cmd/compile: stop interface conversions for generic method calls from allocating

Let T be a type parameter, and say we instantiate it with S, a type
that isn't pointer-like (e.g. a pair of ints, or as in 50182, a
slice). Then to call a method m on a variable of type T, the compiler
does essentially:

var v T = ...
i := (interface{m()})(v)
i.m()

The conversion at that second line allocates, as we need to make the
data word for an interface. And in the general case, that interface
may live an arbitrarily long time. But in this case, we know it
doesn't.

The data word of i has type *S.  When we call i.m, we can't call S.m
directly. It is expecting an S, not a *S. We call through a wrapper
defined on *S, which looks like:

func (p *S) m() {
   var s S = *p
   s.m()
}

The value passed in for p is exactly the data word mentioned above. It
never escapes anywhere - the wrapper copies a type S variable out of
*p and p is dead after that. That means that in the situation where we
build an interface for the explicit purpose of calling a method on it,
and use that built interface nowhere else, the allocation of the data
word for that interface is known to die before the call returns and
thus can be stack allocated.

One tricky case is that although the allocation of the backing store
of the interface conversion doesn't escape, pointers we store *inside*
that allocation might escape (in fact they definitely will, unless we
can devirtualize the receiver).

Fixes #50182

Change-Id: I40e893955c2e6871c54ccecf1b9f0cae17871b0d
Reviewed-on: https://go-review.googlesource.com/c/go/+/378178
Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Dan Scales <danscales@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

0 files changed, 0 insertions, 0 deletions