[PATCH][PR c++/82888] smarter code for default initialization of scalar arrays

[Nathan Froyd]

Default-initialization of scalar arrays in C++ member initialization
lists produced rather slow code, laboriously setting each element of the
array to zero.  It would be much faster to block-initialize the array,
and that's what this patch does.

The patch works for me, but I'm not sure if it's the best way to
accomplish this.  At least two other possibilities come to mind:

1) Detect this case in build_vec_init_expr and act as though the user
   wrote 'member{0}', which the front-end already produces efficient
   code for.

2) Detect this case in build_vec_init, but again, act as though the user
   wrote 'member{0}' and let everything proceed as normal.
   (Alternatively, handle this case prior to calling build_vec_init and
   pass different arguments to build_vec_init.)

Opinions as to the best way forward here?  I'm unsure of whether the
code below is front-end friendly; I see in the gimple dumps that the
solution below adds an extra CLOBBER on 'this' for 'member()', whereas
'member{0}' does not.  It's possible that I'm missing something.

Bootstrapped on x86_64-unknown-linux-gnu, no regressions.

OK for trunk?

-Nathan

gcc/cp/
        PR c++/82888
        * init.c (build_vec_init): Handle default-initialization of array
        types.

gcc/testsuite/
        PR c++/82888
        * g++.dg/init/pr82888.C: New.

diff --git a/gcc/cp/init.c b/gcc/cp/init.c
index c76460d..53d6133 100644
--- a/gcc/cp/init.c
+++ b/gcc/cp/init.c
@@ -4038,6 +4038,15 @@ build_vec_init (tree base, tree maxindex, tree init,
        }
     }
 
+  /* Default-initialize scalar arrays directly.  */
+  if (TREE_CODE (atype) == ARRAY_TYPE
+      && SCALAR_TYPE_P (TREE_TYPE (atype))
+      && !init)
+    {
+      gcc_assert (!from_array);
+      return build2 (MODIFY_EXPR, atype, base, build_constructor (atype, 
NULL));
+    }
+
   /* If we have a braced-init-list or string constant, make sure that the array
      is big enough for all the initializers.  */
   bool length_check = (init
diff --git a/gcc/testsuite/g++.dg/init/pr82888.C 
b/gcc/testsuite/g++.dg/init/pr82888.C
new file mode 100644
index 0000000..9225e23
--- /dev/null
+++ b/gcc/testsuite/g++.dg/init/pr82888.C
@@ -0,0 +1,18 @@
+// { dg-do compile }
+// { dg-options "-fdump-tree-gimple" }
+
+class A
+{
+public:
+  A();
+
+private:
+  unsigned char mStorage[4096];
+};
+
+A::A()
+  : mStorage()
+{}
+
+// { dg-final { scan-tree-dump "this->mStorage = {}" "gimple" } }
+// { dg-final { scan-tree-dump-not "&this->mStorage" "gimple" } }