code review 6940045: cmd/gc: racewalk: fix compiler crash

cmd/gc: racewalk: fix compiler crash
The code:
func main() {
        v := make([]int64, 10)
        i := 1
        _ = v[(i*4)/3]
}
crashes compiler with:

Program received signal SIGSEGV, Segmentation fault.
0x000000000043c274 in walkexpr (np=0x7fffffffc9b8, init=0x0) at src/cmd/gc/walk.c:587
587			*init = concat(*init, n->ninit);
(gdb) bt
#0  0x000000000043c274 in walkexpr (np=0x7fffffffc9b8, init=0x0) at src/cmd/gc/walk.c:587
#1  0x0000000000432d15 in copyexpr (n=0x7ffff7f69a48, t=<optimized out>, init=0x0) at src/cmd/gc/subr.c:2020
#2  0x000000000043f281 in walkdiv (init=0x0, np=0x7fffffffca70) at src/cmd/gc/walk.c:2901
#3  walkexpr (np=0x7ffff7f69760, init=0x0) at src/cmd/gc/walk.c:956
#4  0x000000000043d801 in walkexpr (np=0x7ffff7f69bc0, init=0x0) at src/cmd/gc/walk.c:988
#5  0x000000000043cc9b in walkexpr (np=0x7ffff7f69d38, init=0x0) at src/cmd/gc/walk.c:1068
#6  0x000000000043c50b in walkexpr (np=0x7ffff7f69f50, init=0x0) at src/cmd/gc/walk.c:879
#7  0x000000000043c50b in walkexpr (np=0x7ffff7f6a0c8, init=0x0) at src/cmd/gc/walk.c:879
#8  0x0000000000440a53 in walkexprlist (l=0x7ffff7f6a0c8, init=0x0) at src/cmd/gc/walk.c:357
#9  0x000000000043d0bf in walkexpr (np=0x7fffffffd318, init=0x0) at src/cmd/gc/walk.c:566
#10 0x00000000004402bf in vmkcall (fn=<optimized out>, t=0x0, init=0x0, va=0x7fffffffd368) at src/cmd/gc/walk.c:2275
#11 0x000000000044059a in mkcall (name=<optimized out>, t=0x0, init=0x0) at src/cmd/gc/walk.c:2287
#12 0x000000000042862b in callinstr (np=0x7fffffffd4c8, init=0x7fffffffd568, wr=0, skip=<optimized out>) at src/cmd/gc/racewalk.c:478
#13 0x00000000004288b7 in racewalknode (np=0x7ffff7f68108, init=0x7fffffffd568, wr=0, skip=0) at src/cmd/gc/racewalk.c:287
#14 0x0000000000428781 in racewalknode (np=0x7ffff7f65840, init=0x7fffffffd568, wr=0, skip=0) at src/cmd/gc/racewalk.c:302
#15 0x0000000000428abd in racewalklist (l=0x7ffff7f65840, init=0x0) at src/cmd/gc/racewalk.c:97
#16 0x0000000000428d0b in racewalk (fn=0x7ffff7f5f010) at src/cmd/gc/racewalk.c:63
#17 0x0000000000402b9c in compile (fn=0x7ffff7f5f010) at src/cmd/6g/../gc/pgen.c:67
#18 0x0000000000419f86 in funccompile (n=0x7ffff7f5f010, isclosure=0) at src/cmd/gc/dcl.c:1414
#19 0x0000000000424161 in p9main (argc=<optimized out>, argv=<optimized out>) at src/cmd/gc/lex.c:431
#20 0x0000000000401739 in main (argc=<optimized out>, argv=<optimized out>) at src/lib9/main.c:35

The problem is nil init passed to mkcall().

[dvyukov, 2012-12-13 11:20:17]

Hello golang-dev@googlegroups.com,

I'd like you to review this change to
https://dvyukov%40google.com@code.google.com/p/go/

[rsc, 2012-12-16 23:50:07]

LGTM

[dvyukov, 2012-12-17 08:59:56]

*** Submitted as https://code.google.com/p/go/source/detail?r=9d15015fc6e2 ***

cmd/gc: racewalk: fix compiler crash
The code:
func main() {
        v := make([]int64, 10)
        i := 1
        _ = v[(i*4)/3]
}
crashes compiler with:

Program received signal SIGSEGV, Segmentation fault.
0x000000000043c274 in walkexpr (np=0x7fffffffc9b8, init=0x0) at
src/cmd/gc/walk.c:587
587			*init = concat(*init, n->ninit);
(gdb) bt
#0  0x000000000043c274 in walkexpr (np=0x7fffffffc9b8, init=0x0) at
src/cmd/gc/walk.c:587
#1  0x0000000000432d15 in copyexpr (n=0x7ffff7f69a48, t=<optimized out>,
init=0x0) at src/cmd/gc/subr.c:2020
#2  0x000000000043f281 in walkdiv (init=0x0, np=0x7fffffffca70) at
src/cmd/gc/walk.c:2901
#3  walkexpr (np=0x7ffff7f69760, init=0x0) at src/cmd/gc/walk.c:956
#4  0x000000000043d801 in walkexpr (np=0x7ffff7f69bc0, init=0x0) at
src/cmd/gc/walk.c:988
#5  0x000000000043cc9b in walkexpr (np=0x7ffff7f69d38, init=0x0) at
src/cmd/gc/walk.c:1068
#6  0x000000000043c50b in walkexpr (np=0x7ffff7f69f50, init=0x0) at
src/cmd/gc/walk.c:879
#7  0x000000000043c50b in walkexpr (np=0x7ffff7f6a0c8, init=0x0) at
src/cmd/gc/walk.c:879
#8  0x0000000000440a53 in walkexprlist (l=0x7ffff7f6a0c8, init=0x0) at
src/cmd/gc/walk.c:357
#9  0x000000000043d0bf in walkexpr (np=0x7fffffffd318, init=0x0) at
src/cmd/gc/walk.c:566
#10 0x00000000004402bf in vmkcall (fn=<optimized out>, t=0x0, init=0x0,
va=0x7fffffffd368) at src/cmd/gc/walk.c:2275
#11 0x000000000044059a in mkcall (name=<optimized out>, t=0x0, init=0x0) at
src/cmd/gc/walk.c:2287
#12 0x000000000042862b in callinstr (np=0x7fffffffd4c8, init=0x7fffffffd568,
wr=0, skip=<optimized out>) at src/cmd/gc/racewalk.c:478
#13 0x00000000004288b7 in racewalknode (np=0x7ffff7f68108, init=0x7fffffffd568,
wr=0, skip=0) at src/cmd/gc/racewalk.c:287
#14 0x0000000000428781 in racewalknode (np=0x7ffff7f65840, init=0x7fffffffd568,
wr=0, skip=0) at src/cmd/gc/racewalk.c:302
#15 0x0000000000428abd in racewalklist (l=0x7ffff7f65840, init=0x0) at
src/cmd/gc/racewalk.c:97
#16 0x0000000000428d0b in racewalk (fn=0x7ffff7f5f010) at
src/cmd/gc/racewalk.c:63
#17 0x0000000000402b9c in compile (fn=0x7ffff7f5f010) at
src/cmd/6g/../gc/pgen.c:67
#18 0x0000000000419f86 in funccompile (n=0x7ffff7f5f010, isclosure=0) at
src/cmd/gc/dcl.c:1414
#19 0x0000000000424161 in p9main (argc=<optimized out>, argv=<optimized out>) at
src/cmd/gc/lex.c:431
#20 0x0000000000401739 in main (argc=<optimized out>, argv=<optimized out>) at
src/lib9/main.c:35

The problem is nil init passed to mkcall().

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/6940045