what are files in /data/dalvik-cache, how can i decomplie them?

[Jack Wang]

i have an apk installed on android phone.
i noticed a dex file under /data/dalvik-cache was created with apk similar name, so i adb pull that dex from /data/dalvik-cache to my laptop
and used file against this dex file, and it gives the output as
data@app@com.smile.gifmaker-1.apk@classes.dex: Dalvik dex file (optimized for host) version 036
so i used d2j-dex2jar.sh try to get the corresponding jar, but it failed with:
localhost:dalvic-cache Jack$ d2j-dex2jar.sh data\@app\@com.smile.gifmaker-1.apk\@classes.dex
dex2jar data@app@com.smile.gifmaker-1.apk@classes.dex -> ./data@app@com.smile.gifmaker-1.apk@classes-dex2jar.jar
java.io.IOException: the src file not a .dex or zip file
at com.googlecode.d2j.reader.zip.ZipUtil.readDex(ZipUtil.java:88)
at com.googlecode.d2j.reader.zip.ZipUtil.readDex(ZipUtil.java:57)
at com.googlecode.d2j.reader.zip.ZipUtil.readDex(ZipUtil.java:53)
at com.googlecode.dex2jar.tools.Dex2jarCmd.doCommandLine(Dex2jarCmd.java:104)
at com.googlecode.dex2jar.tools.BaseCmd.doMain(BaseCmd.java:288)
at com.googlecode.dex2jar.tools.Dex2jarCmd.main(Dex2jarCmd.java:32)

so i'm wondering what are exactly this file? how can i get jar/smali out of it?
any clues would be appreciated