#!stacks
"sigpanic" && "runtime.(*gcBits).bytep"

Issue created by stacks.

Crash in GC sweep. 

// gcBits is an alloc/mark bitmap. This is always used as gcBits.x.
type gcBits struct {
    _ sys.NotInHeap
    x uint8
}

// bytep returns a pointer to the n'th byte of b.
func (b *gcBits) bytep(n uintptr) *uint8 {
    return addb(&b.x, n) <--------- SEGV
}
...
func addb(p *byte, n uintptr) *byte {
    return (*byte)(unsafe.Pointer(uintptr(unsafe.Pointer(p)) + n))
}

~~Weirdly, the arm64 code for bytep (when noninlined) shows a MOVB where I would have expected a full-width load. Can someone explain it? [Never mind: it's a nil check on the pointer; the result is discarded.]~~

b is invalid.

(NB, this disassembly is from go1.25 devel on darwin/arm64.)
runtime.(*gcBits).bytep STEXT size=16 args=0x10 locals=0x0 funcid=0x0 align=0x0 leaf
    0x0000 00000    L2509   TEXT    runtime.(*gcBits).bytep(SB), LEAF|NOFRAME|ABIInternal, $0-16
    0x0000 00000    L2509   FUNCDATA    $0, gclocals·g5+hNtRBP6YXNjfog7aZjQ==(SB)
    0x0000 00000    L2509   FUNCDATA    $1, gclocals·g5+hNtRBP6YXNjfog7aZjQ==(SB)
    0x0000 00000    L2509   FUNCDATA    $5, runtime.(*gcBits).bytep.arginfo1(SB)
    0x0000 00000    L2509   FUNCDATA    $6, runtime.(*gcBits).bytep.argliveinfo(SB)
    0x0000 00000    L2509   PCDATA  $3, $1
    0x0000 00000    L2510   PCDATA  $0, $-2
    0x0000 00000    L2510   MOVB    (R0), R27     load b.x, SEGV
    0x0004 00004    L0980   ADD R1, R0, R0            add n 
    0x0008 00008            NOP
    0x0008 00008    L2510   RET (R30)
    0x0000 1b 00 80 39 00 00 01 8b c0 03 5f d6 00 00 00 00  ...9......_.....

This stack dWF4MQ was reported by telemetry:

golang.org/x/tools/gopls@v0.18.1 go1.23.6 linux/amd64 vscode (1)

@golang/compiler