cmd/compile: go1.11beta1 for loop with multiple conditions goes into infinite loop

[bbrks]

I have produced a minimal example of a bug seen when running under go1.11beta1 which causes for loops to go into an infinite loop when the condition contains multiple statements.

This seems to be due to an over-aggressive compiler optimisation, as the issue does not occur if I run with -gcflags '-N -l'

What version of Go are you using (go version)?

13:00 $ go1.11beta1 version
go version go1.11beta1 darwin/amd64

Does this issue reproduce with the latest release?

• go 1.11beta1 - yes
• go 1.11beta1 with -gcflags '-N -l' - no
• go 1.10.2 - no

What operating system and processor architecture are you using (go env)?

12:57 $ go1.11beta1 env
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/benbrooks/Library/Caches/go-build"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/benbrooks/dev/go"
GOPROXY=""
GORACE=""
GOROOT="/Users/benbrooks/sdk/go1.11beta1"
GOTMPDIR=""
GOTOOLDIR="/Users/benbrooks/sdk/go1.11beta1/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/00/8p7hkgx9061b1msn2gmgn9bw0000gn/T/go-build153122456=/tmp/go-build -gno-record-gcc-switches -fno-common"
VGOMODROOT=""

What did you do?

Run this under both 1.10.2 and 1.11beta1 and compare.

https://play.golang.org/p/pdNCtWFnsGj

What did you expect to see?

The loop stop on the third iteration.

What did you see instead?

The loop never finishing.

If it's of any use, here's a diff of the generated instructions when building with and without -gcflags '-N -l' under go1.11beta1

13:32 $ diff -w -d --suppress-common-lines default-main.asm noopt-main.asm
4,40c4,36
<   main.go:3             0x104d94d               764b                    JBE 0x104d99a
<   main.go:3             0x104d94f               4883ec18                SUBQ $0x18, SP
<   main.go:3             0x104d953               48896c2410              MOVQ BP, 0x10(SP)
<   main.go:3             0x104d958               488d6c2410              LEAQ 0x10(SP), BP
<   main.go:3             0x104d95d               31c0                    XORL AX, AX
<   main.go:6             0x104d95f               eb2a                    JMP 0x104d98b
<   main.go:6             0x104d961               4889442408              MOVQ AX, 0x8(SP)
<   main.go:7             0x104d966               e8a549fdff              CALL runtime.printlock(SB)
<   main.go:7             0x104d96b               488b442408              MOVQ 0x8(SP), AX
<   main.go:7             0x104d970               48890424                MOVQ AX, 0(SP)
<   main.go:7             0x104d974               e80751fdff              CALL runtime.printint(SB)
<   main.go:7             0x104d979               e8124cfdff              CALL runtime.printnl(SB)
<   main.go:7             0x104d97e               e80d4afdff              CALL runtime.printunlock(SB)
<   main.go:6             0x104d983               488b442408              MOVQ 0x8(SP), AX
<   main.go:6             0x104d988               48ffc0                  INCQ AX
<   main.go:6             0x104d98b               4885c0                  TESTQ AX, AX
<   main.go:6             0x104d98e               7dd1                    JGE 0x104d961
<   main.go:6             0x104d990               488b6c2410              MOVQ 0x10(SP), BP
<   main.go:6             0x104d995               4883c418                ADDQ $0x18, SP
<   main.go:6             0x104d999               c3                      RET
<   main.go:3             0x104d99a               e82189ffff              CALL runtime.morestack_noctxt(SB)
<   main.go:3             0x104d99f               eb9f                    JMP main.main(SB)
<   :-1                   0x104d9a1               cc                      INT $0x3
<   :-1                   0x104d9a2               cc                      INT $0x3
<   :-1                   0x104d9a3               cc                      INT $0x3
<   :-1                   0x104d9a4               cc                      INT $0x3
<   :-1                   0x104d9a5               cc                      INT $0x3
<   :-1                   0x104d9a6               cc                      INT $0x3
<   :-1                   0x104d9a7               cc                      INT $0x3
<   :-1                   0x104d9a8               cc                      INT $0x3
<   :-1                   0x104d9a9               cc                      INT $0x3
<   :-1                   0x104d9aa               cc                      INT $0x3
<   :-1                   0x104d9ab               cc                      INT $0x3
<   :-1                   0x104d9ac               cc                      INT $0x3
<   :-1                   0x104d9ad               cc                      INT $0x3
<   :-1                   0x104d9ae               cc                      INT $0x3
<   :-1                   0x104d9af               cc                      INT $0x3
---
>   main.go:3             0x104d94d               7676                    JBE 0x104d9c5
>   main.go:3             0x104d94f               4883ec20                SUBQ $0x20, SP
>   main.go:3             0x104d953               48896c2418              MOVQ BP, 0x18(SP)
>   main.go:3             0x104d958               488d6c2418              LEAQ 0x18(SP), BP
>   main.go:4             0x104d95d               48c744240803000000      MOVQ $0x3, 0x8(SP)
>   main.go:6             0x104d966               48c744241000000000      MOVQ $0x0, 0x10(SP)
>   main.go:6             0x104d96f               eb00                    JMP 0x104d971
>   main.go:6             0x104d971               48837c241000            CMPQ $0x0, 0x10(SP)
>   main.go:6             0x104d977               7d02                    JGE 0x104d97b
>   main.go:6             0x104d979               eb48                    JMP 0x104d9c3
>   main.go:6             0x104d97b               488b442408              MOVQ 0x8(SP), AX
>   main.go:6             0x104d980               4839442410              CMPQ AX, 0x10(SP)
>   main.go:6             0x104d985               7c02                    JL 0x104d989
>   main.go:6             0x104d987               eb2e                    JMP 0x104d9b7
>   main.go:7             0x104d989               e88249fdff              CALL runtime.printlock(SB)
>   main.go:7             0x104d98e               488b442410              MOVQ 0x10(SP), AX
>   main.go:7             0x104d993               48890424                MOVQ AX, 0(SP)
>   main.go:7             0x104d997               e8e450fdff              CALL runtime.printint(SB)
>   main.go:7             0x104d99c               e8ef4bfdff              CALL runtime.printnl(SB)
>   main.go:7             0x104d9a1               e8ea49fdff              CALL runtime.printunlock(SB)
>   main.go:7             0x104d9a6               eb00                    JMP 0x104d9a8
>   main.go:6             0x104d9a8               488b442410              MOVQ 0x10(SP), AX
>   main.go:6             0x104d9ad               48ffc0                  INCQ AX
>   main.go:6             0x104d9b0               4889442410              MOVQ AX, 0x10(SP)
>   main.go:6             0x104d9b5               ebba                    JMP 0x104d971
>   main.go:6             0x104d9b7               eb00                    JMP 0x104d9b9
>   main.go:6             0x104d9b9               488b6c2418              MOVQ 0x18(SP), BP
>   main.go:6             0x104d9be               4883c420                ADDQ $0x20, SP
>   main.go:6             0x104d9c2               c3                      RET
>   main.go:6             0x104d9c3               ebf4                    JMP 0x104d9b9
>   main.go:3             0x104d9c5               e8f688ffff              CALL runtime.morestack_noctxt(SB)
>   main.go:3             0x104d9ca               e971ffffff              JMP main.main(SB)
>   :-1                   0x104d9cf               cc                      INT $0x3

[ianlancetaylor]

CC @randall77

May be a dup of #26116.

[randall77]

Yes, this is a dup of #26116. My CL for that issue also fixes this one.