cmd/5g: use MOVB for fixed array nil check
Fixes #4396.
For fixed arrays larger than the unmapped page, agenr would general a nil check by loading the first word of the array. However there is no requirement for the first element of a byte array to be word aligned, so this check causes a trap on ARMv5 hardware (ARMv6 since relaxed that restriction, but it probably still comes at a cost).
Switching the check to MOVB ensures alignment is not an issue. This check is only invoked in a few places in the code where large fixed arrays are embedded into structs, compress/lzw is the biggest offender, and switching to MOVB has no observable performance penalty.
Thanks to Rémy and Daniel Morsing for helping me debug this on IRC last night.
R=remyoudompheng, minux.ma, rsc
CC=golang-dev
https://golang.org/cl/6854063
diff --git a/test/fixedbugs/issue4396a.go b/test/fixedbugs/issue4396a.go
new file mode 100644
index 0000000..11ae1f7
--- /dev/null
+++ b/test/fixedbugs/issue4396a.go
@@ -0,0 +1,27 @@
+// run
+
+// Copyright 2012 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 4396. Arrays of bytes are not required to be
+// word aligned. 5g should use MOVB to load the address
+// of s.g[0] for its nil check.
+//
+// This test _may_ fail on arm, but requires the host to
+// trap unaligned loads. This is generally done with
+//
+// echo "4" > /proc/cpu/alignment
+
+package main
+
+var s = struct {
+ // based on lzw.decoder
+ a, b, c, d, e uint16
+ f [4096]uint8
+ g [4096]uint8
+}{}
+
+func main() {
+ s.g[0] = 1
+}
