runtime: use 4 byte writes in amd64p32 memmove/memclr

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>

diff --git a/src/runtime/asm_amd64p32.s b/src/runtime/asm_amd64p32.s
index e8b14912b9a8dc4d9421beb8a4607802ccdbb238..78b16f4abd0d92356598d1908c603325e808108c 100644 (file)
--- a/src/runtime/asm_amd64p32.s
+++ b/src/runtime/asm_amd64p32.s
@@ -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

diff --git a/src/runtime/memmove_nacl_amd64p32.s b/src/runtime/memmove_nacl_amd64p32.s
index 373607afec194a081671c5560a37d9bb28c64e28..be9e1e55bec0d6cf7222c30652a0a9e4fcacd863 100644 (file)
--- a/src/runtime/memmove_nacl_amd64p32.s
+++ b/src/runtime/memmove_nacl_amd64p32.s
@@ -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