qbit/go
1
0
Fork 0
mirror of https://github.com/golang/go synced 2024-11-11 21:40:21 -07:00
Code Releases Activity

cmd/5g: use MOVB for fixed array nil check

Browse Source 
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
This commit is contained in:
Dave Cheney 2012-12-06 08:01:33 +11:00
parent 9fb96991e6
commit 54e8d504e8
3 changed files with 75 additions and 12 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

31
src/cmd/5g/cgen.c
View File

@ -559,7 +559,6 @@ agen(Node *n, Node *res)
{
Node *nl;
Node n1, n2, n3;
Prog *p1;
int r;
if(debug['g']) {
@ -704,10 +703,13 @@ agen(Node *n, Node *res)
if(nl->type->type->width >= unmappedzero) {
regalloc(&n1, types[tptr], N);
gmove(res, &n1);
p1 = gins(AMOVW, &n1, &n1);
p1->from.type = D_OREG;
p1->from.offset = 0;
regalloc(&n2, types[TUINT8], &n1);
n1.op = OINDREG;
n1.type = types[TUINT8];
n1.xoffset = 0;
gmove(&n1, &n2);
regfree(&n1);
regfree(&n2);
}
nodconst(&n1, types[TINT32], n->xoffset);
regalloc(&n2, n1.type, N);
@ -737,8 +739,7 @@ ret:
void
igen(Node *n, Node *a, Node *res)
{
Node n1;
Prog *p1;
Node n1, n2;
int r;
if(debug['g']) {
@ -785,10 +786,13 @@ igen(Node *n, Node *a, Node *res)
if(n->left->type->type->width >= unmappedzero) {
regalloc(&n1, types[tptr], N);
gmove(a, &n1);
p1 = gins(AMOVW, &n1, &n1);
p1->from.type = D_OREG;
p1->from.offset = 0;
regalloc(&n2, types[TUINT8], &n1);
n1.op = OINDREG;
n1.type = types[TUINT8];
n1.xoffset = 0;
gmove(&n1, &n2);
regfree(&n1);
regfree(&n2);
}
}
a->op = OINDREG;
@ -957,10 +961,13 @@ agenr(Node *n, Node *a, Node *res)
if(isfixedarray(nl->type) && nl->type->width >= unmappedzero) {
regalloc(&n4, types[tptr], N);
gmove(&n3, &n4);
p1 = gins(AMOVW, &n4, &n4);
p1->from.type = D_OREG;
p1->from.offset = 0;
regalloc(&tmp, types[TUINT8], &n4);
n4.op = OINDREG;
n4.type = types[TUINT8];
n4.xoffset = 0;
gmove(&n4, &tmp);
regfree(&n4);
regfree(&tmp);
}
// constant index

27
test/fixedbugs/issue4396a.go Normal file
View File

@ -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
}

29
test/fixedbugs/issue4396b.go Normal file
View File

@ -0,0 +1,29 @@
// 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.
// 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
type T struct {
U uint16
V T2
}
type T2 struct {
pad [4096]byte
A, B byte
}
var s, t = new(T), new(T)
func main() {
var u, v *T2 = &s.V, &t.V
u.B = v.B
}