I'm trying to put together a macro to abstract away some compiler
differences, but I'm having trouble with __builtin_assume_aligned, and
I'm not sure how to generate a reliable test case.
For ICC there is __assume_aligned. This is what I'm looking to
emulate; you just pass it a pointer and the desired alignment:
__assume_aligned(arg, 16)
/* Compiler knows arg is 16-byte aligned */
For MSVC there is __assume, which is a bit of a pain to use, but
basically you pass an expression (which evaluates to true if the
variable is aligned as expected). Something like
__assume((((char*) arg) - ((char*) 0)) % (16) == 0)
/* Compiler knows arg is 16-byte aligned */
GCC 4.7+, OTOH, does things a bit different. It returns a value, and
you're supposed to use the returned value:
void* x = __builtin_assume_aligned(arg, 16);
/* Compiler knows x is 16-byte aligned */
My question is, with __builtin_assume_aligned, does GCC know that *arg*
(not x) is 16-byte aligned? Basically, can I use it like
__assume_aligned?
Also, for 4.5+ (when __builtin_unreachable was added), if I do
something like
#define assume_aligned(arg, align) \
((((char*) ptr) - ((char*) 0)) % (align) == 0) ? \
(0) : __builtin_unreachable()
Would GCC know that arg is aligned?
-Evan
