On Thu, Oct 19, 2023 at 09:39:27AM -0400, Patrick Palka wrote:
> On Tue, 17 Oct 2023, Marek Polacek wrote:
>
> > On Tue, Oct 17, 2023 at 04:49:52PM -0400, Jason Merrill wrote:
> > > On 10/16/23 20:39, Marek Polacek wrote:
> > > > - if (cxx_dialect > cxx17)
> > > > - cp_fold_immediate_r (stmt_p, walk_subtrees, data);
> > > > + if (cxx_dialect >= cxx20)
> > > > + {
> > > > + /* Unfortunately we must handle code like
> > > > + false ? bar () : 42
> > > > + where we have to check bar too. The cp_fold call below could
> > > > + fold the ?: into a constant before we've checked it. */
> > > > + if (code == COND_EXPR)
> > > > + {
> > > > + auto then_fn = cp_fold_r, else_fn = cp_fold_r;
> > > > + /* See if we can figure out if either of the branches is
> > > > dead. If it
> > > > + is, we don't need to do everything that cp_fold_r does. */
> > > > + tree cond = maybe_constant_value (TREE_OPERAND (stmt, 0));
> > > > + if (integer_zerop (cond))
> > > > + then_fn = cp_fold_immediate_r;
> > > > + else if (TREE_CODE (cond) == INTEGER_CST)
> > > > + else_fn = cp_fold_immediate_r;
> > > > +
> > > > + cp_walk_tree (&TREE_OPERAND (stmt, 0), cp_fold_r, data,
> > > > nullptr);
> > >
> > > I wonder about doing this before maybe_constant_value, to hopefully reduce
> > > the redundant calculations? OK either way.
> >
> > Yeah, I was toying with that, I had
> >
> > foo() ? x : y
> >
> > where foo was a constexpr function but the cp_fold_r on op0 didn't eval it
> > to a constant :(.
>
> IIUC that's because cp_fold evaluates constexpr calls only with -O, so
> doing cp_fold_r before maybe_constant_value on the condition should
> still be desirable in that case?
Ah yeah, it depends on whether -fno-inline is on or off as described below.
OK if testing passes? Thanks,
-- >8 --
This patch is an optimization tweak for cp_fold_r. If we cp_fold_r the
COND_EXPR's op0 first, we may be able to evaluate it to a constant if -O,
saving some work. cp_fold has:
3143 if (callee && DECL_DECLARED_CONSTEXPR_P (callee)
3144 && !flag_no_inline)
...
3151 r = maybe_constant_value (x, /*decl=*/NULL_TREE,
flag_no_inline is 1 for -O0:
1124 if (opts->x_optimize == 0)
1125 {
1126 /* Inlining does not work if not optimizing,
1127 so force it not to be done. */
1128 opts->x_warn_inline = 0;
1129 opts->x_flag_no_inline = 1;
1130 }
but otherwise it's 0 and cp_fold will maybe_constant_value calls to
constexpr functions.
gcc/cp/ChangeLog:
* cp-gimplify.cc (cp_fold_r): cp_fold_r the first operand of a COND_EXPR
before calling maybe_constant_value.
---
gcc/cp/cp-gimplify.cc | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)
diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc
index a282c3930a3..746de86dfa6 100644
--- a/gcc/cp/cp-gimplify.cc
+++ b/gcc/cp/cp-gimplify.cc
@@ -1151,14 +1151,16 @@ cp_fold_r (tree *stmt_p, int *walk_subtrees, void
*data_)
{
auto then_fn = cp_fold_r, else_fn = cp_fold_r;
/* See if we can figure out if either of the branches is dead. If it
- is, we don't need to do everything that cp_fold_r does. */
+ is, we don't need to do everything that cp_fold_r does. If we
+ cp_fold_r first, there's a chance it will evaluate the condition to
+ a constant so maybe_constant_value won't have a lot of work to do.
*/
+ cp_walk_tree (&TREE_OPERAND (stmt, 0), cp_fold_r, data, nullptr);
tree cond = maybe_constant_value (TREE_OPERAND (stmt, 0));
if (integer_zerop (cond))
then_fn = cp_fold_immediate_r;
else if (TREE_CODE (cond) == INTEGER_CST)
else_fn = cp_fold_immediate_r;
- cp_walk_tree (&TREE_OPERAND (stmt, 0), cp_fold_r, data, nullptr);
if (TREE_OPERAND (stmt, 1))
cp_walk_tree (&TREE_OPERAND (stmt, 1), then_fn, data,
nullptr);
base-commit: 19cc4b9d74940f29c961e2a5a8b1fa84992d3d30
--
2.41.0
