cmd/compile: make the stack allocator more careful about register args.

Assignment between input parameters causes them to have more than
one "Name", and running this backwards from names to values can end
up confusing (conflating) parameter spill slots.

Around 105a6e9518, this cases a stack overflow running
go test -race encoding/pem
because two slice parameters spill (incorrectly) into the same
stack slots (in the AB?I-defined parameter spill area).

This also tickles a failure in cue, which turned out to be
easier to isolate.

Fixes #45851.
Updates #40724.

Change-Id: I39c56815bd6abb652f1ccbe83c47f4f373a125c3
Reviewed-on: https://go-review.googlesource.com/c/go/+/313212
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>

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

@@ -6,6 +6,7 @@ package ssa
 
 import (
 	"cmd/compile/internal/abi"
+	"cmd/compile/internal/base"
 	"cmd/compile/internal/ir"
 	"cmd/compile/internal/types"
 	"cmd/internal/src"
@@ -1601,7 +1602,10 @@ func (x *expandState) newArgToMemOrRegs(baseArg, toReplace *Value, offset int64,
 			x.f.OwnAux.abiInfo.String())
 		panic(fmt.Errorf("Op/Type mismatch, op=%s, type=%s", op.String(), t.String()))
 	}
-	aux := &AuxNameOffset{baseArg.Aux.(*ir.Name), baseArg.AuxInt + offset}
+	if baseArg.AuxInt != 0 {
+		base.Fatalf("BaseArg %s bound to registers has non-zero AuxInt", baseArg.LongString())
+	}
+	aux := &AuxNameOffset{baseArg.Aux.(*ir.Name), offset}
 	if toReplace != nil && toReplace.Block == baseArg.Block {
 		toReplace.reset(op)
 		toReplace.Aux = aux

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

@@ -145,6 +145,26 @@ func (s *stackAllocState) stackalloc() {
 		// Note: not "range f.NamedValues" above, because
 		// that would be nondeterministic.
 		for _, v := range f.NamedValues[name] {
+			if v.Op == OpArgIntReg || v.Op == OpArgFloatReg {
+				aux := v.Aux.(*AuxNameOffset)
+				// Never let an arg be bound to a differently named thing.
+				if name.N != aux.Name || name.Off != aux.Offset {
+					if f.pass.debug > stackDebug {
+						fmt.Printf("stackalloc register arg %s skipping name %s\n", v, name)
+					}
+					continue
+				}
+			} else if name.N.Class == ir.PPARAM && v.Op != OpArg {
+				// PPARAM's only bind to OpArg
+				if f.pass.debug > stackDebug {
+					fmt.Printf("stackalloc PPARAM name %s skipping non-Arg %s\n", name, v)
+				}
+				continue
+			}
+
+			if f.pass.debug > stackDebug {
+				fmt.Printf("stackalloc value %s to name %s\n", v, name)
+			}
 			names[v.ID] = name
 		}
 	}
@@ -165,6 +185,25 @@ func (s *stackAllocState) stackalloc() {
 			f.setHome(v, loc)
 			continue
 		}
+		// You might think this below would be the right idea, but you would be wrong.
+		// It almost works; as of 105a6e9518 - 2021-04-23,
+		// GOSSAHASH=11011011001011111 == cmd/compile/internal/noder.(*noder).embedded
+		// is compiled incorrectly.  I believe the cause is one of those SSA-to-registers
+		// puzzles that the register allocator untangles; in the event that a register
+		// parameter does not end up bound to a name, "fixing" it is a bad idea.
+		//
+		//if f.DebugTest {
+		//	if v.Op == OpArgIntReg || v.Op == OpArgFloatReg {
+		//		aux := v.Aux.(*AuxNameOffset)
+		//		loc := LocalSlot{N: aux.Name, Type: v.Type, Off: aux.Offset}
+		//		if f.pass.debug > stackDebug {
+		//			fmt.Printf("stackalloc Op%s %s to %s\n", v.Op, v, loc)
+		//		}
+		//		names[v.ID] = loc
+		//		continue
+		//	}
+		//}
+
 	}
 
 	// For each type, we keep track of all the stack slots we

test/fixedbugs/issue45851.go

@@ -0,0 +1,68 @@
+// 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.
+
+// This tickles a stack-allocation bug when the register ABI is enabled.
+// The original report was from cue, internal/core/adt/equality.go,
+// function equalVertex.
+
+// In the failing case, something bad gets passed to equalTerminal.
+
+package main
+
+import "fmt"
+
+type Kind uint16
+type Flag uint16
+
+const (
+	allKinds Kind = 1
+	TopKind  Kind = (allKinds - 1)
+)
+type Value interface {
+	Kind() Kind
+}
+type Vertex struct {
+	BaseValue Value
+	name string
+}
+func (v *Vertex) Kind() Kind {
+	return TopKind
+}
+
+func main() {
+	vA := &Vertex{name:"vA",}
+	vB := &Vertex{name:"vB",}
+	vX := &Vertex{name:"vX",}
+	vA.BaseValue = vX
+	vB.BaseValue = vX
+	_ = equalVertex(vA, vB, Flag(1))
+}
+
+var foo string
+
+//go:noinline
+func (v *Vertex) IsClosedStruct() bool {
+	return true
+}
+
+func equalVertex(x *Vertex, v Value, flags Flag) bool {
+	y, ok := v.(*Vertex)
+	if !ok {
+		return false
+	}
+	v, ok1 := x.BaseValue.(Value)
+	w, ok2 := y.BaseValue.(Value)
+	if !ok1 && !ok2 {
+		return true // both are struct or list.
+	}
+	return equalTerminal(v, w, flags)
+}
+
+//go:noinline
+func equalTerminal(x Value, y Value, flags Flag) bool {
+	foo = fmt.Sprintf("EQclosed %s %s %d\n", x.(*Vertex).name, y.(*Vertex).name, flags)
+	return true
+}