cmd/compile: mark unused values as invalid to prevent problems in expandCalls

Leftover values that have been replaced can cause problems in later
passes (within expandCalls).  For example, a struct select that
itself yields a struct will have a problematic rewrite, if the chance
is presented.

Updates #40724.

Change-Id: I1b445c47c301c3705f7fc0a9d39f1f5c84f4e190
Reviewed-on: https://go-review.googlesource.com/c/go/+/306869
Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>

src/cmd/compile/internal/ssa/expand_calls.go

@@ -941,6 +941,11 @@ func (x *expandState) storeArgOrLoad(pos src.XPos, b *Block, source, mem *Value,
 // stores (or later, register movement).  Extra args for interface and closure calls are ignored,
 // but removed.
 func (x *expandState) rewriteArgs(v *Value, firstArg int) (*Value, []*Value) {
+	if x.debug {
+		x.indent(3)
+		defer x.indent(-3)
+		x.Printf("rewriteArgs(%s; %d)\n", v.LongString(), firstArg)
+	}
 	// Thread the stores on the memory arg
 	aux := v.Aux.(*AuxCall)
 	pos := v.Pos.WithNotStmt()
@@ -969,7 +974,7 @@ func (x *expandState) rewriteArgs(v *Value, firstArg int) (*Value, []*Value) {
 				aOffset = aux.OffsetOfArg(auxI)
 			}
 			if x.debug {
-				x.Printf("storeArg %s, %v, %d\n", a.LongString(), aType, aOffset)
+				x.Printf("...storeArg %s, %v, %d\n", a.LongString(), aType, aOffset)
 			}
 			rc.init(aRegs, aux.abiInfo, result, x.sp)
 			mem = x.storeArgOrLoad(pos, v.Block, a, mem, aType, aOffset, 0, rc)
@@ -1056,6 +1061,7 @@ func expandCalls(f *Func) {
 			}
 		}
 		if isBlockMultiValueExit(b) {
+			x.indent(3)
 			// Very similar to code in rewriteArgs, but results instead of args.
 			v := b.Controls[0]
 			m0 := v.MemoryArg()
@@ -1063,7 +1069,12 @@ func expandCalls(f *Func) {
 			aux := f.OwnAux
 			pos := v.Pos.WithNotStmt()
 			allResults := []*Value{}
+			if x.debug {
+				x.Printf("multiValueExit rewriting %s\n", v.LongString())
+			}
+			var oldArgs []*Value
 			for j, a := range v.Args[:len(v.Args)-1] {
+				oldArgs = append(oldArgs, a)
 				i := int64(j)
 				auxType := aux.TypeOfResult(i)
 				auxBase := b.NewValue2A(v.Pos, OpLocalAddr, types.NewPtr(auxType), aux.NameOfResult(i), x.sp, mem)
@@ -1101,6 +1112,18 @@ func expandCalls(f *Func) {
 			v.AddArg(mem)
 			v.Type = types.NewResults(append(abi.RegisterTypes(aux.abiInfo.OutParams()), types.TypeMem))
 			b.SetControl(v)
+			for _, a := range oldArgs {
+				if a.Uses == 0 {
+					if x.debug {
+						x.Printf("...marking %v unused\n", a.LongString())
+					}
+					a.reset(OpInvalid)
+				}
+			}
+			if x.debug {
+				x.Printf("...multiValueExit new result %s\n", v.LongString())
+			}
+			x.indent(-3)
 		}
 	}
 

test/abi/zombie_struct_select.go

@@ -0,0 +1,36 @@
+// run
+
+// Copyright 2021 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 main
+
+type patchlist struct {
+	head, tail uint32
+}
+
+type frag struct {
+	i   uint32
+	out patchlist
+}
+
+//go:noinline
+//go:registerparams
+func patch(l patchlist, i uint32) {
+}
+
+//go:noinline
+//go:registerparams
+func badbad(f1, f2 frag) frag {
+	// concat of failure is failure
+	if f1.i == 0 || f2.i == 0 { // internal compiler error: 'badbad': incompatible OpArgIntReg [4]: v42 and v26
+		return frag{}
+	}
+	patch(f1.out, f2.i)
+	return frag{f1.i, f2.out}
+}
+
+func main() {
+	badbad(frag{i: 2}, frag{i: 3})
+}