[RFA] Improve initialization of objects when the initializer has trailing zeros.

[Jeff Law via Gcc-patches]

This is an update to a patch originally posted by Takayuki Suwa a few 
months ago.

When we initialize an array from a STRING_CST we perform the 
initialization in two steps.  The first step copies the STRING_CST to 
the destination.  The second step uses clear_storage to initialize 
storage in the array beyond TREE_STRING_LENGTH of the initializer.

Takayuki's patch added a special case when the STRING_CST itself was all 
zeros which would avoid the copy from the STRING_CST and instead do all 
the initialization via clear_storage which is clearly more runtime 
efficient.

Richie had the suggestion that instead of special casing when the entire 
STRING_CST was NULs  to instead identify when the tail of the STRING_CST 
was NULs.   That's more general and handles Takayuki's case as well.

Bootstrapped and regression tested on x86_64-linux-gnu.  Given I rewrote 
Takayuki's patch I think it needs someone else to review rather than 
self-approving.

OK for the trunk?

Jeff

        * expr.cc (store_expr): Identify trailing NULs in a STRING_CST
        initializer and use clear_storage rather than copying the
        NULs to the destination array.

diff --git a/gcc/expr.cc b/gcc/expr.cc
index 62297379ec9..f94d46b969c 100644
--- a/gcc/expr.cc
+++ b/gcc/expr.cc
@@ -6087,6 +6087,17 @@ store_expr (tree exp, rtx target, int call_param_p,
        }
 
       str_copy_len = TREE_STRING_LENGTH (str);
+
+      /* Trailing NUL bytes in EXP will be handled by the call to
+        clear_storage, which is more efficient than copying them from
+        the STRING_CST, so trim those from STR_COPY_LEN.  */
+      while (str_copy_len)
+       {
+         if (TREE_STRING_POINTER (str)[str_copy_len - 1])
+           break;
+         str_copy_len--;
+       }
+
       if ((STORE_MAX_PIECES & (STORE_MAX_PIECES - 1)) == 0)
        {
          str_copy_len += STORE_MAX_PIECES - 1;