[gcc/devel/c++-modules] c++: Tweak reshape_init_array_1 [PR94124]

[gcc/devel/c++-modules] c++: Tweak reshape_init_array_1 [PR94124]

[Nathan Sidwell]

https://gcc.gnu.org/g:4069adf4bbc90d16b603e0308b48499c36b2b637

commit 4069adf4bbc90d16b603e0308b48499c36b2b637
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Thu Mar 12 08:28:05 2020 +0100

    c++: Tweak reshape_init_array_1 [PR94124]
    
    Isn't it wasteful to first copy perhaps a large constructor (recursively)
    and then truncate it to very few elts (zero in this case)?
    
    > We should certainly avoid copying if they're the same.  The code above for
    > only copying the bits that aren't going to be thrown away seems pretty
    > straightforward, might as well use it even if the savings aren't likely to
    > be large.
    
    Calling vec_safe_truncate with the same number of elts the vector already
    has is a nop, so IMHO we just should make sure we only unshare if it
    changed.
    
    2020-03-12  Jakub Jelinek  <jakub@redhat.com>
    
            PR c++/94124
            * decl.c (reshape_init_array_1): Don't unshare constructor if there
            aren't any trailing zero elts, otherwise only unshare the first
            nelts.

Diff:
---
 gcc/cp/ChangeLog |  9 ++++++++-
 gcc/cp/decl.c    | 19 +++++++++++++++----
 2 files changed, 23 insertions(+), 5 deletions(-)

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index 8cde3000e8d..b94e3bb8eb2 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,10 @@
+2020-03-12  Jakub Jelinek  <jakub@redhat.com>
+
+	PR c++/94124
+	* decl.c (reshape_init_array_1): Don't unshare constructor if there
+	aren't any trailing zero elts, otherwise only unshare the first
+	nelts.
+
 2020-03-11  Jason Merrill  <jason@redhat.com>
 
 	PR c++/93907
@@ -27,7 +34,7 @@
 
 2020-03-10  Jason Merrill  <jason@redhat.com>
 
-	PR c++/93956
+	PR c++/93596
 	* pt.c (maybe_aggr_guide): Check BRACE_ENCLOSED_INITIALIZER_P.
 
 2020-03-10  Jason Merrill  <jason@redhat.com>
diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index aa58e5f88ae..d240436d84a 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -6066,10 +6066,21 @@ reshape_init_array_1 (tree elt_type, tree max_index, reshape_iter *d,
 	 overload resolution.  E.g., initializing a class from
 	 {{0}} might be invalid while initializing the same class
 	 from {{}} might be valid.  */
-      if (reuse)
-	new_init = unshare_constructor (new_init);
-
-      vec_safe_truncate (CONSTRUCTOR_ELTS (new_init), nelts);
+      if (reuse && nelts < CONSTRUCTOR_NELTS (new_init))
+	{
+	  vec<constructor_elt, va_gc> *v;
+	  vec_alloc (v, nelts);
+	  for (unsigned int i = 0; i < nelts; i++)
+	    {
+	      constructor_elt elt = *CONSTRUCTOR_ELT (new_init, i);
+	      if (TREE_CODE (elt.value) == CONSTRUCTOR)
+		elt.value = unshare_constructor (elt.value);
+	      v->quick_push (elt);
+	    }
+	  new_init = build_constructor (TREE_TYPE (new_init), v);
+	}
+      else
+	vec_safe_truncate (CONSTRUCTOR_ELTS (new_init), nelts);
     }
 
   return new_init;