Passing enum by-value loses range metadata

[huonw]

Test case update for Rust 1.0:

pub enum Foo {
    A, B, C, D
}

static X: [isize; 4] = [10, 1, 4, 3];

pub fn foo(y: Foo) -> isize {
    X[y as usize]
}

Original report below:

---

pub enum Foo {
    A, B, C, D
}

static X: [int, .. 4] = [10, 1, 4, 3];
pub fn foo(y: Foo) -> int {
    X[y as uint]
}

becomes

; Function Attrs: uwtable
define i64 @_ZN3foo20h19859df9e3d3b0bcsaa4v0.0E(i8) unnamed_addr #0 {
entry-block:
  %1 = zext i8 %0 to i64
  %2 = icmp ugt i8 %0, 3
  br i1 %2, label %cond, label %next, !prof !0

next:                                             ; preds = %entry-block
  %3 = getelementptr inbounds [4 x i64]* @_ZN1X20h61a81bb0de98e867iaa4v0.0E, i64 0, i64 %1
  %4 = load i64* %3, align 8
  ret i64 %4

cond:                                             ; preds = %entry-block
  tail call void @_ZN2rt6unwind17fail_bounds_check20h0f41a1608cc59c5e1199v0.11.preE(i8* getelementptr inbounds ([14 x i8]* @str879, i64 0, i64 0), i64 7, i64 %1, i64 4)
  unreachable
}

even though it's impossible for a Foo to be larger than 3, so the bounds checking can never fail. (Doing a similar thing with e.g. [int, .. 256] and a value of type u8 does eliminate the bounds checks.)

(I guess this is an LLVM bug, but filing it here just in case we're emiting range info incorrectly, or some-such.)

[huonw]

Oh, actually: this is because there is no range metadata when Foo is passed by value: passing it by reference fn foo(x: &Foo) means a load with metadata is emitted and the bounds check is eliminated:

define i64 @_ZN3foo20h043091a9d9208cb0saa4v0.0E(i8* nocapture readonly) unnamed_addr #0 {
entry-block:
  %1 = load i8* %0, align 1, !range !0
  %2 = zext i8 %1 to i64
  %3 = getelementptr inbounds [4 x i64]* @_ZN1X20hcca41332b11ffec7iaa4v0.0E, i64 0, i64 %2
  %4 = load i64* %3, align 8
  ret i64 %4
}

!0 = metadata !{i8 0, i8 4}

Updated title from "indexing fixed-length array doesn't eliminated bounds checks".

[thestinger]

Fixing this would likely require extending the feature in LLVM to be usable on parameters.

[ranma42]

I agree that the !range metadata annotation would be convenient also on values other than those coming from load/call/invoke, but wouldn't it be sufficient to use the llvm.assume intrinsic to specify the constraints on values (in this case, its range)?

[ranma42]

If the enumeration derives Copy and Clone, this seems to be a way to code the function so that the LLVM optimisations generate the desired code:

pub fn foo(y: Foo) -> isize {
    let closure = |z: &Foo| { X[(*z) as usize] };
    closure(&y)
}

; Function Attrs: nounwind readnone uwtable
define i64 @_ZN3foo20h8ac558f51be88dd53aaE(i8) unnamed_addr #1 {
entry-block:
  %1 = zext i8 %0 to i64
  %2 = getelementptr inbounds [4 x i64], [4 x i64]* @_ZN1X20h5e497aaeb62c0700UaaE, i64 0, i64 %1
  %3 = load i64, i64* %2, align 8, !noalias !1
  ret i64 %3
}

This might be fragile as it probably relies on the order in which the LLVM optimiser performs the bound-check optimisation and the closure inlining.

[brson]

Can this be reproed today?

[arielb1]

Sure. Same LLVM problem. I fixed the y as uint case in #36962. The underlying issue still exists.