// Atomically decreases a given *system* memory stat. Same comments as
// mSysStatInc apply.
//go:nosplit
func mSysStatDec(sysStat *uint64, n uintptr) {
if sys.BigEndian != 0 {
atomic.Xadd64(sysStat, -int64(n))
return
}
if val := atomic.Xadduintptr((*uintptr)(unsafe.Pointer(sysStat)), uintptr(-int64(n))); val+n < n {
print("runtime: stat underflow: val ", val, ", n ", n, "\n")
exit(2)
}
}
// Atomically increases a given *system* memory stat. We are counting on this
// stat never overflowing a uintptr, so this function must only be used for
// system memory stats.
//
// The current implementation for little endian architectures is based on
// xadduintptr(), which is less than ideal: xadd64() should really be used.
// Using xadduintptr() is a stop-gap solution until arm supports xadd64() that
// doesn't use locks. (Locks are a problem as they require a valid G, which
// restricts their useability.)
//
// A side-effect of using xadduintptr() is that we need to check for
// overflow errors.
//go:nosplit
func mSysStatInc(sysStat *uint64, n uintptr) {
if _BigEndian != 0 {
atomic.Xadd64(sysStat, int64(n))
return
}
if val := atomic.Xadduintptr((*uintptr)(unsafe.Pointer(sysStat)), n); val < n {
print("runtime: stat overflow: val ", val, ", n ", n, "\n")
exit(2)
}
}
func TestXadduintptr(t *testing.T) {
const N = 20
const iter = 100000
inc := uintptr(100)
total := uintptr(0)
runParallel(N, iter, func() {
atomic.Xadduintptr(&total, inc)
})
if want := uintptr(N * iter * inc); want != total {
t.Fatalf("xadduintpr error, want %d, got %d", want, total)
}
total = 0
runParallel(N, iter, func() {
atomic.Xadduintptr(&total, inc)
atomic.Xadduintptr(&total, uintptr(-int64(inc)))
})
if total != 0 {
t.Fatalf("xadduintpr total error, want %d, got %d", 0, total)
}
}
// Tests that xadduintptr correctly updates 64-bit values. The place where
// we actually do so is mstats.go, functions mSysStat{Inc,Dec}.
func TestXadduintptrOnUint64(t *testing.T) {
if sys.BigEndian != 0 {
// On big endian architectures, we never use xadduintptr to update
// 64-bit values and hence we skip the test. (Note that functions
// mSysStat{Inc,Dec} in mstats.go have explicit checks for
// big-endianness.)
t.Skip("skip xadduintptr on big endian architecture")
}
const inc = 100
val := uint64(0)
atomic.Xadduintptr((*uintptr)(unsafe.Pointer(&val)), inc)
if inc != val {
t.Fatalf("xadduintptr should increase lower-order bits, want %d, got %d", inc, val)
}
}