charon_lib/transform/remove_read_discriminant.rs
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//! The MIR code often contains variables with type `Never`, and we want to get
//! rid of those. We proceed in two steps. First, we remove the instructions
//! `drop(v)` where `v` has type `Never` (it can happen - this module does the
//! filtering). Then, we filter the unused variables ([crate::remove_unused_locals]).
use crate::errors::register_error_or_panic;
use crate::formatter::IntoFormatter;
use crate::llbc_ast::*;
use crate::pretty::FmtWithCtx;
use crate::transform::TransformCtx;
use derive_visitor::visitor_enter_fn_mut;
use derive_visitor::DriveMut;
use itertools::Itertools;
use std::collections::{HashMap, HashSet};
use super::ctx::LlbcPass;
pub struct Transform;
impl Transform {
fn update_block(ctx: &mut TransformCtx<'_>, block: &mut Block) {
// Iterate through the statements.
for i in 0..block.statements.len() {
let suffix = &mut block.statements[i..];
if let [Statement {
content: RawStatement::Assign(dest, Rvalue::Discriminant(p, adt_id)),
span: span1,
..
}, rest @ ..] = suffix
{
// The destination should be a variable
assert!(dest.is_local());
// Lookup the type of the scrutinee
let variants = match ctx.translated.type_decls.get(*adt_id) {
Some(TypeDecl {
kind: TypeDeclKind::Enum(variants),
..
}) => Some(variants),
// This can happen if the type was declared as invisible or opaque.
None
| Some(TypeDecl {
kind: TypeDeclKind::Opaque,
..
}) => {
let name = ctx.translated.item_name(*adt_id).unwrap();
let msg = format!(
"reading the discriminant of an opaque enum. \
Add `--include {}` to the `charon` arguments \
to translate this enum.",
name.fmt_with_ctx(&ctx.into_fmt())
);
register_error_or_panic!(ctx, block.span, msg);
None
}
Some(TypeDecl {
kind:
TypeDeclKind::Struct(..) | TypeDeclKind::Union(..) | TypeDeclKind::Alias(..),
..
}) => {
register_error_or_panic!(
ctx,
block.span,
"reading the discriminant of a non-enum type"
);
None
}
Some(TypeDecl {
kind: TypeDeclKind::Error(..),
..
}) => None,
};
let Some(variants) = variants else {
// An error occurred. We can't keep the `Rvalue::Discriminant` around so we
// `Nop` the discriminant read.
block.statements[i].content = RawStatement::Nop;
return;
};
// We look for a `SwitchInt` just after the discriminant read.
match rest {
[Statement {
content:
RawStatement::Switch(switch @ Switch::SwitchInt(Operand::Move(_), ..)),
..
}, ..] => {
// Convert between discriminants and variant indices. Remark: the discriminant can
// be of any *signed* integer type (`isize`, `i8`, etc.).
let discr_to_id: HashMap<ScalarValue, VariantId> = variants
.iter_indexed_values()
.map(|(id, variant)| (variant.discriminant, id))
.collect();
take_mut::take(switch, |switch| {
let (Operand::Move(op_p), _, targets, otherwise) =
switch.to_switch_int().unwrap()
else {
unreachable!()
};
assert!(op_p.is_local() && op_p.var_id() == dest.var_id());
let mut covered_discriminants: HashSet<ScalarValue> =
HashSet::default();
let targets = targets
.into_iter()
.map(|(v, e)| {
(
v.into_iter()
.filter_map(|discr| {
covered_discriminants.insert(discr);
discr_to_id.get(&discr).or_else(|| {
register_error_or_panic!(
ctx,
block.span,
"Found incorrect discriminant {discr} for enum {adt_id}"
);
None
})
})
.copied()
.collect_vec(),
e,
)
})
.collect_vec();
// Filter the otherwise branch if it is not necessary.
let covers_all = covered_discriminants.len() == discr_to_id.len();
let otherwise = if covers_all { None } else { Some(otherwise) };
// Replace the old switch with a match.
Switch::Match(p.clone(), targets, otherwise)
});
// `Nop` the discriminant read.
block.statements[i].content = RawStatement::Nop;
}
_ => {
// The discriminant read is not followed by a `SwitchInt`. This can happen
// in optimized MIR. We replace `_x = Discr(_y)` with `match _y { 0 => { _x
// = 0 }, 1 => { _x = 1; }, .. }`.
let targets = variants
.iter_indexed_values()
.map(|(id, variant)| {
let discr_value =
Rvalue::Use(Operand::Const(variant.discriminant.to_constant()));
let statement = Statement::new(
*span1,
RawStatement::Assign(dest.clone(), discr_value),
);
(vec![id], statement.into_block())
})
.collect();
block.statements[i].content =
RawStatement::Switch(Switch::Match(p.clone(), targets, None))
}
}
}
}
}
}
impl LlbcPass for Transform {
fn transform_body(&self, ctx: &mut TransformCtx<'_>, b: &mut ExprBody) {
b.body
.drive_mut(&mut visitor_enter_fn_mut(|block: &mut Block| {
Transform::update_block(ctx, block);
}));
}
}