On 5/31/2016 4:04 PM, Yury Norov wrote:
Hi Chris,In path a63c7fa18a (Add sysdeps/unix/sysv/linux/generic/.) you add this: +++ b/sysdeps/unix/sysv/linux/generic/wordsize-32/preadv.c [...] +static ssize_t +do_preadv (int fd, const struct iovec *vector, int count, off_t offset) +{ + assert (sizeof (offset) == 4); + return INLINE_SYSCALL (preadv, __ALIGNMENT_COUNT (5, 6), fd, + vector, count, __ALIGNMENT_ARG + __LONG_LONG_PAIR (offset >> 31, offset)); +} + And this is the code that is picked up if I choose wordsize-32 for my AARCH64/ILP32. So I have questions. 1. What is the assert for? We agreed that all new ABIs will be 64-bit off_t only. I fixed it internally like this: +#ifndef __OFF_T_MATCHES_OFF64_T assert (sizeof (offset) == 4); +#endif There is a bunch of similar assertions in glibc. 2. This one looks weird: __LONG_LONG_PAIR (offset >> 31, offset)) Why 31-bit offset? And why you don't mask 2nd argument? Later in your patch I see this: +static ssize_t +do_preadv64 (int fd, const struct iovec *vector, int count, off64_t offset) +{ + return INLINE_SYSCALL (preadv, __ALIGNMENT_COUNT (5, 6), fd, + vector, count, __ALIGNMENT_ARG + __LONG_LONG_PAIR ((off_t) (offset >> 32), + (off_t) (offset & 0xffffffff))); +} And it looks correct to me. If 1st version is correct as well, I think it should be commented.
I did this work before x32 came out, so I tried to model it more closely on the existing x86 compat API. I agree that a 64-bit off_t model seems reasonable; however, the code does exactly what it does to match x86, namely preadv() takes a 32-bit offset, and preadv64() take a 64-bit offset. The assert() in preadv to force sizeof to be 4 is exactly why in that routine we use (offset >> 31, offset). Since we know offset fits in 32 bits, all we need to do is properly sign-extend it into 64 bits in the high register of the pair, which is what (offset >> 31) does - you end up with only 0 or -1, thus sign-extending the 32-bit signed off_t. Then in preadv64() we actually need to break apart the 64-bit offset into a high 32 bits and a low 32 bits, which is what (offset >> 32, offset & 0xffffffff) does. For a 64-bit off_t you will want to not compile preadv.c at all, and instead make __libc_preadv() and friends be aliases of __libc_preadv64(). -- Chris Metcalf, Mellanox Technologies http://www.mellanox.com
