Skip to content
New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

cmd/compile: allocation escapes to heap with "non-constant size" with constant size >= 64K #41635

kipply opened this issue Sep 26, 2020 · 2 comments


Copy link

@kipply kipply commented Sep 26, 2020

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

go version go1.14.2 darwin/amd64

Does this issue reproduce with the latest release?


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 ALTree changed the title allocation escapes to heap with "non-constant size" with constant size >= 64K cmd/compile: allocation escapes to heap with "non-constant size" with constant size >= 64K Sep 26, 2020
@ALTree ALTree added this to the Go1.16 milestone Sep 26, 2020
@ALTree ALTree self-assigned this Sep 26, 2020
Copy link

@ALTree ALTree commented Sep 26, 2020

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.


Copy link

@gopherbot gopherbot commented Sep 26, 2020

Change mentions this issue: cmd/compile: fix escape reason for MAKESLICE with no cap


@gopherbot gopherbot closed this in e572218 Sep 28, 2020
@golang golang locked and limited conversation to collaborators Sep 28, 2021
Sign up for free to subscribe to this conversation on GitHub. Already have an account? Sign in.
None yet
Linked pull requests

Successfully merging a pull request may close this issue.

None yet
3 participants