The gemara says that one can read shema before and after amud ha'shachar and fulfill both the shema of the evening and shema of the morning. The gemara then has a second lashon that says one can read shema both prior to sunrise and after sunrise and fulfill both the shema of the evening and the shema of the morning. Rabbeinu Yona (2a) says that the Rif holds that these leshonos don't disagree. The first lashon which says that evening shema can be read before amud ha'shachar (at least m'doraysa), and the morning after amud ha'shachar is even in a normal circumstance. The second lashon which talks about before sunrise and after sunrise is speaking about a sha'as ha'dechak where a person could not read shema earlier, so they can read the night time shema before sunrise even after alos ha'shachar, and then again after sunrise. Actually, Rabbeinu Yona says that when the gemara says that the morning shema could be read after sunrise, it is to the exclusion of the evening shema which cannot be read after sunrise, but the ideal time to read the morning shema is just before sunrise as the gemara says on 9b. Tosafos 9a in the second answer also says that one can read the even shema before misheyakir and the morning shema after misheyakir and be yotzei. Even in the lashon that allows the morning shema to be read after alos, Tosafos 8b holds that it cannot be read until misheyakir, and when the gemara says after also ha'shachar, it is לאו דוקא. However, Tosafos 9b implies that even lichatchila one can say shema in the morning after alos. In Shulchan Aruch (o.c. 58) we pasken that ideally one should say the morning shema just before sunrise and daven with sunrise as the vasikin do. If one is unable to do the מצוה מן המובחר, they can say shema as early as misheyakir, and from that point it should be done as soon as possible and must be done before 3 hours of the day. In a sha'as ha'dechak one can say it as early as alos hashachar, provided that they have said the evening shema before alos hashachar (m.b. cites gr"a who doesn't consider it to be a contradiction to say both evening and morning shema after alos, since it is both a time when people are sleeping and a time when people get up.
Rav Moshe has a teshuva where he explains the Mishna in Megilla that says all day mitzvos can be done from alos, but lichatchila should be done after sunrise. Rashi says that m'doraysa the z'man is alos, but m'drabonon they wanted us to wait until sunrise to be sure that it is day. Rav Moshe understands from the Rambam that both the lichatchila and the bidieved are on a Torah level. The Torah considers alos ha'shachar to be day on a bidieved level, and sunrise to be day on a lichtachila level. It seems to me that the language of Rabbeinu Yona 2a strongly supports Rav Moshe's chiddush:
אע"פ שאחר שעלה עמוד השחר יוצא ידי חובה מק"ש של יום, קורא אותו לילה מפני שעיקר היום אינו אלא אחר הנץ החמה ואילך, וכל דבר שמצותו ביום לכתחלה צריך לעשותו מהנץ החמה ואילך, וכיון דלכתחלה אינו יום אע"פ שבדיעבד יוצא מק"ש של יום, קורא אותו לילה