cmd/compile: possible issue in optimization of -(c+-x)

[josharian]

amd64.rules contains this rule:

(NEGQ (ADDQconst [c] (NEGQ x))) && c != -(1<<31) -> (ADDQconst [-c] x)

It was added in https://go-review.googlesource.com/c/go/+/38311/.

I don't understand the constraint c != -(1<<31): Why it is necessary at all, and if it is necessary, why it isn't say -(1<<63). If it does need to be -(1<<63), then we may be generating bad code. As such, tentatively marking Go 1.13.

cc @randall77

[randall77]

ADDQconst on x86 requires that c be an int32.

That's why the restriction is there, so that we don't generate an ADDQconst of 1<<31 (which is invalid) from an ADDQconst of -1<<31 (which is valid).

[josharian]

Ah! Thanks.

[gopherbot]

Change https://golang.org/cl/175778 mentions this issue: cmd/compile: add more Neg/Com optimizations