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54e8d504e8
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
28 lines
628 B
Go
28 lines
628 B
Go
// run
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// Copyright 2012 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Issue 4396. Arrays of bytes are not required to be
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// word aligned. 5g should use MOVB to load the address
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// of s.g[0] for its nil check.
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//
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// This test _may_ fail on arm, but requires the host to
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// trap unaligned loads. This is generally done with
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//
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// echo "4" > /proc/cpu/alignment
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package main
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var s = struct {
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// based on lzw.decoder
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a, b, c, d, e uint16
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f [4096]uint8
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g [4096]uint8
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}{}
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func main() {
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s.g[0] = 1
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}
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