The panic branch of Some(x).unwrap() is not removed for some types

[saethlin]

Demos: https://godbolt.org/z/TMh3Krfc5

The motivating example is:

pub fn bad_vec(s: Vec<u8>) -> Vec<u8> {
    Some(s).expect("bad_vec")
}

example::bad_myvec:
        push    rax
        mov     rax, qword ptr [rsi]
        test    rax, rax
        je      .LBB5_1
        mov     qword ptr [rdi], rax
        movups  xmm0, xmmword ptr [rsi + 8]
        movups  xmmword ptr [rdi + 8], xmm0
        mov     rax, rdi
        pop     rcx
        ret
.LBB5_1:
        lea     rdi, [rip + .L__unnamed_5]
        lea     rdx, [rip + .L__unnamed_6]
        mov     esi, 9
        call    qword ptr [rip + core::option::expect_failed@GOTPCREL]
        ud2

It looks to me like the problem with Vec is that specifically the nonnull attribute does not survive being wrapped in two structs that each contain a second member. If either of RawVec or Vec is missing a usize member, the optimization works.

There is a missed NonZero* optimization in here that I found looking for types that fail to optimize. It's unclear to me if this was reported previously in #49572, or if whatever fixes the problem with Vec will also fix the NonZero* example.

---

I think this is similar but not the same problem as seen in #71257

[scottmcm]

You might be interested in #85646 (which came from #85133), which tried to make some progress here and found some LLVM issues with matching on 2-variant enums (like the Option here).

nox was working on some codegen improvements that might help at some point: https://rust-lang.zulipchat.com/#narrow/stream/189540-t-compiler.2Fwg-mir-opt/topic/Discriminant.20computation.20yields.20bad.20output/near/264996641

[saethlin]

🤔 this is fixed by DataflowConstProp: https://godbolt.org/z/zEKcobr6q

[cjgillot]

The motivating example is handled by #109597
Test:

rust/tests/mir-opt/gvn.rs

Lines 148 to 157 in 5a345b3

	fn wrap_unwrap<T: Copy>(x: T) -> T {
	// CHECK-LABEL: fn wrap_unwrap(
	// CHECK: [[some:_.*]] = Option::<T>::Some(_1);
	// CHECK: switchInt(const 1_isize)
	// CHECK: _0 = _1;
	match Some(x) {
	Some(y) => y,
	None => panic!(),
	}
	}