[llvm-bugs] [Bug 35906] New: Comparisons against last enum are optimizable (found via LLVM classof() analysis)

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=35906

            Bug ID: 35906
           Summary: Comparisons against last enum are optimizable (found
                    via LLVM classof() analysis)
           Product: clang
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: LLVM Codegen
          Assignee: unassignedclangbugs at nondot.org
          Reporter: dave at znu.io
                CC: llvm-bugs at lists.llvm.org

In LLVM and derived projects, exhaustive enums are common. For example, from
the casting machinery, an abstract class might have code roughly like this:

static bool classof(Thing *t) {
  return t->getKind() >= ThingKind::First_AbstractFoo &&
         t->getKind() <= ThingKind::Last_AbstractFoo;
}

If ThingKind::Last_AbstractFoo equals the last enumeration, then "t->getKind()
<= ThingKind::Last_AbstractFoo" will always be true. (Similarly, "t->getKind()
> ThingKind::Last_AbstractFoo" will always fail). That being said, clang
generates the IR for the pointless comparison above.

This seems like an easy optimization opportunity.

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