From d6490fa51090183f780b6ccb3e8ff1aaf0b4431c Mon Sep 17 00:00:00 2001 From: Alan Donovan Date: Thu, 5 Dec 2013 17:00:17 -0500 Subject: [PATCH] go.tools/ssa/ssautil: Switches() utility infers multiway branches. (Note new subpackage ssa/ssautil.) R=gri, axwalk, gri CC=golang-dev https://golang.org/cl/36710043 --- ssa/ssautil/switch.go | 234 +++++++++++++++++++++ ssa/ssautil/switch_test.go | 69 +++++++ ssa/ssautil/testdata/switches.go | 339 +++++++++++++++++++++++++++++++ 3 files changed, 642 insertions(+) create mode 100644 ssa/ssautil/switch.go create mode 100644 ssa/ssautil/switch_test.go create mode 100644 ssa/ssautil/testdata/switches.go diff --git a/ssa/ssautil/switch.go b/ssa/ssautil/switch.go new file mode 100644 index 00000000..4af2da91 --- /dev/null +++ b/ssa/ssautil/switch.go @@ -0,0 +1,234 @@ +// Copyright 2013 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 ssautil + +// This file implements discovery of switch and type-switch constructs +// from low-level control flow. +// +// Many techniques exist for compiling a high-level switch with +// constant cases to efficient machine code. The optimal choice will +// depend on the data type, the specific case values, the code in the +// body of each case, and the hardware. +// Some examples: +// - a lookup table (for a switch that maps constants to constants) +// - a computed goto +// - a binary tree +// - a perfect hash +// - a two-level switch (to partition constant strings by their first byte). + +import ( + "bytes" + "fmt" + "go/token" + + "code.google.com/p/go.tools/go/types" + "code.google.com/p/go.tools/ssa" +) + +// A ConstCase represents a single constant comparison. +// It is part of a Switch. +type ConstCase struct { + Block *ssa.BasicBlock // block performing the comparison + Body *ssa.BasicBlock // body of the case + Value *ssa.Const // case comparand +} + +// A TypeCase represents a single type assertion. +// It is part of a Switch. +type TypeCase struct { + Block *ssa.BasicBlock // block performing the type assert + Body *ssa.BasicBlock // body of the case + Type types.Type // case type + Binding ssa.Value // value bound by this case +} + +// A Switch is a logical high-level control flow operation +// (a multiway branch) discovered by analysis of a CFG containing +// only if/else chains. It is not part of the ssa.Instruction set. +// +// One of ConstCases and TypeCases has length >= 2; +// the other is nil. +// +// In a value switch, the list of cases may contain duplicate constants. +// A type switch may contain duplicate types, or types assignable +// to an interface type also in the list. +// TODO(adonovan): eliminate such duplicates. +// +type Switch struct { + Start *ssa.BasicBlock // block containing start of if/else chain + X ssa.Value // the switch operand + ConstCases []ConstCase // ordered list of constant comparisons + TypeCases []TypeCase // ordered list of type assertions + Default *ssa.BasicBlock // successor if all comparisons fail +} + +func (sw *Switch) String() string { + // We represent each block by the String() of its + // first Instruction, e.g. "print(42:int)". + var buf bytes.Buffer + if sw.ConstCases != nil { + fmt.Fprintf(&buf, "switch %s {\n", sw.X.Name()) + for _, c := range sw.ConstCases { + fmt.Fprintf(&buf, "case %s: %s\n", c.Value, c.Body.Instrs[0]) + } + } else { + fmt.Fprintf(&buf, "switch %s.(type) {\n", sw.X.Name()) + for _, c := range sw.TypeCases { + fmt.Fprintf(&buf, "case %s %s: %s\n", + c.Binding.Name(), c.Type, c.Body.Instrs[0]) + } + } + if sw.Default != nil { + fmt.Fprintf(&buf, "default: %s\n", sw.Default.Instrs[0]) + } + fmt.Fprintf(&buf, "}") + return buf.String() +} + +// Switches examines the control-flow graph of fn and returns the +// set of inferred value and type switches. A value switch tests an +// ssa.Value for equality against two or more compile-time constant +// values. Switches involving link-time constants (addresses) are +// ignored. A type switch type-asserts an ssa.Value against two or +// more types. +// +// The switches are returned in dominance order. +// +// The resulting switches do not necessarily correspond to uses of the +// 'switch' keyword in the source: for example, a single source-level +// switch statement with non-constant cases may result in zero, one or +// many Switches, one per plural sequence of constant cases. +// Switches may even be inferred from if/else- or goto-based control flow. +// (In general, the control flow constructs of the source program +// cannot be faithfully reproduced from the SSA representation.) +// +func Switches(fn *ssa.Function) []Switch { + // Traverse the CFG in dominance order, so we don't + // enter an if/else-chain in the middle. + var switches []Switch + seen := make(map[*ssa.BasicBlock]bool) // TODO(adonovan): opt: use ssa.blockSet + for _, b := range fn.DomPreorder() { + if x, k := isComparisonBlock(b); x != nil { + // Block b starts a switch. + sw := Switch{Start: b, X: x} + valueSwitch(&sw, k, seen) + if len(sw.ConstCases) > 1 { + switches = append(switches, sw) + } + } + + if y, x, T := isTypeAssertBlock(b); y != nil { + // Block b starts a type switch. + sw := Switch{Start: b, X: x} + typeSwitch(&sw, y, T, seen) + if len(sw.TypeCases) > 1 { + switches = append(switches, sw) + } + } + } + return switches +} + +func valueSwitch(sw *Switch, k *ssa.Const, seen map[*ssa.BasicBlock]bool) { + b := sw.Start + x := sw.X + for x == sw.X { + if seen[b] { + break + } + seen[b] = true + + sw.ConstCases = append(sw.ConstCases, ConstCase{ + Block: b, + Body: b.Succs[0], + Value: k, + }) + b = b.Succs[1] + if len(b.Instrs) > 2 { + // Block b contains not just 'if x == k', + // so it may have side effects that + // make it unsafe to elide. + break + } + if len(b.Preds) != 1 { + // Block b has multiple predecessors, + // so it cannot be treated as a case. + break + } + x, k = isComparisonBlock(b) + } + sw.Default = b +} + +func typeSwitch(sw *Switch, y ssa.Value, T types.Type, seen map[*ssa.BasicBlock]bool) { + b := sw.Start + x := sw.X + for x == sw.X { + if seen[b] { + break + } + seen[b] = true + + sw.TypeCases = append(sw.TypeCases, TypeCase{ + Block: b, + Body: b.Succs[0], + Type: T, + Binding: y, + }) + b = b.Succs[1] + if len(b.Instrs) > 4 { + // Block b contains not just + // {TypeAssert; Extract #0; Extract #1; If} + // so it may have side effects that + // make it unsafe to elide. + break + } + if len(b.Preds) != 1 { + // Block b has multiple predecessors, + // so it cannot be treated as a case. + break + } + y, x, T = isTypeAssertBlock(b) + } + sw.Default = b +} + +// isComparisonBlock returns the operands (v, k) if a block ends with +// a comparison v==k, where k is a compile-time constant. +// +func isComparisonBlock(b *ssa.BasicBlock) (v ssa.Value, k *ssa.Const) { + if n := len(b.Instrs); n >= 2 { + if i, ok := b.Instrs[n-1].(*ssa.If); ok { + if binop, ok := i.Cond.(*ssa.BinOp); ok && binop.Block() == b && binop.Op == token.EQL { + if k, ok := binop.Y.(*ssa.Const); ok { + return binop.X, k + } + if k, ok := binop.X.(*ssa.Const); ok { + return binop.Y, k + } + } + } + } + return +} + +// isTypeAssertBlock returns the operands (y, x, T) if a block ends with +// a type assertion "if y, ok := x.(T); ok {". +// +func isTypeAssertBlock(b *ssa.BasicBlock) (y, x ssa.Value, T types.Type) { + if n := len(b.Instrs); n >= 4 { + if i, ok := b.Instrs[n-1].(*ssa.If); ok { + if ext1, ok := i.Cond.(*ssa.Extract); ok && ext1.Block() == b && ext1.Index == 1 { + if ta, ok := ext1.Tuple.(*ssa.TypeAssert); ok && ta.Block() == b { + // hack: relies upon instruction ordering. + if ext0, ok := b.Instrs[n-3].(*ssa.Extract); ok { + return ext0, ta.X, ta.AssertedType + } + } + } + } + } + return +} diff --git a/ssa/ssautil/switch_test.go b/ssa/ssautil/switch_test.go new file mode 100644 index 00000000..4e5119a9 --- /dev/null +++ b/ssa/ssautil/switch_test.go @@ -0,0 +1,69 @@ +// Copyright 2013 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 ssautil_test + +import ( + "go/parser" + "strings" + "testing" + + "code.google.com/p/go.tools/importer" + "code.google.com/p/go.tools/ssa" + "code.google.com/p/go.tools/ssa/ssautil" +) + +func TestSwitches(t *testing.T) { + imp := importer.New(new(importer.Config)) // (uses GCImporter) + f, err := parser.ParseFile(imp.Fset, "testdata/switches.go", nil, parser.ParseComments) + if err != nil { + t.Error(err) + return + } + + mainInfo := imp.CreatePackage("main", f) + + prog := ssa.NewProgram(imp.Fset, 0) + if err := prog.CreatePackages(imp); err != nil { + t.Error(err) + return + } + mainPkg := prog.Package(mainInfo.Pkg) + mainPkg.Build() + + for _, mem := range mainPkg.Members { + if fn, ok := mem.(*ssa.Function); ok { + if fn.Synthetic != "" { + continue // e.g. init() + } + // Each (multi-line) "switch" comment within + // this function must match the printed form + // of a ConstSwitch. + var wantSwitches []string + for _, c := range f.Comments { + if fn.Syntax().Pos() <= c.Pos() && c.Pos() < fn.Syntax().End() { + text := strings.TrimSpace(c.Text()) + if strings.HasPrefix(text, "switch ") { + wantSwitches = append(wantSwitches, text) + } + } + } + + switches := ssautil.Switches(fn) + if len(switches) != len(wantSwitches) { + t.Errorf("in %s, found %d switches, want %d", fn, len(switches), len(wantSwitches)) + } + for i, sw := range switches { + got := sw.String() + if i >= len(wantSwitches) { + continue + } + want := wantSwitches[i] + if got != want { + t.Errorf("in %s, found switch %d: got <<%s>>, want <<%s>>", fn, i, got, want) + } + } + } + } +} diff --git a/ssa/ssautil/testdata/switches.go b/ssa/ssautil/testdata/switches.go new file mode 100644 index 00000000..a2235d62 --- /dev/null +++ b/ssa/ssautil/testdata/switches.go @@ -0,0 +1,339 @@ +// +build ignore + +package main + +// This file is the input to TestFindSwitches in switch_test.go. +// Each multiway conditional with constant or type cases (Switch) +// discovered by FindSwitches is printed, and compared with the +// comments. +// +// The body of each case is printed as the value of its first +// instruction. + +// -------- Value switches -------- + +func SimpleSwitch(x, y int) { + // switch x { + // case 1:int: print(1:int) + // case 2:int: print(23:int) + // case 3:int: print(23:int) + // case 4:int: print(3:int) + // default: x == y + // } + switch x { + case 1: + print(1) + case 2, 3: + print(23) + fallthrough + case 4: + print(3) + default: + print(4) + case y: + print(5) + } + print(6) +} + +func four() int { return 4 } + +// A non-constant case makes a switch "impure", but its pure +// cases form two separate switches. +func SwitchWithNonConstantCase(x int) { + // switch x { + // case 1:int: print(1:int) + // case 2:int: print(23:int) + // case 3:int: print(23:int) + // default: four() + // } + + // switch x { + // case 5:int: print(5:int) + // case 6:int: print(6:int) + // default: print("done":string) + // } + switch x { + case 1: + print(1) + case 2, 3: + print(23) + case four(): + print(3) + case 5: + print(5) + case 6: + print(6) + } + print("done") +} + +// Switches may be found even where the source +// program doesn't have a switch statement. + +func ImplicitSwitches(x, y int) { + // switch x { + // case 1:int: print(12:int) + // case 2:int: print(12:int) + // default: x < 5:int + // } + if x == 1 || 2 == x || x < 5 { + print(12) + } + + // switch x { + // case 3:int: print(34:int) + // case 4:int: print(34:int) + // default: x == y + // } + if x == 3 || 4 == x || x == y { + print(34) + } + + // Not a switch: no consistent variable. + if x == 5 || y == 6 { + print(56) + } + + // Not a switch: only one constant comparison. + if x == 7 || x == y { + print(78) + } +} + +func IfElseBasedSwitch(x int) { + // switch x { + // case 1:int: print(1:int) + // case 2:int: print(2:int) + // default: print("else":string) + // } + if x == 1 { + print(1) + } else if x == 2 { + print(2) + } else { + print("else") + } +} + +func GotoBasedSwitch(x int) { + // switch x { + // case 1:int: print(1:int) + // case 2:int: print(2:int) + // default: print("else":string) + // } + if x == 1 { + goto L1 + } + if x == 2 { + goto L2 + } + print("else") +L1: + print(1) + goto end +L2: + print(2) +end: +} + +func SwitchInAForLoop(x int) { + // switch x { + // case 1:int: print(1:int) + // case 2:int: print(2:int) + // default: print("head":string) + // } +loop: + for { + print("head") + switch x { + case 1: + print(1) + break loop + case 2: + print(2) + break loop + } + } +} + +// This case is a switch in a for-loop, both constructed using goto. +// As before, the default case points back to the block containing the +// switch, but that's ok. +func SwitchInAForLoopUsingGoto(x int) { + // switch x { + // case 1:int: print(1:int) + // case 2:int: print(2:int) + // default: print("head":string) + // } +loop: + print("head") + if x == 1 { + goto L1 + } + if x == 2 { + goto L2 + } + goto loop +L1: + print(1) + goto end +L2: + print(2) +end: +} + +func UnstructuredSwitchInAForLoop(x int) { + // switch x { + // case 1:int: print(1:int) + // case 2:int: x == 1:int + // default: print("end":string) + // } + for { + if x == 1 { + print(1) + return + } + if x == 2 { + continue + } + break + } + print("end") +} + +func CaseWithMultiplePreds(x int) { + for { + if x == 1 { + print(1) + return + } + loop: + // This block has multiple predecessors, + // so can't be treated as a switch case. + if x == 2 { + goto loop + } + break + } + print("end") +} + +func DuplicateConstantsAreNotEliminated(x int) { + // switch x { + // case 1:int: print(1:int) + // case 1:int: print("1a":string) + // case 2:int: print(2:int) + // default: return + // } + if x == 1 { + print(1) + } else if x == 1 { // duplicate => unreachable + print("1a") + } else if x == 2 { + print(2) + } +} + +// Interface values (created by comparisons) are not constants, +// so ConstSwitch.X is never of interface type. +func MakeInterfaceIsNotAConstant(x interface{}) { + if x == "foo" { + print("foo") + } else if x == 1 { + print(1) + } +} + +func ZeroInitializedVarsAreConstants(x int) { + // switch x { + // case 0:int: print(1:int) + // case 2:int: print(2:int) + // default: print("end":string) + // } + var zero int // SSA construction replaces zero with 0 + if x == zero { + print(1) + } else if x == 2 { + print(2) + } + print("end") +} + +// -------- Select -------- + +// NB, potentially fragile reliance on register number. +func SelectDesugarsToSwitch(ch chan int) { + // switch t1 { + // case 0:int: extract t0 #2 + // case 1:int: println(0:int) + // case 2:int: println(1:int) + // default: println("default":string) + // } + select { + case x := <-ch: + println(x) + case <-ch: + println(0) + case ch <- 1: + println(1) + default: + println("default") + } +} + +// -------- Type switches -------- + +// NB, reliance on fragile register numbering. +func SimpleTypeSwitch(x interface{}) { + // switch x.(type) { + // case t3 int: println(x) + // case t7 bool: println(x) + // case t10 string: println(t10) + // default: println(x) + // } + switch y := x.(type) { + case nil: + println(y) + case int, bool: + println(y) + case string: + println(y) + default: + println(y) + } +} + +// NB, potentially fragile reliance on register number. +func DuplicateTypesAreNotEliminated(x interface{}) { + // switch x.(type) { + // case t1 string: println(1:int) + // case t5 interface{}: println(t5) + // case t9 int: println(3:int) + // default: return + // } + switch y := x.(type) { + case string: + println(1) + case interface{}: + println(y) + case int: + println(3) // unreachable! + } +} + +// NB, potentially fragile reliance on register number. +func AdHocTypeSwitch(x interface{}) { + // switch x.(type) { + // case t1 int: println(t1) + // case t5 string: println(t5) + // default: print("default":string) + // } + if i, ok := x.(int); ok { + println(i) + } else if s, ok := x.(string); ok { + println(s) + } else { + print("default") + } +}