cmd/compile: make sure not to use SP as an index register

...because that's an illegal addressing mode.

I double-checked handling of this code, and 387 is the only
place where this check is missing.

Fixes #22429

Change-Id: I2284fe729ea86251c6af2f04076ddf7a5e66367c
Reviewed-on: https://go-review.googlesource.com/73551
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/compile/internal/x86/387.go

@@ -46,6 +46,9 @@ func ssaGenValue387(s *gc.SSAGenState, v *ssa.Value) {
 		case ssa.Op386MOVSSloadidx1, ssa.Op386MOVSDloadidx1:
 			p.From.Scale = 1
 			p.From.Index = v.Args[1].Reg()
+			if p.From.Index == x86.REG_SP {
+				p.From.Reg, p.From.Index = p.From.Index, p.From.Reg
+			}
 		case ssa.Op386MOVSSloadidx4:
 			p.From.Scale = 4
 			p.From.Index = v.Args[1].Reg()
@@ -95,6 +98,9 @@ func ssaGenValue387(s *gc.SSAGenState, v *ssa.Value) {
 		case ssa.Op386MOVSSstoreidx1, ssa.Op386MOVSDstoreidx1:
 			p.To.Scale = 1
 			p.To.Index = v.Args[1].Reg()
+			if p.To.Index == x86.REG_SP {
+				p.To.Reg, p.To.Index = p.To.Index, p.To.Reg
+			}
 		case ssa.Op386MOVSSstoreidx4:
 			p.To.Scale = 4
 			p.To.Index = v.Args[1].Reg()

test/fixedbugs/issue22429.go

+// compile
+
+// Copyright 2017 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.
+
+// Make sure SSA->assembly pass can handle SP as an index register.
+
+package p
+
+type T struct {
+	a,b,c,d float32
+}
+
+func f(a *[8]T, i,j,k int) float32 {
+	b := *a
+	return b[i].a + b[j].b + b[k].c
+}