cmd/compile: avoid dead code in escape analysis

[dsnet]

Using go1.7

Consider the following function:

func Foo(codes []int) int {
    const debug = false
    if debug && !sort.IntsAreSorted(codes) {
        panic("input slice is not sorted")
    }

    var s int
    for _, i := range codes {
        s += i
    }
    return s
}

Currently, codes escapes to the heap. I expect it to not escape since the function where it does escape sort.IntsAreSorted is dead. (the fact that codes escapes at all in sort.IntsAreSorted is a separate concern).

/cc @dr2chase

[josharian]

Ordering the many compiler stages is a tricky problem. It'd also be nice for dead code not to count against a function's budget for inlining. It might be nice to add an early, crude deadcode pass, which implies also doing a bunch of constant folding. And we'd need to make sure all code gets typechecked, dead or not.

[navytux]

Looks to be the same as #12397.

[minux]

Could we just do escape analysis at the very end? After at least inlining and DCE.

[dr2chase]

Right now DCE is in SSA and SSA uses results of escape analysis, so that's moderately tricky. We would need an even-more-generic form of SSA that was (initially) agnostic about whether things were heap or stack allocated.

[minux]

On Feb 1, 2017 10:38 AM, "dr2chase" <notifications@github.com> wrote: Right now DCE is in SSA and SSA uses results of escape analysis, so that's moderately tricky. We would need an even-more-generic form of SSA that was (initially) agnostic about whether things were heap or stack allocated. But running escape analysis after inlining and DCE would be well worth the effort.

[randall77]

On Wed, Feb 1, 2017 at 8:10 AM, Minux Ma ***@***.***> wrote: On Feb 1, 2017 10:38 AM, "dr2chase" ***@***.***> wrote: Right now DCE is in SSA and SSA uses results of escape analysis, so that's moderately tricky. We would need an even-more-generic form of SSA that was (initially) agnostic about whether things were heap or stack allocated. But running escape analysis after inlining and DCE would be well worth the effort.

Escape analysis does run after inlining. I agree that running escape analysis after DCE would be nice. I suspect it is a ton of work, though.

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[gopherbot]

CL https://golang.org/cl/37508 mentions this issue.