cmd/7g: reserve registers R26 to R32

These registers are not available for programs to use. Prior to this
change, the compiler would crash attempting to use ZR as a general
purpose register. Other programs would compile but on execution would
overwrite the G register and cause havoc.

Fixes linux/arm64 build.
Fixes #10304
Fixes #10320

Change-Id: I5cf51d3b77cfe3db7dd6377324950cafb02f8d8b
Reviewed-on: https://go-review.googlesource.com/8456Reviewed-by: Minux Ma <minux@golang.org>

src/cmd/7g/reg.go

@@ -115,6 +115,11 @@ func excludedregs() uint64 {
 	// Exclude registers with fixed functions
 	regbits := uint64(RtoB(arm64.REGRT1) | RtoB(arm64.REGRT2) | RtoB(arm64.REGPR))
 
+	// Exclude R26 - R31.
+	for r := arm64.REGMAX + 1; r <= arm64.REGZERO; r++ {
+		regbits |= RtoB(r)
+	}
+
 	// Also exclude floating point registers with fixed constants
 	regbits |= RtoB(arm64.REG_F27) | RtoB(arm64.REG_F28) | RtoB(arm64.REG_F29) | RtoB(arm64.REG_F30) | RtoB(arm64.REG_F31)
 

test/fixedbugs/issue10320.go

+// run
+
+// Copyright 2015 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.
+
+// Issue 10320: 7g failed to compile a program because it attempted
+// to use ZR as register. Other programs compiled but failed to
+// execute correctly because they clobbered the g register.
+
+package main
+
+func main() {
+	var x00, x01, x02, x03, x04, x05, x06, x07, x08, x09 int
+	var x10, x11, x12, x13, x14, x15, x16, x17, x18, x19 int
+	var x20, x21, x22, x23, x24, x25, x26, x27, x28, x29 int
+	var x30, x31, x32 int
+
+	_ = x00
+	_ = x01
+	_ = x02
+	_ = x03
+	_ = x04
+	_ = x05
+	_ = x06
+	_ = x07
+	_ = x08
+	_ = x09
+
+	_ = x10
+	_ = x11
+	_ = x12
+	_ = x13
+	_ = x14
+	_ = x15
+	_ = x16
+	_ = x17
+	_ = x18
+	_ = x19
+
+	_ = x20
+	_ = x21
+	_ = x22
+	_ = x23
+	_ = x24
+	_ = x25
+	_ = x26
+	_ = x27
+	_ = x28
+	_ = x29
+
+	_ = x30
+	_ = x31
+	_ = x32
+}