Expand description
Global value numbering.
MIR may contain repeated and/or redundant computations. The objective of this pass is to detect such redundancies and re-use the already-computed result when possible.
In a first pass, we compute a symbolic representation of values that are assigned to SSA
locals. This symbolic representation is defined by the Value enum. Each produced instance of
Value is interned as a VnIndex, which allows us to cheaply compute identical values.
From those assignments, we construct a mapping VnIndex -> Vec<(Local, Location)> of available
values, the locals in which they are stored, and the assignment location.
In a second pass, we traverse all (non SSA) assignments x = rvalue and operands. For each
one, we compute the VnIndex of the rvalue. If this VnIndex is associated to a constant, we
replace the rvalue/operand by that constant. Otherwise, if there is an SSA local y
associated to this VnIndex, and if its definition location strictly dominates the assignment
to x, we replace the assignment by x = y.
By opportunity, this pass simplifies some Rvalues based on the accumulated knowledge.
§Operational semantic
Operationally, this pass attempts to prove bitwise equality between locals. Given this MIR:
_a = some value // has VnIndex i
// some MIR
_b = some other value // also has VnIndex iWe consider it to be replacable by:
_a = some value // has VnIndex i
// some MIR
_c = some other value // also has VnIndex i
assume(_a bitwise equal to _c) // follows from having the same VnIndex
_b = _a // follows from the `assume`Which is simplifiable to:
_a = some value // has VnIndex i
// some MIR
_b = _a§Handling of references
We handle references by assigning a different “provenance” index to each Ref/RawPtr rvalue. This ensure that we do not spuriously merge borrows that should not be merged. Meanwhile, we consider all the derefs of an immutable reference to a freeze type to give the same value:
_a = *_b // _b is &Freeze
_c = *_b // replaced by _c = _a§Determinism of constant propagation
When registering a new Value, we attempt to opportunistically evaluate it as a constant.
The evaluated form is inserted in evaluated as an OpTy or None if evaluation failed.
The difficulty is non-deterministic evaluation of MIR constants. Some Const can have
different runtime values each time they are evaluated. This is the case with
Const::Slice which have a new pointer each time they are evaluated, and constants that
contain a fn pointer (AllocId pointing to a GlobalAlloc::Function) pointing to a different
symbol in each codegen unit.
Meanwhile, we want to be able to read indirect constants. For instance:
static A: &'static &'static u8 = &&63;
fn foo() -> u8 {
**A // We want to replace by 63.
}
fn bar() -> u8 {
b"abc"[1] // We want to replace by 'b'.
}The Value::Constant variant stores a possibly unevaluated constant. Evaluating that constant
may be non-deterministic. When that happens, we assign a disambiguator to ensure that we do not
merge the constants. See duplicate_slice test in gvn.rs.
Second, when writing constants in MIR, we do not write Const::Slice or Const
that contain AllocIds.
Structs§
Enums§
- Computing the aggregate’s type can be quite slow, so we only keep the minimal amount of information to reconstruct it when needed.
- Value 🔒