Gecko [ 167.86.127.34 ]

Name	Size	Permission
gen	[ DIR ]	drwxr-xr-x
testdata	[ DIR ]	drwxr-xr-x
README.md	7.76 KB	-rw-r--r--
TODO	950 B	-rw-r--r--
biasedsparsemap.go	2.71 KB	-rw-r--r--
block.go	8.64 KB	-rw-r--r--
branchelim.go	11.41 KB	-rw-r--r--
branchelim_test.go	5.21 KB	-rw-r--r--
cache.go	2.46 KB	-rw-r--r--
check.go	13.9 KB	-rw-r--r--
checkbce.go	956 B	-rw-r--r--
compile.go	16.22 KB	-rw-r--r--
config.go	11.39 KB	-rw-r--r--
copyelim.go	1.82 KB	-rw-r--r--
copyelim_test.go	1.29 KB	-rw-r--r--
critical.go	3.15 KB	-rw-r--r--
cse.go	10.62 KB	-rw-r--r--
cse_test.go	4.21 KB	-rw-r--r--
deadcode.go	9.73 KB	-rw-r--r--
deadcode_test.go	3.49 KB	-rw-r--r--
deadstore.go	8.98 KB	-rw-r--r--
deadstore_test.go	4.09 KB	-rw-r--r--
debug.go	33.92 KB	-rw-r--r--
debug_test.go	28.68 KB	-rw-r--r--
decompose.go	10.91 KB	-rw-r--r--
dom.go	7.98 KB	-rw-r--r--
dom_test.go	13.34 KB	-rw-r--r--
export_test.go	5.68 KB	-rw-r--r--
flagalloc.go	5.49 KB	-rw-r--r--
func.go	22.17 KB	-rw-r--r--
func_test.go	12.85 KB	-rw-r--r--
fuse.go	5.7 KB	-rw-r--r--
fuse_test.go	5.04 KB	-rw-r--r--
html.go	29.04 KB	-rw-r--r--
id.go	576 B	-rw-r--r--
layout.go	3.56 KB	-rw-r--r--
lca.go	3.71 KB	-rw-r--r--
lca_test.go	1.65 KB	-rw-r--r--
likelyadjust.go	15.23 KB	-rw-r--r--
location.go	2.8 KB	-rw-r--r--
loopbce.go	9.85 KB	-rw-r--r--
loopreschedchecks.go	15.36 KB	-rw-r--r--
looprotate.go	2.24 KB	-rw-r--r--
lower.go	1.24 KB	-rw-r--r--
magic.go	13.42 KB	-rw-r--r--
magic_test.go	9.1 KB	-rw-r--r--
nilcheck.go	11.11 KB	-rw-r--r--
nilcheck_test.go	12.07 KB	-rw-r--r--
numberlines.go	8.44 KB	-rw-r--r--
op.go	8.74 KB	-rw-r--r--
opGen.go	839.32 KB	-rw-r--r--
opt.go	290 B	-rw-r--r--
passbm_test.go	3.14 KB	-rw-r--r--
phielim.go	1.47 KB	-rw-r--r--
phiopt.go	3.96 KB	-rw-r--r--
poset.go	37.23 KB	-rw-r--r--
poset_test.go	18.14 KB	-rw-r--r--
print.go	2.98 KB	-rw-r--r--
prove.go	36.68 KB	-rw-r--r--
redblack32.go	8.9 KB	-rw-r--r--
redblack32_test.go	5.96 KB	-rw-r--r--
regalloc.go	76.36 KB	-rw-r--r--
regalloc_test.go	6.41 KB	-rw-r--r--
rewrite.go	33.06 KB	-rw-r--r--
rewrite386.go	514.14 KB	-rw-r--r--
rewrite386splitload.go	3.7 KB	-rw-r--r--
rewriteAMD64.go	1.31 MB	-rw-r--r--
rewriteAMD64splitload.go	4.95 KB	-rw-r--r--
rewriteARM.go	538.26 KB	-rw-r--r--
rewriteARM64.go	837.68 KB	-rw-r--r--
rewriteMIPS.go	203.31 KB	-rw-r--r--
rewriteMIPS64.go	227.84 KB	-rw-r--r--
rewritePPC64.go	661.88 KB	-rw-r--r--
rewriteRISCV64.go	123.11 KB	-rw-r--r--
rewriteS390X.go	904.36 KB	-rw-r--r--
rewriteWasm.go	136.86 KB	-rw-r--r--
rewrite_test.go	3.01 KB	-rw-r--r--
rewritedec.go	10.56 KB	-rw-r--r--
rewritedec64.go	64.01 KB	-rw-r--r--
rewritedecArgs.go	6.6 KB	-rw-r--r--
rewritegeneric.go	1.09 MB	-rw-r--r--
schedule.go	13.99 KB	-rw-r--r--
schedule_test.go	2.91 KB	-rw-r--r--
shift_test.go	4.05 KB	-rw-r--r--
shortcircuit.go	3.97 KB	-rw-r--r--
shortcircuit_test.go	1.31 KB	-rw-r--r--
sizeof_test.go	855 B	-rw-r--r--
softfloat.go	1.94 KB	-rw-r--r--
sparsemap.go	1.98 KB	-rw-r--r--
sparseset.go	1.54 KB	-rw-r--r--
sparsetree.go	7.88 KB	-rw-r--r--
sparsetreemap.go	7.67 KB	-rw-r--r--
stackalloc.go	10.95 KB	-rw-r--r--
stackframe.go	290 B	-rw-r--r--
stmtlines_test.go	2.75 KB	-rw-r--r--
tighten.go	4.21 KB	-rw-r--r--
trim.go	4.24 KB	-rw-r--r--
value.go	10.71 KB	-rw-r--r--
writebarrier.go	17.92 KB	-rw-r--r--
writebarrier_test.go	1.75 KB	-rw-r--r--
xposmap.go	3.29 KB	-rw-r--r--
zcse.go	2.12 KB	-rw-r--r--
zeroextension_test.go	1.66 KB	-rw-r--r--

Code Editor : deadstore.go

// 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/types"
	"cmd/internal/src"
)

// dse does dead-store elimination on the Function.
// Dead stores are those which are unconditionally followed by
// another store to the same location, with no intervening load.
// This implementation only works within a basic block. TODO: use something more global.
func dse(f *Func) {
	var stores []*Value
	loadUse := f.newSparseSet(f.NumValues())
	defer f.retSparseSet(loadUse)
	storeUse := f.newSparseSet(f.NumValues())
	defer f.retSparseSet(storeUse)
	shadowed := f.newSparseMap(f.NumValues())
	defer f.retSparseMap(shadowed)
	for _, b := range f.Blocks {
		// Find all the stores in this block. Categorize their uses:
		//  loadUse contains stores which are used by a subsequent load.
		//  storeUse contains stores which are used by a subsequent store.
		loadUse.clear()
		storeUse.clear()
		stores = stores[:0]
		for _, v := range b.Values {
			if v.Op == OpPhi {
				// Ignore phis - they will always be first and can't be eliminated
				continue
			}
			if v.Type.IsMemory() {
				stores = append(stores, v)
				for _, a := range v.Args {
					if a.Block == b && a.Type.IsMemory() {
						storeUse.add(a.ID)
						if v.Op != OpStore && v.Op != OpZero && v.Op != OpVarDef && v.Op != OpVarKill {
							// CALL, DUFFCOPY, etc. are both
							// reads and writes.
							loadUse.add(a.ID)
						}
					}
				}
			} else {
				for _, a := range v.Args {
					if a.Block == b && a.Type.IsMemory() {
						loadUse.add(a.ID)
					}
				}
			}
		}
		if len(stores) == 0 {
			continue
		}

// find last store in the block
		var last *Value
		for _, v := range stores {
			if storeUse.contains(v.ID) {
				continue
			}
			if last != nil {
				b.Fatalf("two final stores - simultaneous live stores %s %s", last.LongString(), v.LongString())
			}
			last = v
		}
		if last == nil {
			b.Fatalf("no last store found - cycle?")
		}

// Walk backwards looking for dead stores. Keep track of shadowed addresses.
		// A "shadowed address" is a pointer and a size describing a memory region that
		// is known to be written. We keep track of shadowed addresses in the shadowed
		// map, mapping the ID of the address to the size of the shadowed region.
		// Since we're walking backwards, writes to a shadowed region are useless,
		// as they will be immediately overwritten.
		shadowed.clear()
		v := last

walkloop:
		if loadUse.contains(v.ID) {
			// Someone might be reading this memory state.
			// Clear all shadowed addresses.
			shadowed.clear()
		}
		if v.Op == OpStore || v.Op == OpZero {
			var sz int64
			if v.Op == OpStore {
				sz = v.Aux.(*types.Type).Size()
			} else { // OpZero
				sz = v.AuxInt
			}
			if shadowedSize := int64(shadowed.get(v.Args[0].ID)); shadowedSize != -1 && shadowedSize >= sz {
				// Modify the store/zero into a copy of the memory state,
				// effectively eliding the store operation.
				if v.Op == OpStore {
					// store addr value mem
					v.SetArgs1(v.Args[2])
				} else {
					// zero addr mem
					v.SetArgs1(v.Args[1])
				}
				v.Aux = nil
				v.AuxInt = 0
				v.Op = OpCopy
			} else {
				if sz > 0x7fffffff { // work around sparseMap's int32 value type
					sz = 0x7fffffff
				}
				shadowed.set(v.Args[0].ID, int32(sz), src.NoXPos)
			}
		}
		// walk to previous store
		if v.Op == OpPhi {
			// At start of block.  Move on to next block.
			// The memory phi, if it exists, is always
			// the first logical store in the block.
			// (Even if it isn't the first in the current b.Values order.)
			continue
		}
		for _, a := range v.Args {
			if a.Block == b && a.Type.IsMemory() {
				v = a
				goto walkloop
			}
		}
	}
}

// elimDeadAutosGeneric deletes autos that are never accessed. To achieve this
// we track the operations that the address of each auto reaches and if it only
// reaches stores then we delete all the stores. The other operations will then
// be eliminated by the dead code elimination pass.
func elimDeadAutosGeneric(f *Func) {
	addr := make(map[*Value]GCNode) // values that the address of the auto reaches
	elim := make(map[*Value]GCNode) // values that could be eliminated if the auto is
	used := make(map[GCNode]bool)   // used autos that must be kept

// visit the value and report whether any of the maps are updated
	visit := func(v *Value) (changed bool) {
		args := v.Args
		switch v.Op {
		case OpAddr, OpLocalAddr:
			// Propagate the address if it points to an auto.
			n, ok := v.Aux.(GCNode)
			if !ok || n.StorageClass() != ClassAuto {
				return
			}
			if addr[v] == nil {
				addr[v] = n
				changed = true
			}
			return
		case OpVarDef, OpVarKill:
			// v should be eliminated if we eliminate the auto.
			n, ok := v.Aux.(GCNode)
			if !ok || n.StorageClass() != ClassAuto {
				return
			}
			if elim[v] == nil {
				elim[v] = n
				changed = true
			}
			return
		case OpVarLive:
			// Don't delete the auto if it needs to be kept alive.

// We depend on this check to keep the autotmp stack slots
			// for open-coded defers from being removed (since they
			// may not be used by the inline code, but will be used by
			// panic processing).
			n, ok := v.Aux.(GCNode)
			if !ok || n.StorageClass() != ClassAuto {
				return
			}
			if !used[n] {
				used[n] = true
				changed = true
			}
			return
		case OpStore, OpMove, OpZero:
			// v should be eliminated if we eliminate the auto.
			n, ok := addr[args[0]]
			if ok && elim[v] == nil {
				elim[v] = n
				changed = true
			}
			// Other args might hold pointers to autos.
			args = args[1:]
		}

// The code below assumes that we have handled all the ops
		// with sym effects already. Sanity check that here.
		// Ignore Args since they can't be autos.
		if v.Op.SymEffect() != SymNone && v.Op != OpArg {
			panic("unhandled op with sym effect")
		}

if v.Uses == 0 && v.Op != OpNilCheck || len(args) == 0 {
			// Nil check has no use, but we need to keep it.
			return
		}

// If the address of the auto reaches a memory or control
		// operation not covered above then we probably need to keep it.
		// We also need to keep autos if they reach Phis (issue #26153).
		if v.Type.IsMemory() || v.Type.IsFlags() || v.Op == OpPhi || v.MemoryArg() != nil {
			for _, a := range args {
				if n, ok := addr[a]; ok {
					if !used[n] {
						used[n] = true
						changed = true
					}
				}
			}
			return
		}

// Propagate any auto addresses through v.
		node := GCNode(nil)
		for _, a := range args {
			if n, ok := addr[a]; ok && !used[n] {
				if node == nil {
					node = n
				} else if node != n {
					// Most of the time we only see one pointer
					// reaching an op, but some ops can take
					// multiple pointers (e.g. NeqPtr, Phi etc.).
					// This is rare, so just propagate the first
					// value to keep things simple.
					used[n] = true
					changed = true
				}
			}
		}
		if node == nil {
			return
		}
		if addr[v] == nil {
			// The address of an auto reaches this op.
			addr[v] = node
			changed = true
			return
		}
		if addr[v] != node {
			// This doesn't happen in practice, but catch it just in case.
			used[node] = true
			changed = true
		}
		return
	}

iterations := 0
	for {
		if iterations == 4 {
			// give up
			return
		}
		iterations++
		changed := false
		for _, b := range f.Blocks {
			for _, v := range b.Values {
				changed = visit(v) || changed
			}
			// keep the auto if its address reaches a control value
			for _, c := range b.ControlValues() {
				if n, ok := addr[c]; ok && !used[n] {
					used[n] = true
					changed = true
				}
			}
		}
		if !changed {
			break
		}
	}

// Eliminate stores to unread autos.
	for v, n := range elim {
		if used[n] {
			continue
		}
		// replace with OpCopy
		v.SetArgs1(v.MemoryArg())
		v.Aux = nil
		v.AuxInt = 0
		v.Op = OpCopy
	}
}

// elimUnreadAutos deletes stores (and associated bookkeeping ops VarDef and VarKill)
// to autos that are never read from.
func elimUnreadAutos(f *Func) {
	// Loop over all ops that affect autos taking note of which
	// autos we need and also stores that we might be able to
	// eliminate.
	seen := make(map[GCNode]bool)
	var stores []*Value
	for _, b := range f.Blocks {
		for _, v := range b.Values {
			n, ok := v.Aux.(GCNode)
			if !ok {
				continue
			}
			if n.StorageClass() != ClassAuto {
				continue
			}

effect := v.Op.SymEffect()
			switch effect {
			case SymNone, SymWrite:
				// If we haven't seen the auto yet
				// then this might be a store we can
				// eliminate.
				if !seen[n] {
					stores = append(stores, v)
				}
			default:
				// Assume the auto is needed (loaded,
				// has its address taken, etc.).
				// Note we have to check the uses
				// because dead loads haven't been
				// eliminated yet.
				if v.Uses > 0 {
					seen[n] = true
				}
			}
		}
	}

// Eliminate stores to unread autos.
	for _, store := range stores {
		n, _ := store.Aux.(GCNode)
		if seen[n] {
			continue
		}

// replace store with OpCopy
		store.SetArgs1(store.MemoryArg())
		store.Aux = nil
		store.AuxInt = 0
		store.Op = OpCopy
	}
}

${this.title}

Code Editor : deadstore.go