问题:

I'm trying to make a tweak for iOS 7 so that when a device is ARM64 it runs one version and when it is not it runs another (since float is for 32 bit and double is for 64 (If you have a solution for that let me know.)

So it would be like this

if ARM64 {
    \\run double code
}
else {
    \\run float code
}

回答1:

You would do the following

#if __LP64__
    \\You're running on 64 bit
#else
    \\You're running on 32 bit
#endif

回答2:

On arm64 environment, the pointer take 8 bytes.

- (BOOL)isArm64
{
    static BOOL arm64 = NO ;
    static dispatch_once_t once ;
    dispatch_once(&once, ^{
        arm64 = sizeof(int *) == 8 ;
    });
    return arm64 ;
}

回答3:

Looking at the "arm_neon.h" header file, I can see that it is checking the preprocessor directive __arm64. This is on Xcode 6.1.

In addition to that, some ARM NEON intrinsics available on older ARM (32-bit) architectures are not available on ARM64, or are replaced by equivalents which go by a slightly different name.

In particular, vtbl2 is replaced by vtbl1q, because the underlying architecture has emphasized more on 128-bit NEON registers.

If you have some ARM NEON assembly code that don't compile under ARM64, try look up for changes such as this.