1 files changed, 337 insertions, 0 deletions
diff --git a/src/cmd/compile/internal/ssa/nilcheck.go b/src/cmd/compile/internal/ssa/nilcheck.go
new file mode 100644
index 0000000..14f511a
--- /dev/null
+++ b/src/cmd/compile/internal/ssa/nilcheck.go
@@ -0,0 +1,337 @@
+// 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/src"
+	"internal/buildcfg"
+)
+
+// 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 = buildcfg.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.
+	}
+}