tapirl on //go:fix inline and the source-level inliner

Comment on //go:fix inline and the source-level inliner

It looks the following code will be rewritten badly, but no ways to avoid it? If this is true, maybe the blog article should mention this.

    package main
    
    //go:fix inline
    func handle() {
        recover()
    }
    
    func foo() {
        handle()
    }
    
    func main() {
        defer foo()
        panic("bye")
    }

arjvik 5mo

recover()'s semantics make it so that "pointless" use like this can be inlined in a way that changes its semantics, but "correct" use remains unchanged.

Yes, maybe some code uses recover() to check if its being called as a panic handler, and perhaps `go fix` should add a check for this ("error: function to be inlined calls recover()"), but this isn't a particularly common footgun.

tapirl 5mo

... and perhaps `go fix` should add a check for this (

This is an impossible task. For a library function, you can't know whether or not the function is defer called.

Maybe this is not an important problem. But it would be better if the blog article mentions this.

shoo 5mo

Great example, illustrating go1.26.1 go fix source inline transformation breaking program semantics. Raise it as a bug against go fix?

tapirl 5mo

As I have mentioned, no ways to fix it. Because it is hard to know whether or not the handle function is called in a deferred call.

adonovan 5mo

Thanks, that's a bug. We should never inline a function that directly calls recover. I've filed https://go.dev/issue/78193.

arccy 5mo

Or: your buggy code is no longer buggy.

tapirl 5mo

You claim listens right for this specified example. :D

It is just a demo.

tapirl 5mo

Another example (fixable):

    package main

    import "unsafe"

    //go:fix inline
    func foo[T any]() {
        var t T
        _ = 1 / unsafe.Sizeof(t)
    }

    func main() {
        foo[struct{}]()
    }

Go is a language full of details: https://go101.org/details-and-tips/101.html

tapirl 5mo

another:

   package main

   type T = [8]byte
   var a T

   //go:fix inline
   func foo() T {
      return T{}
   }

   func main() {
      if foo() == a {
      }
   }

filed: https://github.com/golang/go/issues/78170 and https://github.com/golang/go/issues/78169

tapirl 5mo

similar:

    package main

    //go:fix inline
    func foo[T [8]byte | [4]uint16]() {
        var v T
        var n byte = 1 << len(v) >> len(v)
        if n == 0 {
            println("T is [8]byte")
        } else {
            println("T is [4]uint16]")
        }
    }

    func main() {
        foo[[8]byte]()
    }

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