diff options
| -rw-r--r-- | compiler/rustc_mir_dataflow/src/impls/liveness.rs | 31 | ||||
| -rw-r--r-- | src/test/ui/mir-dataflow/liveness-enum.rs | 22 | ||||
| -rw-r--r-- | src/test/ui/mir-dataflow/liveness-enum.stderr | 10 |
3 files changed, 63 insertions, 0 deletions
diff --git a/compiler/rustc_mir_dataflow/src/impls/liveness.rs b/compiler/rustc_mir_dataflow/src/impls/liveness.rs index 0039d3188d5..3e2548845e2 100644 --- a/compiler/rustc_mir_dataflow/src/impls/liveness.rs +++ b/compiler/rustc_mir_dataflow/src/impls/liveness.rs @@ -11,6 +11,37 @@ use crate::{AnalysisDomain, Backward, GenKill, GenKillAnalysis}; /// exist. See [this `mir-dataflow` test][flow-test] for an example. You almost never want to use /// this analysis without also looking at the results of [`MaybeBorrowedLocals`]. /// +/// ## Field-(in)sensitivity +/// +/// As the name suggests, this analysis is field insensitive. If a projection of a variable `x` is +/// assigned to (e.g. `x.0 = 42`), it does not "define" `x` as far as liveness is concerned. In fact, +/// such an assignment is currently marked as a "use" of `x` in an attempt to be maximally +/// conservative. +/// +/// ## Enums and `SetDiscriminant` +/// +/// Assigning a literal value to an `enum` (e.g. `Option<i32>`), does not result in a simple +/// assignment of the form `_1 = /*...*/` in the MIR. For example, the following assignment to `x`: +/// +/// ``` +/// x = Some(4); +/// ``` +/// +/// compiles to this MIR +/// +/// ``` +/// ((_1 as Some).0: i32) = const 4_i32; +/// discriminant(_1) = 1; +/// ``` +/// +/// However, `MaybeLiveLocals` **does** mark `x` (`_1`) as "killed" after a statement like this. +/// That's because it treats the `SetDiscriminant` operation as a definition of `x`, even though +/// the writes that actually initialized the locals happened earlier. +/// +/// This makes `MaybeLiveLocals` unsuitable for certain classes of optimization normally associated +/// with a live variables analysis, notably dead-store elimination. It's a dirty hack, but it works +/// okay for the generator state transform (currently the main consumuer of this analysis). +/// /// [`MaybeBorrowedLocals`]: super::MaybeBorrowedLocals /// [flow-test]: https://github.com/rust-lang/rust/blob/a08c47310c7d49cbdc5d7afb38408ba519967ecd/src/test/ui/mir-dataflow/liveness-ptr.rs /// [liveness]: https://en.wikipedia.org/wiki/Live_variable_analysis diff --git a/src/test/ui/mir-dataflow/liveness-enum.rs b/src/test/ui/mir-dataflow/liveness-enum.rs new file mode 100644 index 00000000000..5eb04ae8c8d --- /dev/null +++ b/src/test/ui/mir-dataflow/liveness-enum.rs @@ -0,0 +1,22 @@ +#![feature(core_intrinsics, rustc_attrs)] + +use std::intrinsics::rustc_peek; + +#[rustc_mir(rustc_peek_liveness, stop_after_dataflow)] +fn foo() -> Option<i32> { + let mut x = None; + + // `x` is live here since it is used in the next statement... + rustc_peek(x); + + dbg!(x); + + // But not here, since it is overwritten below + rustc_peek(x); //~ ERROR rustc_peek: bit not set + + x = Some(4); + + x +} + +fn main() {} diff --git a/src/test/ui/mir-dataflow/liveness-enum.stderr b/src/test/ui/mir-dataflow/liveness-enum.stderr new file mode 100644 index 00000000000..483944d731a --- /dev/null +++ b/src/test/ui/mir-dataflow/liveness-enum.stderr @@ -0,0 +1,10 @@ +error: rustc_peek: bit not set + --> $DIR/liveness-enum.rs:15:5 + | +LL | rustc_peek(x); + | ^^^^^^^^^^^^^ + +error: stop_after_dataflow ended compilation + +error: aborting due to 2 previous errors + |