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// Copyright 2016 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package ssa
import "cmd/internal/src"
// trim removes blocks with no code in them.
// These blocks were inserted to remove critical edges.
func trim(f *Func) {
n := 0
for _, b := range f.Blocks {
if !trimmableBlock(b) {
f.Blocks[n] = b
n++
continue
}
bPos := b.Pos
bIsStmt := bPos.IsStmt() == src.PosIsStmt
// Splice b out of the graph. NOTE: `mergePhi` depends on the
// order, in which the predecessors edges are merged here.
p, i := b.Preds[0].b, b.Preds[0].i
s, j := b.Succs[0].b, b.Succs[0].i
ns := len(s.Preds)
p.Succs[i] = Edge{s, j}
s.Preds[j] = Edge{p, i}
for _, e := range b.Preds[1:] {
p, i := e.b, e.i
p.Succs[i] = Edge{s, len(s.Preds)}
s.Preds = append(s.Preds, Edge{p, i})
}
// Attempt to preserve a statement boundary
if bIsStmt {
sawStmt := false
for _, v := range s.Values {
if isPoorStatementOp(v.Op) {
continue
}
if v.Pos.SameFileAndLine(bPos) {
v.Pos = v.Pos.WithIsStmt()
}
sawStmt = true
break
}
if !sawStmt && s.Pos.SameFileAndLine(bPos) {
s.Pos = s.Pos.WithIsStmt()
}
}
// If `s` had more than one predecessor, update its phi-ops to
// account for the merge.
if ns > 1 {
for _, v := range s.Values {
if v.Op == OpPhi {
mergePhi(v, j, b)
}
}
// Remove the phi-ops from `b` if they were merged into the
// phi-ops of `s`.
k := 0
for _, v := range b.Values {
if v.Op == OpPhi {
if v.Uses == 0 {
v.resetArgs()
continue
}
// Pad the arguments of the remaining phi-ops so
// they match the new predecessor count of `s`.
// Since s did not have a Phi op corresponding to
// the phi op in b, the other edges coming into s
// must be loopback edges from s, so v is the right
// argument to v!
args := make([]*Value, len(v.Args))
copy(args, v.Args)
v.resetArgs()
for x := 0; x < j; x++ {
v.AddArg(v)
}
v.AddArg(args[0])
for x := j + 1; x < ns; x++ {
v.AddArg(v)
}
for _, a := range args[1:] {
v.AddArg(a)
}
}
b.Values[k] = v
k++
}
b.Values = b.Values[:k]
}
// Merge the blocks' values.
for _, v := range b.Values {
v.Block = s
}
k := len(b.Values)
m := len(s.Values)
for i := 0; i < k; i++ {
s.Values = append(s.Values, nil)
}
copy(s.Values[k:], s.Values[:m])
copy(s.Values, b.Values)
}
if n < len(f.Blocks) {
f.invalidateCFG()
tail := f.Blocks[n:]
for i := range tail {
tail[i] = nil
}
f.Blocks = f.Blocks[:n]
}
}
// emptyBlock reports whether the block does not contain actual
// instructions
func emptyBlock(b *Block) bool {
for _, v := range b.Values {
if v.Op != OpPhi {
return false
}
}
return true
}
// trimmableBlock reports whether the block can be trimmed from the CFG,
// subject to the following criteria:
// - it should not be the first block
// - it should be BlockPlain
// - it should not loop back to itself
// - it either is the single predecessor of the successor block or
// contains no actual instructions
func trimmableBlock(b *Block) bool {
if b.Kind != BlockPlain || b == b.Func.Entry {
return false
}
s := b.Succs[0].b
return s != b && (len(s.Preds) == 1 || emptyBlock(b))
}
// mergePhi adjusts the number of `v`s arguments to account for merge
// of `b`, which was `i`th predecessor of the `v`s block.
func mergePhi(v *Value, i int, b *Block) {
u := v.Args[i]
if u.Block == b {
if u.Op != OpPhi {
b.Func.Fatalf("value %s is not a phi operation", u.LongString())
}
// If the original block contained u = φ(u0, u1, ..., un) and
// the current phi is
// v = φ(v0, v1, ..., u, ..., vk)
// then the merged phi is
// v = φ(v0, v1, ..., u0, ..., vk, u1, ..., un)
v.SetArg(i, u.Args[0])
v.AddArgs(u.Args[1:]...)
} else {
// If the original block contained u = φ(u0, u1, ..., un) and
// the current phi is
// v = φ(v0, v1, ..., vi, ..., vk)
// i.e. it does not use a value from the predecessor block,
// then the merged phi is
// v = φ(v0, v1, ..., vk, vi, vi, ...)
for j := 1; j < len(b.Preds); j++ {
v.AddArg(v.Args[i])
}
}
}
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