| // Copyright 2015 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/compile/internal/ir" |
| "cmd/internal/objabi" |
| "cmd/internal/src" |
| ) |
| |
| // nilcheckelim eliminates unnecessary nil checks. |
| // runs on machine-independent code. |
| func nilcheckelim(f *Func) { |
| // A nil check is redundant if the same nil check was successful in a |
| // dominating block. The efficacy of this pass depends heavily on the |
| // efficacy of the cse pass. |
| sdom := f.Sdom() |
| |
| // TODO: Eliminate more nil checks. |
| // We can recursively remove any chain of fixed offset calculations, |
| // i.e. struct fields and array elements, even with non-constant |
| // indices: x is non-nil iff x.a.b[i].c is. |
| |
| type walkState int |
| const ( |
| Work walkState = iota // process nil checks and traverse to dominees |
| ClearPtr // forget the fact that ptr is nil |
| ) |
| |
| type bp struct { |
| block *Block // block, or nil in ClearPtr state |
| ptr *Value // if non-nil, ptr that is to be cleared in ClearPtr state |
| op walkState |
| } |
| |
| work := make([]bp, 0, 256) |
| work = append(work, bp{block: f.Entry}) |
| |
| // map from value ID to bool indicating if value is known to be non-nil |
| // in the current dominator path being walked. This slice is updated by |
| // walkStates to maintain the known non-nil values. |
| nonNilValues := make([]bool, f.NumValues()) |
| |
| // make an initial pass identifying any non-nil values |
| for _, b := range f.Blocks { |
| for _, v := range b.Values { |
| // a value resulting from taking the address of a |
| // value, or a value constructed from an offset of a |
| // non-nil ptr (OpAddPtr) implies it is non-nil |
| // We also assume unsafe pointer arithmetic generates non-nil pointers. See #27180. |
| // We assume that SlicePtr is non-nil because we do a bounds check |
| // before the slice access (and all cap>0 slices have a non-nil ptr). See #30366. |
| if v.Op == OpAddr || v.Op == OpLocalAddr || v.Op == OpAddPtr || v.Op == OpOffPtr || v.Op == OpAdd32 || v.Op == OpAdd64 || v.Op == OpSub32 || v.Op == OpSub64 || v.Op == OpSlicePtr { |
| nonNilValues[v.ID] = true |
| } |
| } |
| } |
| |
| for changed := true; changed; { |
| changed = false |
| for _, b := range f.Blocks { |
| for _, v := range b.Values { |
| // phis whose arguments are all non-nil |
| // are non-nil |
| if v.Op == OpPhi { |
| argsNonNil := true |
| for _, a := range v.Args { |
| if !nonNilValues[a.ID] { |
| argsNonNil = false |
| break |
| } |
| } |
| if argsNonNil { |
| if !nonNilValues[v.ID] { |
| changed = true |
| } |
| nonNilValues[v.ID] = true |
| } |
| } |
| } |
| } |
| } |
| |
| // allocate auxiliary date structures for computing store order |
| sset := f.newSparseSet(f.NumValues()) |
| defer f.retSparseSet(sset) |
| storeNumber := make([]int32, f.NumValues()) |
| |
| // perform a depth first walk of the dominee tree |
| for len(work) > 0 { |
| node := work[len(work)-1] |
| work = work[:len(work)-1] |
| |
| switch node.op { |
| case Work: |
| b := node.block |
| |
| // First, see if we're dominated by an explicit nil check. |
| if len(b.Preds) == 1 { |
| p := b.Preds[0].b |
| if p.Kind == BlockIf && p.Controls[0].Op == OpIsNonNil && p.Succs[0].b == b { |
| if ptr := p.Controls[0].Args[0]; !nonNilValues[ptr.ID] { |
| nonNilValues[ptr.ID] = true |
| work = append(work, bp{op: ClearPtr, ptr: ptr}) |
| } |
| } |
| } |
| |
| // Next, order values in the current block w.r.t. stores. |
| b.Values = storeOrder(b.Values, sset, storeNumber) |
| |
| pendingLines := f.cachedLineStarts // Holds statement boundaries that need to be moved to a new value/block |
| pendingLines.clear() |
| |
| // Next, process values in the block. |
| i := 0 |
| for _, v := range b.Values { |
| b.Values[i] = v |
| i++ |
| switch v.Op { |
| case OpIsNonNil: |
| ptr := v.Args[0] |
| if nonNilValues[ptr.ID] { |
| if v.Pos.IsStmt() == src.PosIsStmt { // Boolean true is a terrible statement boundary. |
| pendingLines.add(v.Pos) |
| v.Pos = v.Pos.WithNotStmt() |
| } |
| // This is a redundant explicit nil check. |
| v.reset(OpConstBool) |
| v.AuxInt = 1 // true |
| } |
| case OpNilCheck: |
| ptr := v.Args[0] |
| if nonNilValues[ptr.ID] { |
| // This is a redundant implicit nil check. |
| // Logging in the style of the former compiler -- and omit line 1, |
| // which is usually in generated code. |
| if f.fe.Debug_checknil() && v.Pos.Line() > 1 { |
| f.Warnl(v.Pos, "removed nil check") |
| } |
| if v.Pos.IsStmt() == src.PosIsStmt { // About to lose a statement boundary |
| pendingLines.add(v.Pos) |
| } |
| v.reset(OpUnknown) |
| f.freeValue(v) |
| i-- |
| continue |
| } |
| // Record the fact that we know ptr is non nil, and remember to |
| // undo that information when this dominator subtree is done. |
| nonNilValues[ptr.ID] = true |
| work = append(work, bp{op: ClearPtr, ptr: ptr}) |
| fallthrough // a non-eliminated nil check might be a good place for a statement boundary. |
| default: |
| if v.Pos.IsStmt() != src.PosNotStmt && !isPoorStatementOp(v.Op) && pendingLines.contains(v.Pos) { |
| v.Pos = v.Pos.WithIsStmt() |
| pendingLines.remove(v.Pos) |
| } |
| } |
| } |
| // This reduces the lost statement count in "go" by 5 (out of 500 total). |
| for j := 0; j < i; j++ { // is this an ordering problem? |
| v := b.Values[j] |
| if v.Pos.IsStmt() != src.PosNotStmt && !isPoorStatementOp(v.Op) && pendingLines.contains(v.Pos) { |
| v.Pos = v.Pos.WithIsStmt() |
| pendingLines.remove(v.Pos) |
| } |
| } |
| if pendingLines.contains(b.Pos) { |
| b.Pos = b.Pos.WithIsStmt() |
| pendingLines.remove(b.Pos) |
| } |
| b.truncateValues(i) |
| |
| // Add all dominated blocks to the work list. |
| for w := sdom[node.block.ID].child; w != nil; w = sdom[w.ID].sibling { |
| work = append(work, bp{op: Work, block: w}) |
| } |
| |
| case ClearPtr: |
| nonNilValues[node.ptr.ID] = false |
| continue |
| } |
| } |
| } |
| |
| // All platforms are guaranteed to fault if we load/store to anything smaller than this address. |
| // |
| // This should agree with minLegalPointer in the runtime. |
| const minZeroPage = 4096 |
| |
| // faultOnLoad is true if a load to an address below minZeroPage will trigger a SIGSEGV. |
| var faultOnLoad = objabi.GOOS != "aix" |
| |
| // nilcheckelim2 eliminates unnecessary nil checks. |
| // Runs after lowering and scheduling. |
| func nilcheckelim2(f *Func) { |
| unnecessary := f.newSparseMap(f.NumValues()) // map from pointer that will be dereferenced to index of dereferencing value in b.Values[] |
| defer f.retSparseMap(unnecessary) |
| |
| pendingLines := f.cachedLineStarts // Holds statement boundaries that need to be moved to a new value/block |
| |
| for _, b := range f.Blocks { |
| // Walk the block backwards. Find instructions that will fault if their |
| // input pointer is nil. Remove nil checks on those pointers, as the |
| // faulting instruction effectively does the nil check for free. |
| unnecessary.clear() |
| pendingLines.clear() |
| // Optimization: keep track of removed nilcheck with smallest index |
| firstToRemove := len(b.Values) |
| for i := len(b.Values) - 1; i >= 0; i-- { |
| v := b.Values[i] |
| if opcodeTable[v.Op].nilCheck && unnecessary.contains(v.Args[0].ID) { |
| if f.fe.Debug_checknil() && v.Pos.Line() > 1 { |
| f.Warnl(v.Pos, "removed nil check") |
| } |
| // For bug 33724, policy is that we might choose to bump an existing position |
| // off the faulting load/store in favor of the one from the nil check. |
| |
| // Iteration order means that first nilcheck in the chain wins, others |
| // are bumped into the ordinary statement preservation algorithm. |
| u := b.Values[unnecessary.get(v.Args[0].ID)] |
| if !u.Pos.SameFileAndLine(v.Pos) { |
| if u.Pos.IsStmt() == src.PosIsStmt { |
| pendingLines.add(u.Pos) |
| } |
| u.Pos = v.Pos |
| } else if v.Pos.IsStmt() == src.PosIsStmt { |
| pendingLines.add(v.Pos) |
| } |
| |
| v.reset(OpUnknown) |
| firstToRemove = i |
| continue |
| } |
| if v.Type.IsMemory() || v.Type.IsTuple() && v.Type.FieldType(1).IsMemory() { |
| if v.Op == OpVarKill || v.Op == OpVarLive || (v.Op == OpVarDef && !v.Aux.(*ir.Name).Type().HasPointers()) { |
| // These ops don't really change memory. |
| continue |
| // Note: OpVarDef requires that the defined variable not have pointers. |
| // We need to make sure that there's no possible faulting |
| // instruction between a VarDef and that variable being |
| // fully initialized. If there was, then anything scanning |
| // the stack during the handling of that fault will see |
| // a live but uninitialized pointer variable on the stack. |
| // |
| // If we have: |
| // |
| // NilCheck p |
| // VarDef x |
| // x = *p |
| // |
| // We can't rewrite that to |
| // |
| // VarDef x |
| // NilCheck p |
| // x = *p |
| // |
| // Particularly, even though *p faults on p==nil, we still |
| // have to do the explicit nil check before the VarDef. |
| // See issue #32288. |
| } |
| // This op changes memory. Any faulting instruction after v that |
| // we've recorded in the unnecessary map is now obsolete. |
| unnecessary.clear() |
| } |
| |
| // Find any pointers that this op is guaranteed to fault on if nil. |
| var ptrstore [2]*Value |
| ptrs := ptrstore[:0] |
| if opcodeTable[v.Op].faultOnNilArg0 && (faultOnLoad || v.Type.IsMemory()) { |
| // On AIX, only writing will fault. |
| ptrs = append(ptrs, v.Args[0]) |
| } |
| if opcodeTable[v.Op].faultOnNilArg1 && (faultOnLoad || (v.Type.IsMemory() && v.Op != OpPPC64LoweredMove)) { |
| // On AIX, only writing will fault. |
| // LoweredMove is a special case because it's considered as a "mem" as it stores on arg0 but arg1 is accessed as a load and should be checked. |
| ptrs = append(ptrs, v.Args[1]) |
| } |
| |
| for _, ptr := range ptrs { |
| // Check to make sure the offset is small. |
| switch opcodeTable[v.Op].auxType { |
| case auxSym: |
| if v.Aux != nil { |
| continue |
| } |
| case auxSymOff: |
| if v.Aux != nil || v.AuxInt < 0 || v.AuxInt >= minZeroPage { |
| continue |
| } |
| case auxSymValAndOff: |
| off := ValAndOff(v.AuxInt).Off() |
| if v.Aux != nil || off < 0 || off >= minZeroPage { |
| continue |
| } |
| case auxInt32: |
| // Mips uses this auxType for atomic add constant. It does not affect the effective address. |
| case auxInt64: |
| // ARM uses this auxType for duffcopy/duffzero/alignment info. |
| // It does not affect the effective address. |
| case auxNone: |
| // offset is zero. |
| default: |
| v.Fatalf("can't handle aux %s (type %d) yet\n", v.auxString(), int(opcodeTable[v.Op].auxType)) |
| } |
| // This instruction is guaranteed to fault if ptr is nil. |
| // Any previous nil check op is unnecessary. |
| unnecessary.set(ptr.ID, int32(i), src.NoXPos) |
| } |
| } |
| // Remove values we've clobbered with OpUnknown. |
| i := firstToRemove |
| for j := i; j < len(b.Values); j++ { |
| v := b.Values[j] |
| if v.Op != OpUnknown { |
| if !notStmtBoundary(v.Op) && pendingLines.contains(v.Pos) { // Late in compilation, so any remaining NotStmt values are probably okay now. |
| v.Pos = v.Pos.WithIsStmt() |
| pendingLines.remove(v.Pos) |
| } |
| b.Values[i] = v |
| i++ |
| } |
| } |
| |
| if pendingLines.contains(b.Pos) { |
| b.Pos = b.Pos.WithIsStmt() |
| } |
| |
| b.truncateValues(i) |
| |
| // TODO: if b.Kind == BlockPlain, start the analysis in the subsequent block to find |
| // more unnecessary nil checks. Would fix test/nilptr3.go:159. |
| } |
| } |