3 files changed, 52 insertions, 24 deletions
diff --git a/src/cmd/compile/internal/compare/compare.go b/src/cmd/compile/internal/compare/compare.go
index e165cd6..cb2f84e 100644
--- a/src/cmd/compile/internal/compare/compare.go
+++ b/src/cmd/compile/internal/compare/compare.go
@@ -148,7 +148,7 @@ func calculateCostForType(t *types.Type) int64 {
 		return EqStructCost(t)
 	case types.TSLICE:
 		// Slices are not comparable.
-		base.Fatalf("eqStructFieldCost: unexpected slice type")
+		base.Fatalf("calculateCostForType: unexpected slice type")
 	case types.TARRAY:
 		elemCost := calculateCostForType(t.Elem())
 		cost = t.NumElem() * elemCost
@@ -371,6 +371,11 @@ func eqmem(p, q ir.Node, field int, size int64) ir.Node {
 }
 
 func eqmemfunc(size int64, t *types.Type) (fn *ir.Name, needsize bool) {
+	if !base.Ctxt.Arch.CanMergeLoads && t.Alignment() < int64(base.Ctxt.Arch.Alignment) && t.Alignment() < t.Size() {
+		// We can't use larger comparisons if the value might not be aligned
+		// enough for the larger comparison. See issues 46283 and 67160.
+		size = 0
+	}
 	switch size {
 	case 1, 2, 4, 8, 16:
 		buf := fmt.Sprintf("memequal%d", int(size)*8)
diff --git a/src/cmd/compile/internal/noder/irgen.go b/src/cmd/compile/internal/noder/irgen.go
index e0b7bb9..145bcc8 100644
--- a/src/cmd/compile/internal/noder/irgen.go
+++ b/src/cmd/compile/internal/noder/irgen.go
@@ -34,7 +34,13 @@ func checkFiles(m posMap, noders []*noder) (*types2.Package, *types2.Info) {
 	posBaseMap := make(map[*syntax.PosBase]*syntax.File)
 	for i, p := range noders {
 		files[i] = p.file
-		posBaseMap[p.file.Pos().Base()] = p.file
+		// The file.Pos() is the position of the package clause.
+		// If there's a //line directive before that, file.Pos().Base()
+		// refers to that directive, not the file itself.
+		// Make sure to consistently map back to file base, here and
+		// when we look for a file in the conf.Error handler below,
+		// otherwise the file may not be found (was go.dev/issue/67141).
+		posBaseMap[fileBase(p.file.Pos())] = p.file
 	}
 
 	// typechecking
@@ -68,13 +74,12 @@ func checkFiles(m posMap, noders []*noder) (*types2.Package, *types2.Info) {
 		terr := err.(types2.Error)
 		msg := terr.Msg
 		if versionErrorRx.MatchString(msg) {
-			posBase := terr.Pos.Base()
-			for !posBase.IsFileBase() { // line directive base
-				posBase = posBase.Pos().Base()
-			}
+			posBase := fileBase(terr.Pos)
 			fileVersion := info.FileVersions[posBase]
 			file := posBaseMap[posBase]
-			if file.GoVersion == fileVersion {
+			if file == nil {
+				// This should never happen, but be careful and don't crash.
+			} else if file.GoVersion == fileVersion {
 				// If we have a version error caused by //go:build, report it.
 				msg = fmt.Sprintf("%s (file declares //go:build %s)", msg, fileVersion)
 			} else {
@@ -149,6 +154,15 @@ func checkFiles(m posMap, noders []*noder) (*types2.Package, *types2.Info) {
 	return pkg, info
 }
 
+// fileBase returns a file's position base given a position in the file.
+func fileBase(pos syntax.Pos) *syntax.PosBase {
+	base := pos.Base()
+	for !base.IsFileBase() { // line directive base
+		base = base.Pos().Base()
+	}
+	return base
+}
+
 // A cycleFinder detects anonymous interface cycles (go.dev/issue/56103).
 type cycleFinder struct {
 	cyclic map[*types2.Interface]bool
diff --git a/src/cmd/compile/internal/walk/assign.go b/src/cmd/compile/internal/walk/assign.go
index fc3b858..63b6a1d 100644
--- a/src/cmd/compile/internal/walk/assign.go
+++ b/src/cmd/compile/internal/walk/assign.go
@@ -623,21 +623,23 @@ func isAppendOfMake(n ir.Node) bool {
 //	    panicmakeslicelen()
 //	  }
 //	  s := l1
-//	  n := len(s) + l2
-//	  // Compare n and s as uint so growslice can panic on overflow of len(s) + l2.
-//	  // cap is a positive int and n can become negative when len(s) + l2
-//	  // overflows int. Interpreting n when negative as uint makes it larger
-//	  // than cap(s). growslice will check the int n arg and panic if n is
-//	  // negative. This prevents the overflow from being undetected.
-//	  if uint(n) <= uint(cap(s)) {
-//	    s = s[:n]
-//	  } else {
-//	    s = growslice(T, s.ptr, n, s.cap, l2, T)
+//	  if l2 != 0 {
+//	    n := len(s) + l2
+//	    // Compare n and s as uint so growslice can panic on overflow of len(s) + l2.
+//	    // cap is a positive int and n can become negative when len(s) + l2
+//	    // overflows int. Interpreting n when negative as uint makes it larger
+//	    // than cap(s). growslice will check the int n arg and panic if n is
+//	    // negative. This prevents the overflow from being undetected.
+//	    if uint(n) <= uint(cap(s)) {
+//	      s = s[:n]
+//	    } else {
+//	      s = growslice(T, s.ptr, n, s.cap, l2, T)
+//	    }
+//	    // clear the new portion of the underlying array.
+//	    hp := &s[len(s)-l2]
+//	    hn := l2 * sizeof(T)
+//	    memclr(hp, hn)
 //	  }
-//	  // clear the new portion of the underlying array.
-//	  hp := &s[len(s)-l2]
-//	  hn := l2 * sizeof(T)
-//	  memclr(hp, hn)
 //	}
 //	s
 //
@@ -671,11 +673,18 @@ func extendSlice(n *ir.CallExpr, init *ir.Nodes) ir.Node {
 	s := typecheck.TempAt(base.Pos, ir.CurFunc, l1.Type())
 	nodes = append(nodes, ir.NewAssignStmt(base.Pos, s, l1))
 
+	// if l2 != 0 {
+	// Avoid work if we're not appending anything. But more importantly,
+	// avoid allowing hp to be a past-the-end pointer when clearing. See issue 67255.
+	nifnz := ir.NewIfStmt(base.Pos, ir.NewBinaryExpr(base.Pos, ir.ONE, l2, ir.NewInt(base.Pos, 0)), nil, nil)
+	nifnz.Likely = true
+	nodes = append(nodes, nifnz)
+
 	elemtype := s.Type().Elem()
 
 	// n := s.len + l2
 	nn := typecheck.TempAt(base.Pos, ir.CurFunc, types.Types[types.TINT])
-	nodes = append(nodes, ir.NewAssignStmt(base.Pos, nn, ir.NewBinaryExpr(base.Pos, ir.OADD, ir.NewUnaryExpr(base.Pos, ir.OLEN, s), l2)))
+	nifnz.Body = append(nifnz.Body, ir.NewAssignStmt(base.Pos, nn, ir.NewBinaryExpr(base.Pos, ir.OADD, ir.NewUnaryExpr(base.Pos, ir.OLEN, s), l2)))
 
 	// if uint(n) <= uint(s.cap)
 	nuint := typecheck.Conv(nn, types.Types[types.TUINT])
@@ -697,7 +706,7 @@ func extendSlice(n *ir.CallExpr, init *ir.Nodes) ir.Node {
 			l2)),
 	}
 
-	nodes = append(nodes, nif)
+	nifnz.Body = append(nifnz.Body, nif)
 
 	// hp := &s[s.len - l2]
 	// TODO: &s[s.len] - hn?
@@ -723,7 +732,7 @@ func extendSlice(n *ir.CallExpr, init *ir.Nodes) ir.Node {
 		// if growslice isn't called do we need to do the zeroing ourselves.
 		nif.Body = append(nif.Body, clr...)
 	} else {
-		nodes = append(nodes, clr...)
+		nifnz.Body = append(nifnz.Body, clr...)
 	}
 
 	typecheck.Stmts(nodes)