add SWAP macro

Add a macro for exchanging the values of variables.  It allows users
to avoid repetition and takes care of the temporary variable for them.
It also makes sure that the storage sizes of its two parameters are the
same.  Its memcpy(1) calls are optimized away by current compilers.

Also add a conservative semantic patch for transforming only swaps of
variables of the same type.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/contrib/coccinelle/swap.cocci b/contrib/coccinelle/swap.cocci
new file mode 100644 (file)
index 0000000..a0934d1
--- /dev/null
+++ b/contrib/coccinelle/swap.cocci
@@ -0,0 +1,28 @@
+@ swap_with_declaration @
+type T;
+identifier tmp;
+T a, b;
+@@
+- T tmp = a;
++ T tmp;
++ tmp = a;
+  a = b;
+  b = tmp;
+
+@ swap @
+type T;
+T tmp, a, b;
+@@
+- tmp = a;
+- a = b;
+- b = tmp;
++ SWAP(a, b);
+
+@ extends swap @
+identifier unused;
+@@
+  {
+  ...
+- T unused;
+  ... when != unused
+  }

diff --git a/git-compat-util.h b/git-compat-util.h
index 87237b092b51df6255552f0756359eddf3b12042..66cd466eeaca10eb4ebd8cb00becbe5afa9117c8 100644 (file)
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -527,6 +527,16 @@ static inline int ends_with(const char *str, const char *suffix)
        return strip_suffix(str, suffix, &len);
 }
 
+#define SWAP(a, b) do {                                                \
+       void *_swap_a_ptr = &(a);                               \
+       void *_swap_b_ptr = &(b);                               \
+       unsigned char _swap_buffer[sizeof(a)];                  \
+       memcpy(_swap_buffer, _swap_a_ptr, sizeof(a));           \
+       memcpy(_swap_a_ptr, _swap_b_ptr, sizeof(a) +            \
+              BUILD_ASSERT_OR_ZERO(sizeof(a) == sizeof(b)));   \
+       memcpy(_swap_b_ptr, _swap_buffer, sizeof(a));           \
+} while (0)
+
 #if defined(NO_MMAP) || defined(USE_WIN32_MMAP)
 
 #ifndef PROT_READ