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package ssa
// Simple block optimisations to simplify the control flow graph.
// TODO(adonovan): instead of creating several "unreachable" blocks
// per function in the Builder, reuse a single one (e.g. at Blocks[1])
// to reduce garbage.
import (
"fmt"
"os"
)
// If true, perform sanity checking and show progress at each
// successive iteration of optimizeBlocks. Very verbose.
const debugBlockOpt = false
func hasPhi(b *BasicBlock) bool {
_, ok := b.Instrs[0].(*Phi)
return ok
}
// prune attempts to prune block b if it is unreachable (i.e. has no
// predecessors other than itself), disconnecting it from the CFG.
// The result is true if the optimisation was applied. i is the block
// index within the function.
//
func prune(f *Function, i int, b *BasicBlock) bool {
if i == 0 {
return false // don't prune entry block
}
if len(b.Preds) == 0 || len(b.Preds) == 1 && b.Preds[0] == b {
// Disconnect it from its successors.
for _, c := range b.Succs {
c.removePred(b)
}
if debugBlockOpt {
fmt.Fprintln(os.Stderr, "prune", b.Name)
}
// Delete b.
f.Blocks[i] = nil
return true
}
return false
}
// jumpThreading attempts to apply simple jump-threading to block b,
// in which a->b->c become a->c if b is just a Jump.
// The result is true if the optimisation was applied.
// i is the block index within the function.
//
func jumpThreading(f *Function, i int, b *BasicBlock) bool {
if i == 0 {
return false // don't apply to entry block
}
if b.Instrs == nil {
fmt.Println("empty block ", b.Name)
return false
}
if _, ok := b.Instrs[0].(*Jump); !ok {
return false // not just a jump
}
c := b.Succs[0]
if c == b {
return false // don't apply to degenerate jump-to-self.
}
if hasPhi(c) {
return false // not sound without more effort
}
for j, a := range b.Preds {
a.replaceSucc(b, c)
// If a now has two edges to c, replace its degenerate If by Jump.
if len(a.Succs) == 2 && a.Succs[0] == c && a.Succs[1] == c {
jump := new(Jump)
jump.SetBlock(a)
a.Instrs[len(a.Instrs)-1] = jump
a.Succs = a.Succs[:1]
c.removePred(b)
} else {
if j == 0 {
c.replacePred(b, a)
} else {
c.Preds = append(c.Preds, a)
}
}
if debugBlockOpt {
fmt.Fprintln(os.Stderr, "jumpThreading", a.Name, b.Name, c.Name)
}
}
f.Blocks[i] = nil
return true
}
// fuseBlocks attempts to apply the block fusion optimisation to block
// a, in which a->b becomes ab if len(a.Succs)==len(b.Preds)==1.
// The result is true if the optimisation was applied.
//
func fuseBlocks(f *Function, a *BasicBlock) bool {
if len(a.Succs) != 1 {
return false
}
b := a.Succs[0]
if len(b.Preds) != 1 {
return false
}
// Eliminate jump at end of A, then copy all of B across.
a.Instrs = append(a.Instrs[:len(a.Instrs)-1], b.Instrs...)
for _, instr := range b.Instrs {
instr.SetBlock(a)
}
// A inherits B's successors
a.Succs = append(a.succs2[:0], b.Succs...)
// Fix up Preds links of all successors of B.
for _, c := range b.Succs {
c.replacePred(b, a)
}
if debugBlockOpt {
fmt.Fprintln(os.Stderr, "fuseBlocks", a.Name, b.Name)
}
// Make b unreachable. Subsequent pruning will reclaim it.
b.Preds = nil
return true
}
// optimizeBlocks() performs some simple block optimizations on a
// completed function: dead block elimination, block fusion, jump
// threading.
//
func optimizeBlocks(f *Function) {
// Loop until no further progress.
changed := true
for changed {
changed = false
if debugBlockOpt {
f.DumpTo(os.Stderr)
MustSanityCheck(f, nil)
}
for i, b := range f.Blocks {
// f.Blocks will temporarily contain nils to indicate
// deleted blocks; we remove them at the end.
if b == nil {
continue
}
// Prune unreachable blocks (including all empty blocks).
if prune(f, i, b) {
changed = true
continue // (b was pruned)
}
// Fuse blocks. b->c becomes bc.
if fuseBlocks(f, b) {
changed = true
}
// a->b->c becomes a->c if b contains only a Jump.
if jumpThreading(f, i, b) {
changed = true
continue // (b was disconnected)
}
}
}
// Eliminate nils from Blocks.
j := 0
for _, b := range f.Blocks {
if b != nil {
f.Blocks[j] = b
j++
}
}
// Nil out b.Blocks[j:] to aid GC.
for i := j; i < len(f.Blocks); i++ {
f.Blocks[i] = nil
}
f.Blocks = f.Blocks[:j]
}
|
