On 9/3/2021 9:05 AM, Andrew MacLeod via Gcc-patches wrote:
On 9/3/21 10:41 AM, Aldy Hernandez wrote:
[Andrew, do you see any problem with using the minus relational code
here? It seems like everything matches up.]
I've seen cases in the upcoming jump threader enhancements where we see
a difference of two pointers that are known to be equivalent, and yet we
fail to return 0 for the range. This is because we have no working
range-op entry for POINTER_DIFF_EXPR. The entry we currently have is
a mere placeholder to avoid ignoring POINTER_DIFF_EXPR's so
adjust_pointer_diff_expr() could get a whack at it here:
// def = __builtin_memchr (arg, 0, sz)
// n = def - arg
//
// The range for N can be narrowed to [0, PTRDIFF_MAX - 1].
In theory... but do the non-equality relations make sense? ie ptr1 <
ptr2 ? Perhaps there is no harm in those.. Probably undefined
behaviour if they are not related so we can do whatever...
I'm pretty sure they're in the realm of undefined behavior if the
pointers point to different objects. There was some code that would
have cared about this, but I think it mostly got cleaned up in response
to Martin S's diagnostic work over the last couple years.
Jeff
