gdbsupport: add array_view copy function

An assertion was recently added to array_view::operator[] to ensure we
don't do out of bounds accesses.  However, when the array_view is copied
to or from using memcpy, it bypasses that safety.

To address this, add a `copy` free function that copies data from an
array view to another, ensuring that the destination and source array
views have the same size.  When copying to or from parts of an
array_view, we are expected to use gdb::array_view::slice, which does
its own bounds check.  With all that, any copy operation that goes out
of bounds should be caught by an assertion at runtime.

copy is implemented using std::copy and std::copy_backward, which, at
least on libstdc++, appears to pick memmove when copying trivial data.
So in the end there shouldn't be much difference vs using a bare memcpy,
as we do right now.  When copying non-trivial data, std::copy and
std::copy_backward assigns each element in a loop.

To properly support overlapping ranges, we must use std::copy or
std::copy_backward, depending on whether the destination is before the
source or vice-versa.  std::copy and std::copy_backward don't support
copying exactly overlapping ranges (where the source range is equal to
the destination range).  But in this case, no copy is needed anyway, so
we do nothing.

The order of parameters of the new copy function is based on std::copy
and std::copy_backward, where the source comes before the destination.

Change a few randomly selected spots to use the new function, to show
how it can be used.

Add a test for the new function, testing both with arrays of a trivial
type (int) and of a non-trivial type (foo).  Test non-overlapping
ranges as well as three kinds of overlapping ranges: source before dest,
dest before source, and dest == source.

Change-Id: Ibeaca04e0028410fd44ce82f72e60058d6230a03

1 files changed, 15 insertions, 0 deletions

diff --git a/gdbsupport/array-view.h b/gdbsupport/array-view.h
index edf6655..5f1f46b 100644
--- a/gdbsupport/array-view.h
+++ b/gdbsupport/array-view.h
@@ -19,6 +19,7 @@
 #define COMMON_ARRAY_VIEW_H
 
 #include "traits.h"
+#include <algorithm>
 #include <type_traits>
 
 /* An array_view is an abstraction that provides a non-owning view
@@ -206,6 +207,20 @@ private:
   size_type m_size;
 };
 
+/* Copy the contents referenced by the array view SRC to the array view DEST.
+
+   The two array views must have the same length.  */
+
+template <typename U, typename T>
+void copy (gdb::array_view<U> src, gdb::array_view<T> dest)
+{
+  gdb_assert (dest.size () == src.size ());
+  if (dest.data () < src.data ())
+    std::copy (src.begin (), src.end (), dest.begin ());
+  else if (dest.data () > src.data ())
+    std::copy_backward (src.begin (), src.end (), dest.end ());
+}
+
 /* Compare LHS and RHS for (deep) equality.  That is, whether LHS and
    RHS have the same sizes, and whether each pair of elements of LHS
    and RHS at the same position compares equal.  */