Crate rucc_cost 

What things cost on a target, and every tuning constant the optimizer has.

This is document 40 of spec/optimizer/, made real. The crate exists because of the failure that document opens with: a compiler where each pass has its own idea of what an operation costs is a compiler where two passes undo each other, and neither of them is wrong. Putting the numbers in one place does not make them right, but it makes them one thing that can be measured and changed, instead of thirty things that have to be found first.

What is in here

Cycles and Bytes, which are separate types so that a cost in time is never compared against a threshold in space. Cost, which is a time and a complexity compared lexicographically with an explicit infinity, from GCC’s comp_cost. CostTable, which a target fills in completely or not at all. TuneFlag, which is the half of a cost model that is a boolean rather than a number. And heuristics, which is the file every threshold in every pass has to come from.

Two tables, not one table and a policy

Section 40.3 reads ix86_cur_cost() at gcc/config/i386/i386.h:269 and takes the design from it: optimizing for size is a different cost table, not a weighting applied to the same one. -Os selects the second table and every pass then goes on asking the same questions in the same way. It makes -Os behaviour inspectable as data, and it means no pass has to remember to ask whether it is optimizing for size, which is the sort of thing a pass forgets in exactly one of its five decisions.

What is not in here

Anything derived from a function. Register pressure, block frequency and branch predictability are all things section 40.6 wants computed once per function and shared, and all three need the IR, so they belong with the analyses rather than with the target description. This crate is below the IR on purpose.

Re-exports

pub use cost::Complexity;

pub use cost::Cost;

pub use cycles::Bytes;

pub use cycles::Cycles;

pub use table::AddrMode;

pub use table::Builder;

pub use table::CostTable;

pub use table::Width;

pub use tune::TuneFlag;

pub use tune::Tuning;

Modules

cost

A cost is a pair, and the second half of it is a preference the first half cannot express.

cycles

The two units a cost is measured in, and the reason they are two types.

heuristics

Every threshold any pass consults, with where it came from.

table

The table a target fills in, and the machinery that makes filling it in completely mandatory.

tune

The half of a cost model that is not a number.

x86_64

The two cost tables for x86-64, and what the target answers about tuning.

Enums

Goal

Which of a target’s two tables is wanted.

Traits

TargetCosts

What a pass asks a target about costs, per section 40.12.

Functions

for_arch

The costs for a target, or nothing for one nobody has written a table for.