[dev.regabi] all: merge master (bf0f7c9) into dev.regabi

This merge involved two merge conflicts:

1. walk's ascompatee code has been substantially refactored on
dev.regabi, so CL 285633 is ported to the new style.

2. The os.TestDirFS workaround added in CL 286213 can be removed now
that #42637 has been fixed by CL 285720.

Conflicts:

- src/cmd/compile/internal/gc/walk.go
- src/os/os_test.go

Merge List:

+ 2021-01-25 bf0f7c9d78 doc/go1.16: mention os.DirFS in os section
+ 2021-01-25 deaf29a8a8 cmd/compile: fix order-of-assignment issue w/ defers
+ 2021-01-25 ad2ca26a52 doc/go1.16: mention os.DirEntry and types moved from os to io/fs
+ 2021-01-25 a51921fa5b doc/go1.16: mention new testing/iotest functions
+ 2021-01-25 e6b6d107f7 doc/go1.16: mention deprecation of io/ioutil
+ 2021-01-25 96a276363b doc/go1.16: mention go/build changes
+ 2021-01-25 3d85c69a0b html/template: revert "avoid race when escaping updates template"
+ 2021-01-25 54514c6b28 cmd/go: fix TestScript/cgo_path, cgo_path_space when CC set
+ 2021-01-25 6de8443f3b doc/asm: add a section on go_asm.h, clean up go_tls.h section
+ 2021-01-25 54b251f542 lib/time, time/tzdata: update tzdata to 2021a
+ 2021-01-25 ff82cc971a os: force consistent mtime before running fstest on directory on Windows
+ 2021-01-25 044f937a73 doc/go1.16: fix WalkDir and Walk links
+ 2021-01-23 b634f5d97a doc/go1.16: add crypto/x509 memory optimization
+ 2021-01-23 9897655c61 doc/go1.16: reword ambiguously parsable sentence
+ 2021-01-23 cd99385ff4 cmd/internal/obj/arm64: fix VMOVQ instruction encoding error
+ 2021-01-23 66ee8b158f runtime: restore cgo_import_dynamic for libc.so on openbsd
+ 2021-01-22 25c39e4fb5 io/ioutil: fix example test for WriteFile to allow it to run in the playground
+ 2021-01-22 eb21b31e48 runtime: define dummy msanmove
+ 2021-01-22 3a778ff50f runtime: check for g0 stack last in signal handler
+ 2021-01-22 a2cef9b544 cmd/go: don't lookup the path for CC when invoking cgo

Change-Id: I651949f9eb18b57e3c996c4f3b2b3bf458bc5d97

src/cmd/compile/internal/walk/assign.go

@@ -289,11 +289,14 @@ func ascompatee(op ir.Op, nl, nr []ir.Node) []ir.Node {
 	}
 
 	var assigned ir.NameSet
-	var memWrite bool
+	var memWrite, deferResultWrite bool
 
 	// affected reports whether expression n could be affected by
 	// the assignments applied so far.
 	affected := func(n ir.Node) bool {
+		if deferResultWrite {
+			return true
+		}
 		return ir.Any(n, func(n ir.Node) bool {
 			if n.Op() == ir.ONAME && assigned.Has(n.(*ir.Name)) {
 				return true
@@ -369,21 +372,40 @@ func ascompatee(op ir.Op, nl, nr []ir.Node) []ir.Node {
 
 		appendWalkStmt(&late, convas(ir.NewAssignStmt(base.Pos, lorig, r), &late))
 
-		if name != nil && ir.IsBlank(name) {
-			// We can ignore assignments to blank.
-			continue
-		}
-		if op == ir.ORETURN && types.OrigSym(name.Sym()) == nil {
-			// We can also ignore assignments to anonymous result
-			// parameters. These can't appear in expressions anyway.
+		// Check for reasons why we may need to compute later expressions
+		// before this assignment happens.
+
+		if name == nil {
+			// Not a direct assignment to a declared variable.
+			// Conservatively assume any memory access might alias.
+			memWrite = true
 			continue
 		}
 
-		if name != nil && name.OnStack() && !name.Addrtaken() {
-			assigned.Add(name)
-		} else {
-			memWrite = true
+		if name.Class == ir.PPARAMOUT && ir.CurFunc.HasDefer() {
+			// Assignments to a result parameter in a function with defers
+			// becomes visible early if evaluation of any later expression
+			// panics (#43835).
+			deferResultWrite = true
+			continue
 		}
+
+		if sym := types.OrigSym(name.Sym()); sym == nil || sym.IsBlank() {
+			// We can ignore assignments to blank or anonymous result parameters.
+			// These can't appear in expressions anyway.
+			continue
+		}
+
+		if name.Addrtaken() || !name.OnStack() {
+			// Global variable, heap escaped, or just addrtaken.
+			// Conservatively assume any memory access might alias.
+			memWrite = true
+			continue
+		}
+
+		// Local, non-addrtaken variable.
+		// Assignments can only alias with direct uses of this variable.
+		assigned.Add(name)
 	}
 
 	early.Append(late.Take()...)