){
> > > > if constexpr(sizeof(T) == sizeof(int)){
> > > > return v;
> > > > }else{
> > > > return 0;
> > > > }
> > > > }
> > > >
> > > > and call f('a'), then the condition is false, meaning that we won't
> > > > instantiate
> > > > the then-branch, as per tsubst_expr/IF_STMT:
> > > > 17284 if (IF_STMT_CONSTEXPR_P (t) && integer_zerop (tmp))
> > > > 17285 /* Don't instantiate the THEN_CLAUSE. */;
> > > > so we'll never get round to mark_exp_read-ing the decls used in the
> > > > then-branch, causing finish_function to emit "parameter set but not
> > > > used"
> > > > warnings.
> > > >
> > > > It's unclear how to best deal with this. Marking the decls DECL_READ_P
> > > > while
> > > > parsing doesn't seem like a viable approach
> > >
> > > Why not?
> >
> > Well, while parsing, we're in a template and so the condition won't be
> > evaluated until tsubst_expr. So we can't tell which branch is dead.
>
> But if a decl is used on one branch, we shouldn't warn even if it isn't used
> on the selected branch.
I didn't want to mark the decls as read multiple times but we do it anyway
so that's no longer my concern. So...
Bootstrapped/regtested on x86_64-linux, ok for trunk?
2019-08-27 Marek Polacek <pola...@redhat.com>
PR c++/81676 - bogus -Wunused warnings in constexpr if.
* semantics.c (maybe_mark_exp_read_r): New function.
(finish_if_stmt): Call it on THEN_CLAUSE and ELSE_CLAUSE.
* g++.dg/cpp1z/constexpr-if31.C: New test.
* g++.dg/cpp1z/constexpr-if32.C: New test.
diff --git gcc/cp/semantics.c gcc/cp/semantics.c
index 1f7745933f9..2ba8e635ab7 100644
--- gcc/cp/semantics.c
+++ gcc/cp/semantics.c
@@ -774,6 +774,18 @@ finish_else_clause (tree if_stmt)
ELSE_CLAUSE (if_stmt) = pop_stmt_list (ELSE_CLAUSE (if_stmt));
}
+/* Callback for cp_walk_tree to mark all {VAR,PARM}_DECLs in a tree as
+ read. */
+
+static tree
+maybe_mark_exp_read_r (tree *tp, int *, void *)
+{
+ tree t = *tp;
+ if (VAR_P (t) || TREE_CODE (t) == PARM_DECL)
+ mark_exp_read (t);
+ return NULL_TREE;
+}
+
/* Finish an if-statement. */
void
@@ -781,6 +793,16 @@ finish_if_stmt (tree if_stmt)
{
tree scope = IF_SCOPE (if_stmt);
IF_SCOPE (if_stmt) = NULL;
+ if (IF_STMT_CONSTEXPR_P (if_stmt))
+ {
+ /* Prevent various -Wunused warnings. We might not instantiate
+ either of these branches, so we would not mark the variables
+ used in that branch as read. */
+ cp_walk_tree_without_duplicates (&THEN_CLAUSE (if_stmt),
+ maybe_mark_exp_read_r, NULL);
+ cp_walk_tree_without_duplicates (&ELSE_CLAUSE (if_stmt),
+ maybe_mark_exp_read_r, NULL);
+ }
add_stmt (do_poplevel (scope));
}
diff --git gcc/testsuite/g++.dg/cpp1z/constexpr-if31.C
gcc/testsuite/g++.dg/cpp1z/constexpr-if31.C
new file mode 100644
index 00000000000..02140cff9fd
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp1z/constexpr-if31.C
@@ -0,0 +1,79 @@
+// PR c++/81676 - bogus -Wunused warnings in constexpr if.
+// { dg-do compile { target c++17 } }
+// { dg-options "-Wall -Wextra" }
+
+template <typename T> int
+f1 (T v)
+{
+ T x = 0;
+ if constexpr(sizeof(T) == sizeof(int))
+ return v + x;
+ else
+ return 0;
+}
+
+template <typename T> int
+f2 (T v) // { dg-warning "unused parameter .v." }
+{
+ T x = 0;
+ if constexpr(sizeof(T) == sizeof(int))
+ return x;
+ else
+ return 0;
+}
+
+template <typename T> int
+f3 (T v)
+{
+ T x = 0; // { dg-warning "unused variable .x." }
+ if constexpr(sizeof(T) == sizeof(int))
+ return v;
+ else
+ return 0;
+}
+
+template <typename T> int
+f4 (T v)
+{
+ T x = 0;
+ if constexpr(sizeof(T) == sizeof(int))
+ return 0;
+ else
+ return v + x;
+}
+
+template <typename T> int
+f5 (T v) // { dg-warning "unused parameter .v." }
+{
+ T x = 0;
+ if constexpr(sizeof(T) == sizeof(int))
+ return 0;
+ else
+ return x;
+}
+
+template <typename T> int
+f6 (T v)
+{
+ T x = 0; // { dg-warning "unused variable .x." }
+ if constexpr(sizeof(T) == sizeof(int))
+ return 0;
+ else
+ return v;
+}
+
+int main()
+{
+ f1(0);
+ f1('a');
+ f2(0);
+ f2('a');
+ f3(0);
+ f3('a');
+ f4(0);
+ f4('a');
+ f5(0);
+ f5('a');
+ f6(0);
+ f6('a');
+}
diff --git gcc/testsuite/g++.dg/cpp1z/constexpr-if32.C
gcc/testsuite/g++.dg/cpp1z/constexpr-if32.C
new file mode 100644
index 00000000000..13a6039fce6
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp1z/constexpr-if32.C
@@ -0,0 +1,16 @@
+// PR c++/81676 - bogus -Wunused warnings in constexpr if.
+// { dg-do compile { target c++17 } }
+// { dg-options "-Wall -Wextra" }
+
+int main()
+{
+ auto f = [](auto a, auto b) {
+ if constexpr (sizeof(b) == 1) {
+ return a;
+ } else {
+ return b;
+ }
+ };
+
+ return f(1, 1) + f(1, 'a');
+}
