cmd/compile: "x[i] op= y" evaluates x[i] more than once

[mdempsky]

The Go spec says "An assignment operation x op= y where op is a binary arithmetic operator is equivalent to x = x op (y) but evaluates x only once."

I think for both of these sample programs the only valid outputs are "2 2" and "12 2":

• Slices: https://play.golang.org/p/p4fFBLx0xM
• Maps: https://play.golang.org/p/-lKZXXnndS

An output of "11 2" indicates that m[0] was evaluated twice: once indexing into the old slice/map to read the value 1, and then once into the new to store the value 11.

Notably, gccgo outputs "12 2" for slices, but cmd/compile outputs "11 2" for slices, and both output "11 2" for maps.

/cc @griesemer @ianlancetaylor

[mdempsky]

Slightly simpler repro cases: the only valid outputs (IMO) are "-1 2" and "1 -2":

• Slices: https://play.golang.org/p/9sluA-AaH1
• Maps: https://play.golang.org/p/SZo6hHIdun

An output of "1 -1" indicates the same failure as above.

[gopherbot]

Change https://golang.org/cl/60091 mentions this issue: cmd/compile: fix evaluation order for OASOP