vm: Fix address hints of 0 with MAP_32BIT

[alc]

Summary

Also, I want to rename min_addr to default_addr, which I contend better reflects what it represents. The min_addr is not a minimum address in the same way that max_addr is actually a maximum address that can be allocated. For example, a non-zero hint can be less than min_addr and be allocated.

[markj]

sys/vm/vm_mmap.c

1638

Should we try to avoid the unnecessary calculation when MAP_32BIT is set?

#ifdef MAP_32BIT
if ((flags & MAP_32BIT) != 0)
    default_addr = 0;
else
#endif
    default_addr = round_page(...

[alc]

sys/vm/vm_mmap.c

1638

My assumption is that MAP_32BIT is rarely used. Handling MAP_32BIT in this way leads the compiler to generate a conditional move instruction rather than a conditional branch.

[dchagin]

Thank you for fixing this

sys/vm/vm_mmap.c

1633

btw, Is this cast really necessary?

[alc]

sys/vm/vm_mmap.c

1633

Yes, it is. vm_daddr_t is a caddr_t. The implementation of round_page() is machine-dependent. On some architectures, it internally casts its argument to an unsigned long. On the other architectures, where it doesn't have that internal cast, there will be a compilation error.