compiler

Expression Evaluation

Currently builder.eval always creates a new variable. When the expression is a constant or a single variable, using builder.eval to evaluate an expression causes an unnecessary variable assignment, which impacts performance significantly.

The naming of builder.eval is semantically incorrect because creating a new variable is out of scope of "eval". However, the assumption is taken too widely to fix. So we introduce a new method builder.eval_expr for the real "evaluation".

builder.eval_expr returns a right value, which is either a constant or a variable. An instruction can use a right value for reading a value directly.

Currently builder.eval_expr only supports SymbolicVar.

RVar

RVar represents the right value of SymbolicVar.

CR Variable

To unify static/dynamic programs in eDSL, we introduce a new concept, CR variable("C" for compile time and "R" for runtime). A CR variable behaves like a normal variable in eDSL(e.g. it supports assignment), but it could be either a constant or a variable in runtime.

Currently, there are 2 types of CR variables:

Usize

Usize could be a constant native field or a Var at right time.

Array

Array has 2 variants: Fixed and Dyn.

Fixed is a logical array only exists in compile time.

• A static program can only use Fixed to represent an array.
• Fixed only supports constant index access.
• Try to use Fixed if possible because most of its operations don't cost instructions.

Dyn is an array on heap.

• A static program cannot use Dyn.
• When initializing a Dyn, the length could be either a constant or a variable.
• The length of Dyn is fixed after initialization.
• Dyn supports dynamic index access.

Branches

In static programs, only constant branches are allowed.

Loops

Constant Loops

When both start and end of a loop are constant, the loop is a constant loop. The loop body will be unrolled. This optimization saves 1 instruction per iteration.

In static programs, only constant loops are allowed.

Break Support

!!Attention!!: Break support for constant loops is not perfect. It brings some restrictions which require developers' awareness.:

• If you want to use break in a possibly constant loop, you need to use .for_each_may_break instead of .for_each.
• If you want to use break in a branch inside a loop, you need to use .then_may_break/.then_or_else_may_break instead of .for_each/.then_or_else.
• Inside a constant loop, you cannot use a non-constant branch to break.