cmd/gc: Slice access of const causes strange string errors (Bug introduced in go1.3, doesn't happen in go <= 1.2)

[gopherbot]

by psnim2000:

What does 'go version' print?
go version go1.3 darwin/amd64 (first discovered in go version go1.3 linux/amd64)

What steps reproduce the problem?
If possible, include a link to a program on play.golang.org.
http://play.golang.org/p/RwqaHkITff
 
What happened?
See attached program. 
If you run this program on go1.3, you will get something like.
Hello, playground PLCM7w9WX0F4Fs9Yz9.0699t A5

If you run this program on go1.2, you will get something like
Hello, playground PLCM2IE95QPT6ZLUAX218417FLX3449NEJ5D

What should have happened instead?
The code shouldn't return any characters after [A-Z0-9]
If you notice the output under go1.3, the value outside of [A-Z0-9] shouldn't be
possible. I'm unsure if the output value is even valid UTF-8.

Please provide any additional information below.
This error was uncovered after upgrading a service to go1.3. We tested the function with
go1 and go1.2, and the problem doesn't occur.

[ianlancetaylor]

Comment 1:

This looks like a compiler bug.

Labels changed: added repo-main, release-go1.3.1.

[remyoudompheng]

Comment 2:

The generated code is wrong for b % 36.
The theoretical equivalent expression is: b - 36*(b/36) where b/36 is represented by 
uint16(b) * 57 >> 11 (2048/57 = 35.9298...)
The generated code is:
    0x007f 00127 (z.go:7)   MOVBQZX (SI),BP    => BP = b
        0x0083 00131 (z.go:7)   MOVQ    BP,DI      => DI = b too.
    0x0086 00134 (z.go:8)   CMPQ    CX,R9
    0x0089 00137 (z.go:8)   JCC $1,325
    0x008f 00143 (z.go:8)   LEAQ    (R10)(CX*1),BX
    0x0093 00147 (z.go:8)   MOVQ    $57,R8
    0x009a 00154 (z.go:8)   MOVQ    BP,AX
    0x009d 00157 (z.go:8)   MULB    R8B,       => AH:AL = uint16(b)*57
    0x00a0 00160 (z.go:8)   MOVB    AH,DL      => b/36 is DL>>3
                                                   => we are missing a MOVB DL, BPB here
    0x00a2 00162 (z.go:8)   SHRB    $3,BPB     => BP>>3 is not what we want.
    0x00a6 00166 (z.go:8)   MOVQ    $36,R8
    0x00ad 00173 (z.go:8)   IMULQ   R8,BP
    0x00b1 00177 (z.go:8)   MOVQ    DI,R8
    0x00b4 00180 (z.go:8)   SUBQ    BP,R8      => R8 = DI - 36*BP
    0x00b7 00183 (z.go:8)   MOVBQZX R8B,BP
    0x00bb 00187 (z.go:8)   LEAQ    go.string."0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"+16(SB)(BP*1),R8
The wrong code results in wrong index (> 36) taken in the constant string.

Status changed to Started.

[remyoudompheng]

Comment 3:

I could bisect the issue up to:
changeset:   19052:bcc8475a7ada
user:        Daniel Morsing <daniel.morsing@gmail.com>
date:        Tue Feb 11 20:25:40 2014 +0000
summary:     cmd/6g, cmd/8g, cmd/5g: make the undefined instruction have no successors
Not sure how it causes the bug.

[remyoudompheng]

Comment 4:

https://golang.org/cl/104560044

[gopherbot]

Comment 5:

CL https://golang.org/cl/104560044 mentions this issue.

[gopherbot]

Comment 6:

CL https://golang.org/cl/124950043 mentions this issue.

[rsc]

Comment 7:

This issue was closed by revision 5b63ce4.

Status changed to Fixed.

[rsc]

Comment 8:

No workaround, small fix. Approved for Go 1.3.1.

[rsc]

Comment 9:

Also need CL 129720043 for 386 fix.

[adg]

Comment 10:

This issue was closed by revision 8246129e698d.

[adg]

Comment 11:

Applied to release-branch.go1.3.