// It clobbers FLAGS. It does not clobber any general-purpose registers,
// but may clobber others (e.g., SSE registers).
// Typical use would be, when doing *(CX+88) = AX
//     CMPL    $0, runtime.writeBarrier(SB)
//     JEQ     dowrite
//     CALL    runtime.gcBatchBarrier2(SB)
//     MOVL    AX, (DI)
//     MOVL    88(CX), DX
//     MOVL    DX, 4(DI)
// dowrite:
//     MOVL    AX, 88(CX)