enums with disjoint ranges should emit more precise `llvm.range` metadata

[Kmeakin]

I tried this code:
https://godbolt.org/z/4KKGvWs8d

#[derive(Copy, Clone)]
pub enum Foo {
    Foo1 = 1,
    Foo2 = 2,
    Foo4 = 4,
    Foo8 = 8,
}

#[no_mangle]
pub fn load(foo: &Foo) -> u32 {
    *foo as u32
}

I expected to see this happen: The metadata attatched to the load should specify that the loaded value is either 1, 2, 4 or 8:

define noundef range(i32 1, 8) i32 @load(ptr noalias nocapture noundef readonly align 1 dereferenceable(1) %foo) unnamed_addr #0 {
  %0 = load i8, ptr %foo, align 1, !range !2, !noundef !3
  %_0 = zext nneg i8 %0 to i32
  ret i32 %_0
}

!2 = !{
  i8 1, i8 3, ; 1 or 2
  i8 4, i8 5, ; 4
  i8 8, i8 9, ; 8
 }

Instead, this happened: the range metadata gives a still correct, but less accurate range of [1,9):

define noundef range(i32 1, 8) i32 @load(ptr noalias nocapture noundef readonly align 1 dereferenceable(1) %foo) unnamed_addr #0 {
  %0 = load i8, ptr %foo, align 1, !range !2, !noundef !3
  %_0 = zext nneg i8 %0 to i32
  ret i32 %_0
}

!2 = !{i8 1, i8 9}

Similarly, the range attribute for the return value of load (and the range attribute for a value of type Foo if it is passed by value) is also less precise than it could be. But it appears from reading the LLVM LangRef that range attributes can only specify a single range, rather than a union of disjoint ranges like llvm.range metadata can.

Meta

rustc --version --verbose:

rustc 1.85.0-nightly (5e1440ae5 2024-12-01)
binary: rustc
commit-hash: 5e1440ae514d98ddfcbf1607acb64d41e07ef616
commit-date: 2024-12-01
host: x86_64-unknown-linux-gnu
release: 1.85.0-nightly
LLVM version: 19.1.4

Backtrace

<backtrace>

[Kmeakin]

@rustbot label +I-slow

[dianqk]

LLVM supports multiple ranges, but such metadata is usually merged in to one range by LLVM's passes. :(