cmd/compile: remove CLOBBERDEAD experiment

This experiment is less effective and less needed since the
introduction of stack objects.

We can't clobber stack objects because we don't know statically
whether they are live or not.

We don't really need this experiment that much any more, as it was
primarily used to test the complicated ambiguously-live logic in the
liveness analysis, which has been removed in favor of stack objects.

It is also ~infeasible to maintain once we have safepoints everywhere.

Fixes #27326

Change-Id: I3bdde480b93dd508d048703055d4586b496176af
Reviewed-on: https://go-review.googlesource.com/c/151317
Run-TryBot: Keith Randall <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: randall77

src/cmd/compile/internal/gc/main.go

@@ -1345,8 +1345,8 @@ func concurrentBackendAllowed() bool {
 	if Debug_vlog || debugstr != "" || debuglive > 0 {
 		return false
 	}
-	// TODO: Test and delete these conditions.
-	if objabi.Fieldtrack_enabled != 0 || objabi.Clobberdead_enabled != 0 {
+	// TODO: Test and delete this condition.
+	if objabi.Fieldtrack_enabled != 0 {
 		return false
 	}
 	// TODO: fix races and enable the following flags

src/cmd/compile/internal/gc/plive.go

@@ -19,11 +19,8 @@ import (
 	"cmd/compile/internal/types"
 	"cmd/internal/obj"
 	"cmd/internal/objabi"
-	"cmd/internal/src"
 	"crypto/md5"
-	"crypto/sha1"
 	"fmt"
-	"os"
 	"strings"
 )
 
@@ -632,7 +629,7 @@ func (lv *Liveness) pointerMap(liveout bvec, vars []*Node, args, locals bvec) {
 
 // markUnsafePoints finds unsafe points and computes lv.unsafePoints.
 func (lv *Liveness) markUnsafePoints() {
-	if compiling_runtime || lv.f.NoSplit || objabi.Clobberdead_enabled != 0 {
+	if compiling_runtime || lv.f.NoSplit {
 		// No complex analysis necessary. Do this on the fly
 		// in issafepoint.
 		return
@@ -791,7 +788,7 @@ func (lv *Liveness) issafepoint(v *ssa.Value) bool {
 	// go:nosplit functions are similar. Since safe points used to
 	// be coupled with stack checks, go:nosplit often actually
 	// means "no safe points in this function".
-	if compiling_runtime || lv.f.NoSplit || objabi.Clobberdead_enabled != 0 {
+	if compiling_runtime || lv.f.NoSplit {
 		return v.Op.IsCall()
 	}
 	switch v.Op {
@@ -1051,161 +1048,6 @@ func (lv *Liveness) epilogue() {
 	}
 }
 
-func (lv *Liveness) clobber() {
-	// The clobberdead experiment inserts code to clobber all the dead variables (locals and args)
-	// before and after every safepoint. This experiment is useful for debugging the generation
-	// of live pointer bitmaps.
-	if objabi.Clobberdead_enabled == 0 {
-		return
-	}
-	var varSize int64
-	for _, n := range lv.vars {
-		varSize += n.Type.Size()
-	}
-	if len(lv.stackMaps) > 1000 || varSize > 10000 {
-		// Be careful to avoid doing too much work.
-		// Bail if >1000 safepoints or >10000 bytes of variables.
-		// Otherwise, giant functions make this experiment generate too much code.
-		return
-	}
-	if h := os.Getenv("GOCLOBBERDEADHASH"); h != "" {
-		// Clobber only functions where the hash of the function name matches a pattern.
-		// Useful for binary searching for a miscompiled function.
-		hstr := ""
-		for _, b := range sha1.Sum([]byte(lv.fn.funcname())) {
-			hstr += fmt.Sprintf("%08b", b)
-		}
-		if !strings.HasSuffix(hstr, h) {
-			return
-		}
-		fmt.Printf("\t\t\tCLOBBERDEAD %s\n", lv.fn.funcname())
-	}
-	if lv.f.Name == "forkAndExecInChild" || lv.f.Name == "wbBufFlush" {
-		// forkAndExecInChild calls vfork (on linux/amd64, anyway).
-		// The code we add here clobbers parts of the stack in the child.
-		// When the parent resumes, it is using the same stack frame. But the
-		// child has clobbered stack variables that the parent needs. Boom!
-		// In particular, the sys argument gets clobbered.
-		// Note to self: GOCLOBBERDEADHASH=011100101110
-		//
-		// runtime.wbBufFlush must not modify its arguments. See the comments
-		// in runtime/mwbbuf.go:wbBufFlush.
-		return
-	}
-
-	var oldSched []*ssa.Value
-	for _, b := range lv.f.Blocks {
-		// Copy block's values to a temporary.
-		oldSched = append(oldSched[:0], b.Values...)
-		b.Values = b.Values[:0]
-
-		// Clobber all dead variables at entry.
-		if b == lv.f.Entry {
-			for len(oldSched) > 0 && len(oldSched[0].Args) == 0 {
-				// Skip argless ops. We need to skip at least
-				// the lowered ClosurePtr op, because it
-				// really wants to be first. This will also
-				// skip ops like InitMem and SP, which are ok.
-				b.Values = append(b.Values, oldSched[0])
-				oldSched = oldSched[1:]
-			}
-			clobber(lv, b, lv.stackMaps[0])
-		}
-
-		// Copy values into schedule, adding clobbering around safepoints.
-		for _, v := range oldSched {
-			if !lv.issafepoint(v) {
-				b.Values = append(b.Values, v)
-				continue
-			}
-			before := true
-			if v.Op.IsCall() && v.Aux != nil && v.Aux.(*obj.LSym) == typedmemmove {
-				// Can't put clobber code before the call to typedmemmove.
-				// The variable to-be-copied is marked as dead
-				// at the callsite. That is ok, though, as typedmemmove
-				// is marked as nosplit, and the first thing it does
-				// is to call memmove (also nosplit), after which
-				// the source value is dead.
-				// See issue 16026.
-				before = false
-			}
-			if before {
-				clobber(lv, b, lv.stackMaps[lv.livenessMap.Get(v).stackMapIndex])
-			}
-			b.Values = append(b.Values, v)
-			clobber(lv, b, lv.stackMaps[lv.livenessMap.Get(v).stackMapIndex])
-		}
-	}
-}
-
-// clobber generates code to clobber all dead variables (those not marked in live).
-// Clobbering instructions are added to the end of b.Values.
-func clobber(lv *Liveness, b *ssa.Block, live bvec) {
-	for i, n := range lv.vars {
-		if !live.Get(int32(i)) {
-			clobberVar(b, n)
-		}
-	}
-}
-
-// clobberVar generates code to trash the pointers in v.
-// Clobbering instructions are added to the end of b.Values.
-func clobberVar(b *ssa.Block, v *Node) {
-	clobberWalk(b, v, 0, v.Type)
-}
-
-// b = block to which we append instructions
-// v = variable
-// offset = offset of (sub-portion of) variable to clobber (in bytes)
-// t = type of sub-portion of v.
-func clobberWalk(b *ssa.Block, v *Node, offset int64, t *types.Type) {
-	if !types.Haspointers(t) {
-		return
-	}
-	switch t.Etype {
-	case TPTR,
-		TUNSAFEPTR,
-		TFUNC,
-		TCHAN,
-		TMAP:
-		clobberPtr(b, v, offset)
-
-	case TSTRING:
-		// struct { byte *str; int len; }
-		clobberPtr(b, v, offset)
-
-	case TINTER:
-		// struct { Itab *tab; void *data; }
-		// or, when isnilinter(t)==true:
-		// struct { Type *type; void *data; }
-		// Note: the first word isn't a pointer. See comment in plive.go:onebitwalktype1.
-		clobberPtr(b, v, offset+int64(Widthptr))
-
-	case TSLICE:
-		// struct { byte *array; int len; int cap; }
-		clobberPtr(b, v, offset)
-
-	case TARRAY:
-		for i := int64(0); i < t.NumElem(); i++ {
-			clobberWalk(b, v, offset+i*t.Elem().Size(), t.Elem())
-		}
-
-	case TSTRUCT:
-		for _, t1 := range t.Fields().Slice() {
-			clobberWalk(b, v, offset+t1.Offset, t1.Type)
-		}
-
-	default:
-		Fatalf("clobberWalk: unexpected type, %v", t)
-	}
-}
-
-// clobberPtr generates a clobber of the pointer at offset offset in v.
-// The clobber instruction is added at the end of b.
-func clobberPtr(b *ssa.Block, v *Node, offset int64) {
-	b.NewValue0IA(src.NoXPos, ssa.OpClobber, types.TypeVoid, offset, v)
-}
-
 // Compact coalesces identical bitmaps from lv.livevars into the sets
 // lv.stackMapSet and lv.regMaps.
 //
@@ -1553,7 +1395,6 @@ func liveness(e *ssafn, f *ssa.Func, pp *Progs) LivenessMap {
 	lv.prologue()
 	lv.solve()
 	lv.epilogue()
-	lv.clobber()
 	if debuglive > 0 {
 		lv.showlive(nil, lv.stackMaps[0])
 		for _, b := range f.Blocks {

src/cmd/go/internal/work/gc.go

@@ -174,7 +174,7 @@ CheckFlags:
 	}
 
 	// TODO: Test and delete these conditions.
-	if objabi.Fieldtrack_enabled != 0 || objabi.Preemptibleloops_enabled != 0 || objabi.Clobberdead_enabled != 0 {
+	if objabi.Fieldtrack_enabled != 0 || objabi.Preemptibleloops_enabled != 0 {
 		canDashC = false
 	}
 

src/cmd/internal/objabi/util.go

@@ -104,7 +104,6 @@ var (
 	framepointer_enabled     int = 1
 	Fieldtrack_enabled       int
 	Preemptibleloops_enabled int
-	Clobberdead_enabled      int
 )
 
 // Toolchain experiments.
@@ -118,7 +117,6 @@ var exper = []struct {
 	{"fieldtrack", &Fieldtrack_enabled},
 	{"framepointer", &framepointer_enabled},
 	{"preemptibleloops", &Preemptibleloops_enabled},
-	{"clobberdead", &Clobberdead_enabled},
 }
 
 var defaultExpstring = Expstring()