go/src/cmd/compile/internal/ssa/shortcircuit.go

// shortcircuit finds situations where branch directions
// are always correlated and rewrites the CFG to take
// advantage of that fact.
// This optimization is useful for compiling && and || expressions.
func shortcircuit(f *Func) { … }

// shortcircuitBlock checks for a CFG in which an If block
// has as its control value a Phi that has a ConstBool arg.
// In some such cases, we can rewrite the CFG into a flatter form.
//
// (1) Look for a CFG of the form
//
//	p   other pred(s)
//	 \ /
//	  b
//	 / \
//	t   other succ
//
// in which b is an If block containing a single phi value with a single use (b's Control),
// which has a ConstBool arg.
// p is the predecessor corresponding to the argument slot in which the ConstBool is found.
// t is the successor corresponding to the value of the ConstBool arg.
//
// Rewrite this into
//
//	p   other pred(s)
//	|  /
//	| b
//	|/ \
//	t   u
//
// and remove the appropriate phi arg(s).
//
// (2) Look for a CFG of the form
//
//	p   q
//	 \ /
//	  b
//	 / \
//	t   u
//
// in which b is as described in (1).
// However, b may also contain other phi values.
// The CFG will be modified as described in (1).
// However, in order to handle those other phi values,
// for each other phi value w, we must be able to eliminate w from b.
// We can do that though a combination of moving w to a different block
// and rewriting uses of w to use a different value instead.
// See shortcircuitPhiPlan for details.
func shortcircuitBlock(b *Block) bool { … }

// shortcircuitPhiPlan returns a function to handle non-ctl phi values in b,
// where b is as described in shortcircuitBlock.
// The returned function accepts a value v
// and the index i of v in v.Block: v.Block.Values[i] == v.
// If the returned function moves v to a different block, it will use v.moveTo.
// cidx is the index in ctl of the ConstBool arg.
// ti is the index in b.Succs of the always taken branch when arriving from p.
// If shortcircuitPhiPlan returns nil, there is no plan available,
// and the CFG modifications must not proceed.
// The returned function assumes that shortcircuitBlock has completed its CFG modifications.
func shortcircuitPhiPlan(b *Block, ctl *Value, cidx int, ti int64) func(*Value, int) { … }

// replaceUses replaces all uses of old in b with new.
func (b *Block) replaceUses(old, new *Value) { … }

// moveTo moves v to dst, adjusting the appropriate Block.Values slices.
// The caller is responsible for ensuring that this is safe.
// i is the index of v in v.Block.Values.
func (v *Value) moveTo(dst *Block, i int) { … }