allocRawBytes(nbytes) {
	        // always aligns to 64bit
	        nbytes = ((nbytes + 7) >> 3) << 3;
	        if (this.stackTop + nbytes > this.buffer.byteLength) {
	            const newSize = Math.max(this.buffer.byteLength * 2, this.stackTop + nbytes);
	            const oldU8 = this.viewU8;
	            this.buffer = new ArrayBuffer(newSize);
	            this.updateViews();
	            this.viewU8.set(oldU8);
	            if (this.basePtr != 0) {
	                this.cFreeSpace(this.basePtr);
	            }
	            this.basePtr = this.cAllocSpace(newSize);
	        }
	        const retOffset = this.stackTop;
	        this.stackTop += nbytes;
	        return retOffset;
	    }
Allocate space by number of bytes
@param nbytes Number of bytes.
@note This function always allocate space that aligns to 64bit.