Regression: Inlined loops not fully evaluating with `const`-eable dependencies

[sno2]

I tried this code:

pub fn arr() -> usize {
  let values = [23, 16, 54, 3, 60, 9];
  let mut acc = 0;
  for item in values { 
    acc += item;
  }
  acc
}

I expected to see this happen: The loop should be inlined because values is const-known. For example, here is the generated assembly on Rust 1.63.0 (Godbolt) and Rust Beta (Godbolt):

example::arr:
        mov     eax, 165
        ret

Instead, this happened: On nightly, the loop is inlined. However, the entire function is not computed despite the stable and beta Rust inlining it. Here is the generated assembly on Nightly (Godbolt):

.LCPI0_0:
        .quad   23
        .quad   16
.LCPI0_1:
        .quad   54
        .quad   3
.LCPI0_2:
        .quad   60
        .quad   9
example::arr:
        sub     rsp, 72
        movaps  xmm0, xmmword ptr [rip + .LCPI0_0]
        movaps  xmmword ptr [rsp], xmm0
        movaps  xmm0, xmmword ptr [rip + .LCPI0_1]
        movaps  xmmword ptr [rsp + 16], xmm0
        movaps  xmm0, xmmword ptr [rip + .LCPI0_2]
        movaps  xmmword ptr [rsp + 32], xmm0
        mov     qword ptr [rsp + 56], 6
        xor     ecx, ecx
        xor     edx, edx
        jmp     .LBB0_1
.LBB0_3:
        lea     rdi, [rcx + 1]
        mov     qword ptr [rsp + 48], rdi
        mov     rdx, qword ptr [rsp + 8*rcx]
        mov     esi, 1
        mov     rcx, rdi
        add     rdx, rax
        test    rsi, rsi
        je      .LBB0_5
.LBB0_1:
        mov     rax, rdx
        cmp     rcx, 5
        jbe     .LBB0_3
        xor     esi, esi
        add     rdx, rax
        test    rsi, rsi
        jne     .LBB0_1
.LBB0_5:
        add     rsp, 72
        ret

Meta

n/a - disclosed at usages

Keywords: const inline loop, const loop, const loop not evaluated

Relevant discord thread: https://discord.com/channels/273534239310479360/592856094527848449/1013050670942793808

[Rageking8]

@rustbot label regression-from-stable-to-nightly

[steffahn]

@rustbot label I-slow

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-high t-compiler

[nikic]

Partial fix: llvm/llvm-project@43e7d9a This at least allows the loop to be unrolled, though it only gets fully constant folded in the backend.