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Module comments for reset-reuse

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Figuring out what this module was doing, and why, took me a bit less
than half an hour. We should document what's happening for others in the
future so they don't need to follow up on Zulip necessarily.
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ayazhafiz 2022-01-10 20:16:56 -05:00
parent 76d4d5d664
commit e655ab7d3b
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15
compiler/mono/src/reset_reuse.rs
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//! This module inserts reset/reuse statements into the mono IR. These statements provide an
//! opportunity to reduce memory pressure by reusing memory slots of non-shared values. From the
//! introduction of the relevant paper:
//!
//! > [We] have added two additional instructions to our IR: `let y = reset x` and
//! > `let z = (reuse y in ctor_i w)`. The two instructions are used together; if `x`
//! > is a shared value, then `y` is set to a special reference, and the reuse instruction
//! > just allocates a new constructor value `ctor_i w`. If `x` is not shared, then reset
//! > decrements the reference counters of the components of `x`, and `y` is set to `x`.
//! > Then, reuse reuses the memory cell used by `x` to store the constructor value `ctor_i w`.
//!
//! See also
//! - [Counting Immutable Beans](https://arxiv.org/pdf/1908.05647.pdf) (Ullrich and Moura, 2020)
//! - [The lean implementation](https://github.com/leanprover/lean4/blob/master/src/Lean/Compiler/IR/ResetReuse.lean)
use crate::inc_dec::{collect_stmt, occurring_variables_expr, JPLiveVarMap, LiveVarSet};
use crate::ir::{
BranchInfo, Call, Expr, ListLiteralElement, Proc, Stmt, UpdateModeId, UpdateModeIds,