proposal: cmd/compile: add `GOEXPERIMENT=2pointers-closures`

[Jorropo]

Proposal Details

Add a GOEXPERIMENT 2pointers-closures which changes unsafe.Sizeof(func(){}) from unsafe.Sizeof(uintptr(0)) to unsafe.Sizeof(uintptr(0)) * 2.

This is an ABI breaking change I want to try targeted at optimizing the the typescript compiler.
It would transform closure := ptr.method from a heap allocation at worst.
To an LEAQ and a MOV.

basically use a representation for closures that is nearly identical to a single method interface
so the above pseudo code would compile to:

closure := abi.closure{&type.method.descriptor, ptr}

where type.method.descriptor is a .rodata objecting contains the method's PC and all of reflect's required info.

[Jorropo]

It might be a bit faster in normal cases, the biggest downside is that in theory an ABI0 assembly function might do something with an object storing a closure and then everything would break.

[gabyhelp]

Related Issues

• cmd/gc: use two-word representation for func #4081 (closed)

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