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 87237b0..66cd466 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