What steps will reproduce the problem?
1. run go test cmd/gofmt (long test, GOMAXPROCS=4)
2.
3.

What is the expected output? What do you see instead?
% go test -v cmd/gofmt
=== RUN TestRewrite-4
--- PASS: TestRewrite-4 (0.04 seconds)
=== RUN TestAll-4
panic: illegal base or size

goroutine 6 [running]:
go/token.(*FileSet).AddFile(0xf840044b00, 0xf8403e7180, 0x3b, 0x111db0050f5a0, 0x0, ...)
    /home/mikioh/go/src/pkg/go/token/position.go:333 +0x217
go/parser.(*parser).init(0xf8405c96c0, 0xf840044b00, 0xf8403e7180, 0xf80000003b,
0xf84065c000, ...)
    /home/mikioh/go/src/pkg/go/parser/parser.go:66 +0x7b
go/parser.ParseFile(0xf840044b00, 0xf8403e7180, 0x1fe000000003b, 0x4ca348, 0xf840713ba0,
...)
    /home/mikioh/go/src/pkg/go/parser/interface.go:91 +0x130
cmd/gofmt.parse(0xf8403e7180, 0x3b, 0xf84065c000, 0x1fe00000111db, 0x11100, ...)
    /home/mikioh/go/src/cmd/gofmt/gofmt.go:259 +0x82
cmd/gofmt.testFile(0xf84006b1c0, 0xf84007dc60, 0xf84007dcc0, 0xf8403e7180, 0xf80000003b,
...)
    /home/mikioh/go/src/cmd/gofmt/long_test.go:61 +0x328
cmd/gofmt.testFiles(0xf84006b1c0, 0xf840115000, 0xf8400f7050, 0x0, 0x0, ...)
    /home/mikioh/go/src/cmd/gofmt/long_test.go:94 +0xb0
created by cmd/gofmt.TestAll
    /home/mikioh/go/src/cmd/gofmt/long_test.go:146 +0x18d

goroutine 1 [chan receive]:
testing.RunTests(0x400c00, 0x609b38, 0x200000002, 0x1, 0x5fe4a8, ...)
    /home/mikioh/go/src/pkg/testing/testing.go:350 +0x79f
testing.Main(0x400c00, 0x609b38, 0x200000002, 0x60d940, 0x0, ...)
    /home/mikioh/go/src/pkg/testing/testing.go:285 +0x7a
main.main()
    /tmp/go-build244483973/cmd/gofmt/_test/_testmain.go:45 +0x91

goroutine 2 [syscall]:
created by runtime.main
    /home/mikioh/go/src/pkg/runtime/proc.c:221

goroutine 4 [chan receive]:
cmd/gofmt.TestAll(0xf84006b1c0, 0x2fb8cc0)
    /home/mikioh/go/src/cmd/gofmt/long_test.go:151 +0x1c5
testing.tRunner(0xf84006b1c0, 0x609b50, 0x0, 0x0)
    /home/mikioh/go/src/pkg/testing/testing.go:273 +0x6f
created by testing.RunTests
    /home/mikioh/go/src/pkg/testing/testing.go:349 +0x77c

goroutine 5 [runnable]:
cmd/gofmt._func_006(0x80075efa8, 0x80075efb0, 0x4adf81, 0xf8402bbed0, 0xf80000002b, ...)
    /home/mikioh/go/src/cmd/gofmt/long_test.go:108 +0x123
path/filepath.walk(0xf8402bbed0, 0x20000002b, 0xf840001af0, 0xf840623370, 0xf84012a040,
...)
    /home/mikioh/go/src/pkg/path/filepath/path.go:302 +0x6f
path/filepath.walk(0xf840071210, 0x200000023, 0xf840001af0, 0xf840623050, 0xf84012a040,
...)
    /home/mikioh/go/src/pkg/path/filepath/path.go:320 +0x396
path/filepath.walk(0xf840088120, 0x20000001d, 0xf840001af0, 0xf8402dd8c0, 0xf84012a040,
...)
    /home/mikioh/go/src/pkg/path/filepath/path.go:320 +0x396
path/filepath.walk(0xf8403f6f60, 0x200000017, 0xf840001af0, 0xf84020b320, 0xf84012a040,
...)
    /home/mikioh/go/src/pkg/path/filepath/path.go:320 +0x396
path/filepath.walk(0xf840169e00, 0x200000013, 0xf840001af0, 0xf840134370, 0xf84012a040,
...)
    /home/mikioh/go/src/pkg/path/filepath/path.go:320 +0x396
path/filepath.walk(0x7fffffffef31, 0x80000000f, 0xf840001af0, 0xf8400f7e10,
0xf84012a040, ...)
    /home/mikioh/go/src/pkg/path/filepath/path.go:320 +0x396
path/filepath.Walk(0x7fffffffef31, 0xf, 0xf84012a040, 0x80075efb0, 0xf84012a040, ...)
    /home/mikioh/go/src/pkg/path/filepath/path.go:337 +0xd1
cmd/gofmt.genFilenames(0xf84006b1c0, 0xf840115000, 0x0, 0x0)
    /home/mikioh/go/src/cmd/gofmt/long_test.go:124 +0x194
created by cmd/gofmt.TestAll
    /home/mikioh/go/src/cmd/gofmt/long_test.go:141 +0x123

goroutine 7 [running]:
created by cmd/gofmt.TestAll
    /home/mikioh/go/src/cmd/gofmt/long_test.go:146 +0x18d
exit status 2
FAIL    cmd/gofmt   2.691s

Please use labels and text to provide additional information.
It's race in test code.
Also FreeBSD is very good for detecting race issues.