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runtime: use 4 byte writes in amd64p32 memmove/memclr

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Currently, amd64p32's memmove and memclr use 8 byte writes as much as
possible and 1 byte writes for the tail of the object. However, if an
object ends with a 4 byte pointer at an 8 byte aligned offset, this
may copy/zero the pointer field one byte at a time, allowing the
garbage collector to observe a partially copied pointer.

Fix this by using 4 byte writes instead of 8 byte writes.

Updates #12552.

Change-Id: I13324fd05756fb25ae57e812e836f0a975b5595c
Reviewed-on: https://go-review.googlesource.com/15370
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
This commit is contained in:
Austin Clements 2015-10-02 18:17:54 -04:00
parent 44078a3228
commit 9f6df6c940
2 changed files with 16 additions and 13 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

6
src/runtime/asm_amd64p32.s
View File

@ -620,12 +620,12 @@ TEXT runtime·memclr(SB),NOSPLIT,$0-8
MOVL ptr+0(FP), DI
MOVL n+4(FP), CX
MOVQ CX, BX
ANDQ $7, BX
SHRQ $3, CX
ANDQ $3, BX
SHRQ $2, CX
MOVQ $0, AX
CLD
REP
STOSQ
STOSL
MOVQ BX, CX
REP
STOSB

23
src/runtime/memmove_nacl_amd64p32.s
View File

@ -4,6 +4,9 @@
#include "textflag.h"
// This could use MOVSQ, but we use MOVSL so that if an object ends in
// a 4 byte pointer, we copy it as a unit instead of byte by byte.
TEXT runtime·memmove(SB), NOSPLIT, $0-12
MOVL to+0(FP), DI
MOVL from+4(FP), SI
@ -14,9 +17,9 @@ TEXT runtime·memmove(SB), NOSPLIT, $0-12
forward:
MOVL BX, CX
SHRL $3, CX
ANDL $7, BX
REP; MOVSQ
SHRL $2, CX
ANDL $3, BX
REP; MOVSL
MOVL BX, CX
REP; MOVSB
RET
@ -32,13 +35,13 @@ back:
STD
MOVL BX, CX
SHRL $3, CX
ANDL $7, BX
SUBL $8, DI
SUBL $8, SI
REP; MOVSQ
ADDL $7, DI
ADDL $7, SI
SHRL $2, CX
ANDL $3, BX
SUBL $4, DI
SUBL $4, SI
REP; MOVSL
ADDL $3, DI
ADDL $3, SI
MOVL BX, CX
REP; MOVSB
CLD