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Auto merge of #116012 - cjgillot:gvn-const, r=oli-obk

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Implement constant propagation on top of MIR SSA analysis

This implements the idea I proposed in https://github.com/rust-lang/rust/pull/110719#issuecomment-1718324700

Based on https://github.com/rust-lang/rust/pull/109597

The value numbering "GVN" pass formulates each rvalue that appears in MIR with an abstract form (the `Value` enum), and assigns an integer `VnIndex` to each. This abstract form can be used to deduplicate values, reusing an earlier local that holds the same value instead of recomputing. This part is proposed in #109597.

From this abstract representation, we can perform more involved simplifications, for example in https://github.com/rust-lang/rust/pull/111344.

With the abstract representation `Value`, we can also attempt to evaluate each to a constant using the interpreter. This builds a `VnIndex -> OpTy` map. From this map, we can opportunistically replace an operand or a rvalue with a constant if their value has an associated `OpTy`.

The most relevant commit is [Evaluated computed values to constants.](2767c4912e)"

r? `@oli-obk`
This commit is contained in:
bors 2023-12-30 03:45:58 +00:00
commit 8d76d07666
209 changed files with 1094 additions and 1587 deletions
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5
compiler/rustc_infer/src/infer/relate/generalize.rs
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@ -1,6 +1,7 @@
use std::mem;
use rustc_data_structures::sso::SsoHashMap;
use rustc_data_structures::stack::ensure_sufficient_stack;
use rustc_hir::def_id::DefId;
use rustc_middle::infer::unify_key::{ConstVarValue, ConstVariableValue};
use rustc_middle::ty::error::TypeError;
@ -215,7 +216,9 @@ where
let old_ambient_variance = self.ambient_variance;
self.ambient_variance = self.ambient_variance.xform(variance);
debug!(?self.ambient_variance, "new ambient variance");
let r = self.relate(a, b)?;
// Recursive calls to `relate` can overflow the stack. For example a deeper version of
// `ui/associated-consts/issue-93775.rs`.
let r = ensure_sufficient_stack(|| self.relate(a, b))?;
self.ambient_variance = old_ambient_variance;
Ok(r)
}