What is The compiler's -json optimization logging

[Falco Wockenfuß]

Hi,

sorry if this is a dumb question, but the Release Notes for go 1.15 have the Note:

The compiler's -json optimization logging now reports large (>= 128 byte) copies and includes explanations of escape analysis decisions. 

But I didn't find anything about "-json" as a flag for go build or similar and google dind't yield anything for go compiler json optimization logging or similar.

Can anyone clarify what this json optimiziation logging is and how it can be used/invoked ? 

[Ian Lance Taylor]

[ + drchase ]

It does seem that this option should be mentioned in cmd/compile/doc.go.

To use it, try "go build -gcflags=json=0,/my/json/dir".

Ian

[Peter Weinberger (温博格)]

And gopls will compute them as diagnostics. For instance, if using vscode and gopls, then in setting.json:

"gopls": {

  "codelens": {

    "gc_details": true,

  }

}

and a clickable 'Toggle gc annotation details' should appear just above the package statement.

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[David Chase]

There is also a command-line proof-of-concept that combines it with profiling information to show you only the missed/unabled optimizations that are also hot spots in the profile.

https://github.com/dr2chase/gc-lsp-tools

There's a lot of checks and things that can't be optimized away, but very few have any effect on a typical program's performance.