235 lines
6.8 KiB
Go
235 lines
6.8 KiB
Go
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// Copyright 2013 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 ssautil
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// This file implements discovery of switch and type-switch constructs
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// from low-level control flow.
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//
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// Many techniques exist for compiling a high-level switch with
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// constant cases to efficient machine code. The optimal choice will
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// depend on the data type, the specific case values, the code in the
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// body of each case, and the hardware.
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// Some examples:
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// - a lookup table (for a switch that maps constants to constants)
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// - a computed goto
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// - a binary tree
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// - a perfect hash
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// - a two-level switch (to partition constant strings by their first byte).
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import (
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"bytes"
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"fmt"
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"go/token"
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"go/types"
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"honnef.co/go/tools/ssa"
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)
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// A ConstCase represents a single constant comparison.
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// It is part of a Switch.
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type ConstCase struct {
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Block *ssa.BasicBlock // block performing the comparison
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Body *ssa.BasicBlock // body of the case
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Value *ssa.Const // case comparand
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}
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// A TypeCase represents a single type assertion.
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// It is part of a Switch.
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type TypeCase struct {
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Block *ssa.BasicBlock // block performing the type assert
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Body *ssa.BasicBlock // body of the case
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Type types.Type // case type
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Binding ssa.Value // value bound by this case
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}
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// A Switch is a logical high-level control flow operation
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// (a multiway branch) discovered by analysis of a CFG containing
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// only if/else chains. It is not part of the ssa.Instruction set.
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//
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// One of ConstCases and TypeCases has length >= 2;
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// the other is nil.
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//
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// In a value switch, the list of cases may contain duplicate constants.
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// A type switch may contain duplicate types, or types assignable
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// to an interface type also in the list.
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// TODO(adonovan): eliminate such duplicates.
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//
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type Switch struct {
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Start *ssa.BasicBlock // block containing start of if/else chain
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X ssa.Value // the switch operand
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ConstCases []ConstCase // ordered list of constant comparisons
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TypeCases []TypeCase // ordered list of type assertions
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Default *ssa.BasicBlock // successor if all comparisons fail
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}
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func (sw *Switch) String() string {
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// We represent each block by the String() of its
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// first Instruction, e.g. "print(42:int)".
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var buf bytes.Buffer
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if sw.ConstCases != nil {
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fmt.Fprintf(&buf, "switch %s {\n", sw.X.Name())
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for _, c := range sw.ConstCases {
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fmt.Fprintf(&buf, "case %s: %s\n", c.Value, c.Body.Instrs[0])
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}
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} else {
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fmt.Fprintf(&buf, "switch %s.(type) {\n", sw.X.Name())
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for _, c := range sw.TypeCases {
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fmt.Fprintf(&buf, "case %s %s: %s\n",
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c.Binding.Name(), c.Type, c.Body.Instrs[0])
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}
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}
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if sw.Default != nil {
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fmt.Fprintf(&buf, "default: %s\n", sw.Default.Instrs[0])
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}
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fmt.Fprintf(&buf, "}")
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return buf.String()
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}
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// Switches examines the control-flow graph of fn and returns the
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// set of inferred value and type switches. A value switch tests an
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// ssa.Value for equality against two or more compile-time constant
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// values. Switches involving link-time constants (addresses) are
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// ignored. A type switch type-asserts an ssa.Value against two or
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// more types.
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//
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// The switches are returned in dominance order.
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//
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// The resulting switches do not necessarily correspond to uses of the
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// 'switch' keyword in the source: for example, a single source-level
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// switch statement with non-constant cases may result in zero, one or
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// many Switches, one per plural sequence of constant cases.
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// Switches may even be inferred from if/else- or goto-based control flow.
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// (In general, the control flow constructs of the source program
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// cannot be faithfully reproduced from the SSA representation.)
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//
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func Switches(fn *ssa.Function) []Switch {
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// Traverse the CFG in dominance order, so we don't
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// enter an if/else-chain in the middle.
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var switches []Switch
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seen := make(map[*ssa.BasicBlock]bool) // TODO(adonovan): opt: use ssa.blockSet
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for _, b := range fn.DomPreorder() {
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if x, k := isComparisonBlock(b); x != nil {
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// Block b starts a switch.
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sw := Switch{Start: b, X: x}
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valueSwitch(&sw, k, seen)
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if len(sw.ConstCases) > 1 {
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switches = append(switches, sw)
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}
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}
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if y, x, T := isTypeAssertBlock(b); y != nil {
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// Block b starts a type switch.
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sw := Switch{Start: b, X: x}
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typeSwitch(&sw, y, T, seen)
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if len(sw.TypeCases) > 1 {
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switches = append(switches, sw)
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}
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}
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}
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return switches
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}
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func valueSwitch(sw *Switch, k *ssa.Const, seen map[*ssa.BasicBlock]bool) {
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b := sw.Start
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x := sw.X
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for x == sw.X {
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if seen[b] {
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break
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}
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seen[b] = true
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sw.ConstCases = append(sw.ConstCases, ConstCase{
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Block: b,
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Body: b.Succs[0],
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Value: k,
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})
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b = b.Succs[1]
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if len(b.Instrs) > 2 {
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// Block b contains not just 'if x == k',
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// so it may have side effects that
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// make it unsafe to elide.
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break
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}
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if len(b.Preds) != 1 {
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// Block b has multiple predecessors,
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// so it cannot be treated as a case.
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break
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}
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x, k = isComparisonBlock(b)
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}
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sw.Default = b
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}
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func typeSwitch(sw *Switch, y ssa.Value, T types.Type, seen map[*ssa.BasicBlock]bool) {
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b := sw.Start
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x := sw.X
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for x == sw.X {
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if seen[b] {
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break
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}
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seen[b] = true
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sw.TypeCases = append(sw.TypeCases, TypeCase{
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Block: b,
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Body: b.Succs[0],
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Type: T,
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Binding: y,
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})
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b = b.Succs[1]
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if len(b.Instrs) > 4 {
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// Block b contains not just
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// {TypeAssert; Extract #0; Extract #1; If}
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// so it may have side effects that
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// make it unsafe to elide.
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break
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}
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if len(b.Preds) != 1 {
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// Block b has multiple predecessors,
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// so it cannot be treated as a case.
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break
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}
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y, x, T = isTypeAssertBlock(b)
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}
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sw.Default = b
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}
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// isComparisonBlock returns the operands (v, k) if a block ends with
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// a comparison v==k, where k is a compile-time constant.
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//
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func isComparisonBlock(b *ssa.BasicBlock) (v ssa.Value, k *ssa.Const) {
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if n := len(b.Instrs); n >= 2 {
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if i, ok := b.Instrs[n-1].(*ssa.If); ok {
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if binop, ok := i.Cond.(*ssa.BinOp); ok && binop.Block() == b && binop.Op == token.EQL {
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if k, ok := binop.Y.(*ssa.Const); ok {
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return binop.X, k
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}
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if k, ok := binop.X.(*ssa.Const); ok {
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return binop.Y, k
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}
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}
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}
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}
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return
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}
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// isTypeAssertBlock returns the operands (y, x, T) if a block ends with
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// a type assertion "if y, ok := x.(T); ok {".
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//
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func isTypeAssertBlock(b *ssa.BasicBlock) (y, x ssa.Value, T types.Type) {
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if n := len(b.Instrs); n >= 4 {
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if i, ok := b.Instrs[n-1].(*ssa.If); ok {
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if ext1, ok := i.Cond.(*ssa.Extract); ok && ext1.Block() == b && ext1.Index == 1 {
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if ta, ok := ext1.Tuple.(*ssa.TypeAssert); ok && ta.Block() == b {
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// hack: relies upon instruction ordering.
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if ext0, ok := b.Instrs[n-3].(*ssa.Extract); ok {
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return ext0, ta.X, ta.AssertedType
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}
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}
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}
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}
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}
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return
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}
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