mirror of https://github.com/golang/go.git
324 lines
8.1 KiB
Go
324 lines
8.1 KiB
Go
// Copyright 2016 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package ssa
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// phiopt eliminates boolean Phis based on the previous if.
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//
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// Main use case is to transform:
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// x := false
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// if b {
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// x = true
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// }
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// into x = b.
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//
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// In SSA code this appears as
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//
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// b0
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// If b -> b1 b2
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// b1
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// Plain -> b2
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// b2
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// x = (OpPhi (ConstBool [true]) (ConstBool [false]))
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//
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// In this case we can replace x with a copy of b.
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func phiopt(f *Func) {
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sdom := f.Sdom()
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for _, b := range f.Blocks {
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if len(b.Preds) != 2 || len(b.Values) == 0 {
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// TODO: handle more than 2 predecessors, e.g. a || b || c.
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continue
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}
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pb0, b0 := b, b.Preds[0].b
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for len(b0.Succs) == 1 && len(b0.Preds) == 1 {
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pb0, b0 = b0, b0.Preds[0].b
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}
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if b0.Kind != BlockIf {
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continue
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}
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pb1, b1 := b, b.Preds[1].b
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for len(b1.Succs) == 1 && len(b1.Preds) == 1 {
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pb1, b1 = b1, b1.Preds[0].b
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}
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if b1 != b0 {
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continue
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}
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// b0 is the if block giving the boolean value.
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// reverse is the predecessor from which the truth value comes.
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var reverse int
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if b0.Succs[0].b == pb0 && b0.Succs[1].b == pb1 {
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reverse = 0
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} else if b0.Succs[0].b == pb1 && b0.Succs[1].b == pb0 {
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reverse = 1
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} else {
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b.Fatalf("invalid predecessors\n")
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}
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for _, v := range b.Values {
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if v.Op != OpPhi {
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continue
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}
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// Look for conversions from bool to 0/1.
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if v.Type.IsInteger() {
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phioptint(v, b0, reverse)
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}
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if !v.Type.IsBoolean() {
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continue
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}
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// Replaces
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// if a { x = true } else { x = false } with x = a
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// and
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// if a { x = false } else { x = true } with x = !a
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if v.Args[0].Op == OpConstBool && v.Args[1].Op == OpConstBool {
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if v.Args[reverse].AuxInt != v.Args[1-reverse].AuxInt {
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ops := [2]Op{OpNot, OpCopy}
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v.reset(ops[v.Args[reverse].AuxInt])
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v.AddArg(b0.Controls[0])
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if f.pass.debug > 0 {
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f.Warnl(b.Pos, "converted OpPhi to %v", v.Op)
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}
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continue
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}
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}
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// Replaces
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// if a { x = true } else { x = value } with x = a || value.
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// Requires that value dominates x, meaning that regardless of a,
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// value is always computed. This guarantees that the side effects
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// of value are not seen if a is false.
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if v.Args[reverse].Op == OpConstBool && v.Args[reverse].AuxInt == 1 {
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if tmp := v.Args[1-reverse]; sdom.IsAncestorEq(tmp.Block, b) {
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v.reset(OpOrB)
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v.SetArgs2(b0.Controls[0], tmp)
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if f.pass.debug > 0 {
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f.Warnl(b.Pos, "converted OpPhi to %v", v.Op)
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}
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continue
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}
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}
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// Replaces
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// if a { x = value } else { x = false } with x = a && value.
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// Requires that value dominates x, meaning that regardless of a,
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// value is always computed. This guarantees that the side effects
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// of value are not seen if a is false.
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if v.Args[1-reverse].Op == OpConstBool && v.Args[1-reverse].AuxInt == 0 {
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if tmp := v.Args[reverse]; sdom.IsAncestorEq(tmp.Block, b) {
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v.reset(OpAndB)
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v.SetArgs2(b0.Controls[0], tmp)
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if f.pass.debug > 0 {
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f.Warnl(b.Pos, "converted OpPhi to %v", v.Op)
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}
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continue
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}
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}
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}
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}
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// strengthen phi optimization.
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// Main use case is to transform:
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// x := false
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// if c {
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// x = true
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// ...
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// }
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// into
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// x := c
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// if x { ... }
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//
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// For example, in SSA code a case appears as
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// b0
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// If c -> b, sb0
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// sb0
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// If d -> sd0, sd1
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// sd1
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// ...
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// sd0
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// Plain -> b
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// b
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// x = (OpPhi (ConstBool [true]) (ConstBool [false]))
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//
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// In this case we can also replace x with a copy of c.
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//
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// The optimization idea:
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// 1. block b has a phi value x, x = OpPhi (ConstBool [true]) (ConstBool [false]),
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// and len(b.Preds) is equal to 2.
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// 2. find the common dominator(b0) of the predecessors(pb0, pb1) of block b, and the
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// dominator(b0) is a If block.
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// Special case: one of the predecessors(pb0 or pb1) is the dominator(b0).
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// 3. the successors(sb0, sb1) of the dominator need to dominate the predecessors(pb0, pb1)
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// of block b respectively.
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// 4. replace this boolean Phi based on dominator block.
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//
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// b0(pb0) b0(pb1) b0
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// | \ / | / \
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// | sb1 sb0 | sb0 sb1
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// | ... ... | ... ...
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// | pb1 pb0 | pb0 pb1
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// | / \ | \ /
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// b b b
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//
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var lca *lcaRange
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for _, b := range f.Blocks {
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if len(b.Preds) != 2 || len(b.Values) == 0 {
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// TODO: handle more than 2 predecessors, e.g. a || b || c.
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continue
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}
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for _, v := range b.Values {
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// find a phi value v = OpPhi (ConstBool [true]) (ConstBool [false]).
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// TODO: v = OpPhi (ConstBool [true]) (Arg <bool> {value})
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if v.Op != OpPhi {
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continue
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}
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if v.Args[0].Op != OpConstBool || v.Args[1].Op != OpConstBool {
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continue
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}
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if v.Args[0].AuxInt == v.Args[1].AuxInt {
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continue
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}
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pb0 := b.Preds[0].b
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pb1 := b.Preds[1].b
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if pb0.Kind == BlockIf && pb0 == sdom.Parent(b) {
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// special case: pb0 is the dominator block b0.
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// b0(pb0)
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// | \
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// | sb1
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// | ...
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// | pb1
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// | /
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// b
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// if another successor sb1 of b0(pb0) dominates pb1, do replace.
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ei := b.Preds[0].i
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sb1 := pb0.Succs[1-ei].b
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if sdom.IsAncestorEq(sb1, pb1) {
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convertPhi(pb0, v, ei)
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break
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}
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} else if pb1.Kind == BlockIf && pb1 == sdom.Parent(b) {
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// special case: pb1 is the dominator block b0.
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// b0(pb1)
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// / |
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// sb0 |
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// ... |
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// pb0 |
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// \ |
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// b
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// if another successor sb0 of b0(pb0) dominates pb0, do replace.
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ei := b.Preds[1].i
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sb0 := pb1.Succs[1-ei].b
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if sdom.IsAncestorEq(sb0, pb0) {
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convertPhi(pb1, v, 1-ei)
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break
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}
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} else {
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// b0
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// / \
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// sb0 sb1
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// ... ...
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// pb0 pb1
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// \ /
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// b
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//
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// Build data structure for fast least-common-ancestor queries.
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if lca == nil {
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lca = makeLCArange(f)
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}
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b0 := lca.find(pb0, pb1)
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if b0.Kind != BlockIf {
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break
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}
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sb0 := b0.Succs[0].b
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sb1 := b0.Succs[1].b
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var reverse int
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if sdom.IsAncestorEq(sb0, pb0) && sdom.IsAncestorEq(sb1, pb1) {
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reverse = 0
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} else if sdom.IsAncestorEq(sb1, pb0) && sdom.IsAncestorEq(sb0, pb1) {
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reverse = 1
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} else {
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break
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}
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if len(sb0.Preds) != 1 || len(sb1.Preds) != 1 {
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// we can not replace phi value x in the following case.
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// if gp == nil || sp < lo { x = true}
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// if a || b { x = true }
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// so the if statement can only have one condition.
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break
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}
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convertPhi(b0, v, reverse)
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}
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}
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}
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}
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func phioptint(v *Value, b0 *Block, reverse int) {
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a0 := v.Args[0]
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a1 := v.Args[1]
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if a0.Op != a1.Op {
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return
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}
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switch a0.Op {
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case OpConst8, OpConst16, OpConst32, OpConst64:
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default:
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return
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}
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negate := false
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switch {
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case a0.AuxInt == 0 && a1.AuxInt == 1:
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negate = true
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case a0.AuxInt == 1 && a1.AuxInt == 0:
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default:
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return
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}
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if reverse == 1 {
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negate = !negate
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}
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a := b0.Controls[0]
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if negate {
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a = v.Block.NewValue1(v.Pos, OpNot, a.Type, a)
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}
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v.AddArg(a)
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cvt := v.Block.NewValue1(v.Pos, OpCvtBoolToUint8, v.Block.Func.Config.Types.UInt8, a)
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switch v.Type.Size() {
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case 1:
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v.reset(OpCopy)
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case 2:
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v.reset(OpZeroExt8to16)
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case 4:
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v.reset(OpZeroExt8to32)
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case 8:
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v.reset(OpZeroExt8to64)
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default:
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v.Fatalf("bad int size %d", v.Type.Size())
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}
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v.AddArg(cvt)
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f := b0.Func
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if f.pass.debug > 0 {
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f.Warnl(v.Block.Pos, "converted OpPhi bool -> int%d", v.Type.Size()*8)
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}
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}
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// b is the If block giving the boolean value.
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// v is the phi value v = (OpPhi (ConstBool [true]) (ConstBool [false])).
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// reverse is the predecessor from which the truth value comes.
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func convertPhi(b *Block, v *Value, reverse int) {
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f := b.Func
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ops := [2]Op{OpNot, OpCopy}
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v.reset(ops[v.Args[reverse].AuxInt])
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v.AddArg(b.Controls[0])
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if f.pass.debug > 0 {
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f.Warnl(b.Pos, "converted OpPhi to %v", v.Op)
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}
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}
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