[Bug tree-optimization/93917] New: VRP forgets range of value read from memory

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93917

            Bug ID: 93917
           Summary: VRP forgets range of value read from memory
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: glisse at gcc dot gnu.org
  Target Milestone: ---

First a case that works:

void f(int n){
  if(n<0)__builtin_unreachable();
}

EVRP assigns a range to n, and VRP1 folds the comparison to false.

Let's add an indirection:

void f(int*n){
  if(*n<0)__builtin_unreachable();
}

EVRP still assigns a range to *n, but now VRP1 seems to forget that range, does
not fold the comparison, and reassigns a range to *n. And again in VRP2.
__builtin_unreachable survives all the way to FAB, which in my original code
prevents any loop optimization. I know of plans to fold __builtin_unreachable
earlier always, which would likely solve the issue, but independently of that,
it seems that VRP is dropping some interesting information present in old
ranges, possibly because a memory read is not considered an interesting
statement.