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runtime: gccheckmark does nothing? #69074

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@MikeMitchellWebDev

Description

@MikeMitchellWebDev

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

$ go version
go version go1.23.0 darwin/amd64

Does this issue reproduce with the latest release?

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

go env Output
$ go env
GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/Users/mm/Library/Caches/go-build'
GOENV='/Users/mm/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='/Users/mm/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='/Users/mm/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/local/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/local/go/pkg/tool/darwin_amd64'
GOVCS=''
GOVERSION='go1.23.0'
GODEBUG=''
GOTELEMETRY='local'
GOTELEMETRYDIR='/Users/mm/Library/Application Support/go/telemetry'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='/usr/bin/clang'
CXX='clang++'
CGO_ENABLED='1'
GOMOD='/Users/mm/go/src/github.com/MikeMitchellWebDev/gc_knobs/go.mod'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/5y/wtzzmjlj5v52pg7wr8ptbg_m0000gp/T/go-build534087183=/tmp/go-build -gno-record-gcc-switches -fno-common'
uname -v: Darwin Kernel Version 20.6.0: Thu Jul  6 22:12:47 PDT 2023; root:xnu-7195.141.49.702.12~1/RELEASE_X86_64
ProductName:	macOS
ProductVersion:	11.7.10
BuildVersion:	20G1427
lldb --version: lldb-1300.0.42.3
Swift version 5.5.2-dev

What did you do?

2 things: First I ran a program with GODEBUG=gccheckmark=1. When setCheckmark wasn't called, I tried to build the go source code after manually setting var useCheckmark = true

What did you expect to see?

It depends on whether I had simply run a program with GODEBUG=gccheckmark=1 or altered the source code as described above.

  1. setCheckmark to be called at some point

  2. The source code to build after I set useCheckmark = true

What did you see instead?

  1. With the source code "as is", setCheckmark is never called. There is a very narrow window in which useCheckmark is set to true inside gcMarkTermination, so this line of code in mgcmark.go never runs
	if useCheckmark {
		if setCheckmark(obj, base, off, mbits) {
  1. When I manually set useCheckmark = true, the source code wouldn't build. There's always a fatal error "fatal error: checkmark found unmarked object"

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    NeedsInvestigationSomeone must examine and confirm this is a valid issue and not a duplicate of an existing one.compiler/runtimeIssues related to the Go compiler and/or runtime.

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