cmd/compile: allocation escapes to heap with "non-constant size" with constant size >= 64K

[kipply]

What version of Go are you using (go version)?

go version go1.14.2 darwin/amd64

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

// skipping

What did you do?

package main

func main() {
  thing := make([]byte, 65536) // also ran with 65535, which is < 64KiB 
}

Ran with go run -gcflags '-m -m' alloc.go

What did you expect to see?

message should say it escapes to heap because of a max value (not sure if this is an escape analysis impl determined value, or more related to my architecture or setup of the stack, or a mix of those)

What did you see instead?

./alloc.go:7:13: make([]byte, 65537) escapes to heap:
./alloc.go:7:13:   flow: {heap} = &{storage for make([]byte, 65537)}:
./alloc.go:7:13:     from make([]byte, 65537) (non-constant size) at ./alloc.go:7:13

it will not escape when I use 65535 instead of 65536.

[ALTree]

Thanks for reporting this. The problem is at the following check in escape.go:

why := "too large for stack"
if n.Op == OMAKESLICE && (!Isconst(n.Left, CTINT) || !Isconst(n.Right, CTINT)) {
	why = "non-constant size"
}

In OMAKESLICE nodes, n.Left is the length and n.Right is the capacity. In a call with implied capacity like make([]byte, 65536), n.Right will be nil, so Isconst(n.Right, CTINT) will return false, and we print non-constant size instead of the correct message.

[gopherbot]

Change https://golang.org/cl/257641 mentions this issue: cmd/compile: fix escape reason for MAKESLICE with no cap