Hi René,
On Sat, 28 Jan 2017, René Scharfe wrote:
> diff --git a/git-compat-util.h b/git-compat-util.h
> index 87237b092b..66cd466eea 100644
> --- 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)
It may seem as a matter of taste, or maybe not: I prefer this without the
_swap_a_ptr (and I would also prefer not to use identifiers starting with
an underscore, as section 7.1.3 Reserved Identifiers of the C99 standard
says they are reserved):
+#define SWAP(a, b) do { \
+ unsigned char swap_buffer_[sizeof(a)]; \
+ memcpy(swap_buffer_, &(a), sizeof(a)); \
+ memcpy(&(a), &(b), sizeof(a) + \
+ BUILD_ASSERT_OR_ZERO(sizeof(a) == sizeof(b))); \
+ memcpy(&(b), swap_buffer_, sizeof(a)); \
+} while (0)
One idea to address the concern that not all C compilers people use to
build Git may optimize away those memcpy()s: we could also introduce a
SWAP_PRIMITIVE_TYPE (or SWAP2 or SIMPLE_SWAP or whatever) that accepts
only primitive types. But since __typeof__() is not portable...
Ciao,
Dscho
