loopvar

Documentation

Overview

Package loopvar applies the proper variable capture, according to experiment, flags, language version, etc.

Index

• func LogTransformations(transformed []VarAndLoop)
• type VarAndLoop
  • func ForCapture(fn *ir.Func) []VarAndLoop

Functions

func LogTransformations

func LogTransformations(transformed []VarAndLoop)

Types

type VarAndLoop

type VarAndLoop struct {
	Name    *ir.Name
	Loop    ir.Node  // the *ir.RangeStmt or *ir.ForStmt. Used for identity and position
	LastPos src.XPos // the last position observed within Loop
}

func ForCapture

func ForCapture(fn *ir.Func) []VarAndLoop

ForCapture transforms for and range loops that declare variables that might be captured by a closure or escaped to the heap, using a syntactic check that conservatively overestimates the loops where capture occurs, but still avoids transforming the (large) majority of loops. It returns the list of names subject to this change, that may (once transformed) be heap allocated in the process. (This allows checking after escape analysis to call out any such variables, in case it causes allocation/performance problems).

The decision to transform loops is normally encoded in the For/Range loop node field DistinctVars but is also dependent on base.LoopVarHash, and some values of base.Debug.LoopVar (which is set per-package). Decisions encoded in DistinctVars are preserved across inlining, so if package a calls b.F and loops in b.F are transformed, then they are always transformed, whether b.F is inlined or not.

Per-package, the debug flag settings that affect this transformer:

base.LoopVarHash != nil => use hash setting to govern transformation. note that LoopVarHash != nil sets base.Debug.LoopVar to 1 (unless it is >= 11, for testing/debugging).

base.Debug.LoopVar == 11 => transform ALL loops ignoring syntactic/potential escape. Do not log, can be in addition to GOEXPERIMENT.

The effect of GOEXPERIMENT=loopvar is to change the default value (0) of base.Debug.LoopVar to 1 for all packages.