Re: [PATCH 3/2] c++: INIT_EXPR trial constexpr folding

On Fri, 14 Aug 2026, Patrick Palka wrote:

> On Thu, 6 Aug 2026, Patrick Palka wrote:
> 
> > Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK
> > for trunk only?
> 
> Ping.  This is a quick follow up to the PR126483 series, that fixes the
> is_constant_evaluated3b.C optimization regression that it'd incidentally
> cause.
> 
> > 
> > -- >8 --
> > 
> > This patch adds trial constexpr folding of INIT_EXPR that mirrors the
> > existing TARGET_EXPR folding added by r15-6052.  Since local copy-init
> > is represented as INIT_EXPR, this addresses the xfails in
> > is_constant_evaluated3{a,b}.C.
> > 
> > gcc/cp/ChangeLog:
> > 
> > 	* cp-gimplify.cc (cp_fold_r) <case INIT_EXPR>: Add trial
> > 	maybe_constant_init logic like for TARGET_EXPR.
> > 
> > gcc/testsuite/ChangeLog:
> > 
> > 	* g++.dg/opt/is_constant_evaluated3a.C: Remove xfail.
> > 	* g++.dg/opt/is_constant_evaluated3b.C: Likewise.
> > ---
> >  gcc/cp/cp-gimplify.cc                             | 15 ++++++++++++++-
> >  .../g++.dg/opt/is_constant_evaluated3a.C          |  2 +-
> >  .../g++.dg/opt/is_constant_evaluated3b.C          |  2 +-
> >  3 files changed, 16 insertions(+), 3 deletions(-)
> > 
> > diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc
> > index 23a1b7ea2ccd..927062f0121c 100644
> > --- a/gcc/cp/cp-gimplify.cc
> > +++ b/gcc/cp/cp-gimplify.cc
> > @@ -1736,7 +1736,20 @@ cp_fold_r (tree *stmt_p, int *walk_subtrees, void *data_)
> >  	 reference transition.  */
> >      case INIT_EXPR:
> >        if (data->flags & ff_genericize)
> > -	cp_genericize_init_expr (stmt_p);
> > +	{
> > +	  if (!flag_no_inline)
> > +	    {
> > +	      tree to = TREE_OPERAND (*stmt_p, 0);
> > +	      tree &from = TREE_OPERAND (*stmt_p, 1);

When the initializer of the INIT_EXPR is itself a TARGET_EXPR, I wonder
if we should just punt here and let the TARGET_EXPR case handle folding
it (as a prvalue)?  Otherwise we'll end up trial constant evaluating
twice, redundantly.

IIUC that'd give a different result for self-referential initializers
such as

    struct A { A* p = this; };

    int main() {
      A a = A{};
    }

where the prvalue folding would make p point to some unnamed temporary,
and the INIT_EXPR folding would make p point to a, but either way the
result wouldn't be TREE_CONSTANT so we'd discard the result.

> > +	      tree folded = maybe_constant_init (from, to,
> > +						 (data->flags & ff_mce_false
> > +						  ? mce_false : mce_unknown));
> > +	      if (folded != from && TREE_CONSTANT (folded))
> > +		from = folded;
> > +	    }
> > +
> > +	  cp_genericize_init_expr (stmt_p);
> > +	}
> >        break;
> >  
> >      case TARGET_EXPR:
> > diff --git a/gcc/testsuite/g++.dg/opt/is_constant_evaluated3a.C b/gcc/testsuite/g++.dg/opt/is_constant_evaluated3a.C
> > index 4ec3ecdf7b5d..e5c8b7f57895 100644
> > --- a/gcc/testsuite/g++.dg/opt/is_constant_evaluated3a.C
> > +++ b/gcc/testsuite/g++.dg/opt/is_constant_evaluated3a.C
> > @@ -17,7 +17,7 @@ constexpr void f() {
> >    A a5{};
> >  }
> >  
> > -// { dg-final { scan-tree-dump "a1 = {\\.n=42, \\.m=0}" "original" { xfail *-*-* } } }
> > +// { dg-final { scan-tree-dump "a1 = {\\.n=42, \\.m=0}" "original" } }
> >  // { dg-final { scan-tree-dump "a2 = {\\.n=42, \\.m=0}" "original" } }
> >  // { dg-final { scan-tree-dump "a3 = {\\.n=42, \\.m=0}" "original" } }
> >  // { dg-final { scan-tree-dump "a4 = {\\.n=42, \\.m=0}" "original" } }
> > diff --git a/gcc/testsuite/g++.dg/opt/is_constant_evaluated3b.C b/gcc/testsuite/g++.dg/opt/is_constant_evaluated3b.C
> > index ac6d21d632e1..ff708e23832b 100644
> > --- a/gcc/testsuite/g++.dg/opt/is_constant_evaluated3b.C
> > +++ b/gcc/testsuite/g++.dg/opt/is_constant_evaluated3b.C
> > @@ -17,7 +17,7 @@ auto f = [] {
> >    A a5{};
> >  };
> >  
> > -// { dg-final { scan-tree-dump "a1 = {\\.n=42, \\.m=0}" "original" { xfail *-*-* } } }
> > +// { dg-final { scan-tree-dump "a1 = {\\.n=42, \\.m=0}" "original" } }
> >  // { dg-final { scan-tree-dump "a2 = {\\.n=42, \\.m=0}" "original" } }
> >  // { dg-final { scan-tree-dump "a3 = {\\.n=42, \\.m=0}" "original" } }
> >  // { dg-final { scan-tree-dump "a4 = {\\.n=42, \\.m=0}" "original" } }
> > -- 
> > 2.55.0.481.ga97fcc37c2
> > 
> > 
>