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

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);
> +	      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
> 
>