On Sat, 12 Apr 2025, Jason Merrill wrote:
> On 4/11/25 4:36 PM, Patrick Palka wrote:
> > Boostrapped and regtested on x86_64-pc-linux- gnu, does this look OK
> > for GCC 15 perhaps?
> >
> > -- >8 --
> >
> > This patch improves upon r15-6052-g12de1942a0a673 by performing prvalue
> > folding with mce_false rather than mce_unknown when it's safe to do so
> > (i.e. the caller passed ff_mce_false), so that we can also fold temporary
> > initializers that call is_constant_evaluated etc.
> >
> > In passing I noticed constexpr-prvalue1.C could more precisely verify
> > the optimization happened by inspecting what the front end spits out
> > instead of the final assembly.
>
> I'm not sure that specificity is necessary, but OK.
Since we're compiling with -O the middle-end inlines the constructor
call, so the scan-assembler test succeeds even without r15-6052.
>
> > PR c++/116416
> >
> > gcc/cp/ChangeLog:
> >
> > * constexpr.cc (maybe_constant_init_1): Generalize type of
> > of manifestly_const_eval parameter from bool to mce_value.
> > (maybe_constant_init): Define 3-parameter version taking a
> > manifestly_const_eval instead of bool parameter.
> > (cxx_constant_init): Adjust maybe_constant_init_1.
> > * cp-gimplify.cc (cp_fold_r) <case TARGET_EXPR>: Pass mce_false
> > to maybe_constant_init during prvalue folding if ff_mce_false is
> > set.
> > * cp-tree.h (maybe_constant_init): Declare new overload.
> >
> > gcc/testsuite/ChangeLog:
> >
> > * g++.dg/cpp1y/constexpr-prvalue1.C: Adjust to instead inspect
> > the 'original' dump.
> > * g++.dg/cpp1y/constexpr-prvalue1a.C: New test.
> > ---
> > gcc/cp/constexpr.cc | 16 +++++++---
> > gcc/cp/cp-gimplify.cc | 4 ++-
> > gcc/cp/cp-tree.h | 1 +
> > .../g++.dg/cpp1y/constexpr-prvalue1.C | 11 ++++---
> > .../g++.dg/cpp1y/constexpr-prvalue1a.C | 32 +++++++++++++++++++
> > 5 files changed, 53 insertions(+), 11 deletions(-)
> > create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-prvalue1a.C
> >
> > diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> > index 497f64f3ceaa..0242425370f4 100644
> > --- a/gcc/cp/constexpr.cc
> > +++ b/gcc/cp/constexpr.cc
> > @@ -9679,7 +9679,7 @@ fold_non_dependent_init (tree t,
> > static tree
> > maybe_constant_init_1 (tree t, tree decl, bool allow_non_constant,
> > - bool manifestly_const_eval)
> > + mce_value manifestly_const_eval)
> > {
> > if (!t)
> > return t;
> > @@ -9709,13 +9709,13 @@ maybe_constant_init_1 (tree t, tree decl, bool
> > allow_non_constant,
> > bool is_static = (decl && DECL_P (decl)
> > && (TREE_STATIC (decl) || DECL_EXTERNAL (decl)));
> > if (is_static)
> > - manifestly_const_eval = true;
> > + manifestly_const_eval = mce_true;
> > - if (cp_unevaluated_operand && !manifestly_const_eval)
> > + if (cp_unevaluated_operand && manifestly_const_eval != mce_true)
> > return fold_to_constant (t);
> > t = cxx_eval_outermost_constant_expr (t, allow_non_constant,
> > !is_static,
> > - mce_value (manifestly_const_eval),
> > + manifestly_const_eval,
> > false, decl);
> > }
> > if (TREE_CODE (t) == TARGET_EXPR)
> > @@ -9731,6 +9731,12 @@ maybe_constant_init_1 (tree t, tree decl, bool
> > allow_non_constant,
> > tree
> > maybe_constant_init (tree t, tree decl, bool manifestly_const_eval)
> > +{
> > + return maybe_constant_init_1 (t, decl, true, mce_value
> > (manifestly_const_eval));
> > +}
> > +
> > +tree
> > +maybe_constant_init (tree t, tree decl, mce_value manifestly_const_eval)
> > {
> > return maybe_constant_init_1 (t, decl, true, manifestly_const_eval);
> > }
> > @@ -9740,7 +9746,7 @@ maybe_constant_init (tree t, tree decl, bool
> > manifestly_const_eval)
> > tree
> > cxx_constant_init (tree t, tree decl)
> > {
> > - return maybe_constant_init_1 (t, decl, false, true);
> > + return maybe_constant_init_1 (t, decl, false, mce_true);
> > }
> > #if 0
> > diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc
> > index 550cea29dd29..8dfa5ac2b88d 100644
> > --- a/gcc/cp/cp-gimplify.cc
> > +++ b/gcc/cp/cp-gimplify.cc
> > @@ -1480,7 +1480,9 @@ cp_fold_r (tree *stmt_p, int *walk_subtrees, void
> > *data_)
> > *walk_subtrees = 0;
> > if (!flag_no_inline)
> > {
> > - tree folded = maybe_constant_init (init, TARGET_EXPR_SLOT
> > (stmt));
> > + tree folded = maybe_constant_init (init, TARGET_EXPR_SLOT
> > (stmt),
> > + (data->flags & ff_mce_false
> > + ? mce_false : mce_unknown));
> > if (folded != init && TREE_CONSTANT (folded))
> > init = folded;
> > }
> > diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
> > index 927f51b116b3..55f986e25c14 100644
> > --- a/gcc/cp/cp-tree.h
> > +++ b/gcc/cp/cp-tree.h
> > @@ -8837,6 +8837,7 @@ extern void cxx_constant_dtor (tree,
> > tree);
> > extern tree cxx_constant_init (tree, tree =
> > NULL_TREE);
> > extern tree maybe_constant_value (tree, tree = NULL_TREE,
> > mce_value = mce_unknown);
> > extern tree maybe_constant_init (tree, tree =
> > NULL_TREE, bool = false);
> > +extern tree maybe_constant_init (tree, tree,
> > mce_value);
> > extern tree fold_non_dependent_expr (tree,
> > tsubst_flags_t =
> > tf_warning_or_error,
> > bool = false, tree =
> > NULL_TREE);
> > diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-prvalue1.C
> > b/gcc/testsuite/g++.dg/cpp1y/constexpr-prvalue1.C
> > index ad31e3001167..a17fed786fe8 100644
> > --- a/gcc/testsuite/g++.dg/cpp1y/constexpr-prvalue1.C
> > +++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-prvalue1.C
> > @@ -1,6 +1,6 @@
> > // PR c++/116416
> > // { dg-do compile { target c++14 } }
> > -// { dg-options "-O" }
> > +// { dg-options "-O -fdump-tree-original" }
> > struct Str {
> > constexpr Str() {}
> > @@ -17,14 +17,15 @@ extern void callback(Str str);
> > void
> > func1()
> > {
> > - callback(Str{"Test"});
> > + callback(Str{"Test1"});
> > }
> > void
> > func2()
> > {
> > - Str str{"Test"};
> > + Str str{"Test2"};
> > callback(str);
> > }
> > -// Check that we don't call Str::Str(char const*)
> > -// { dg-final { scan-assembler-not "_ZN3StrC1EPKc" } }
> > +// Check that the front end folds both constructor calls.
> > +// { dg-final { scan-tree-dump "{.str=\\(const char \\*\\) \"Test1\",
> > .length=5}" "original" } }
> > +// { dg-final { scan-tree-dump "{.str=\\(const char \\*\\) \"Test2\",
> > .length=5}" "original" } }
> > diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-prvalue1a.C
> > b/gcc/testsuite/g++.dg/cpp1y/constexpr-prvalue1a.C
> > new file mode 100644
> > index 000000000000..3f6ef97f9bf4
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-prvalue1a.C
> > @@ -0,0 +1,32 @@
> > +// PR c++/116416
> > +// A version of constexpr-prvalue1.C that calls
> > __builtin_is_constant_evaluated.
> > +// { dg-do compile { target c++14 } }
> > +// { dg-options "-O -fdump-tree-original" }
> > +
> > +struct Str {
> > + constexpr Str() {}
> > + constexpr Str(const char *instr) {
> > + str = instr; length = 0;
> > + for (auto index = 0; instr[index]; ++index) {
> > + length += __builtin_is_constant_evaluated() ? 1 : 1;
> > + }
> > + }
> > + const char *str = nullptr;
> > + int length = 0;
> > +};
> > +extern void callback(Str str);
> > +void
> > +func1()
> > +{
> > + callback(Str{"Test1"});
> > +}
> > +void
> > +func2()
> > +{
> > + Str str{"Test2"};
> > + callback(str);
> > +}
> > +
> > +// Check that the front end folds both constructor calls.
> > +// { dg-final { scan-tree-dump "{.str=\\(const char \\*\\) \"Test1\",
> > .length=5}" "original" } }
> > +// { dg-final { scan-tree-dump "{.str=\\(const char \\*\\) \"Test2\",
> > .length=5}" "original" } }
>
>
