If I'm forced to guess, the following would be what is emphasized and its possible questions that prompted the sentence.
- 俺はご飯を食う - neutral statement
- ご飯を俺は食う ← 何を食べますか What do you eat?
- ご飯を食う俺は ← 何をしますか What do you do?
But, in my opinion, word order is not usually used for emphasizing. For 2 and 3, more natural responses would be (especially if the speaker calls himself 俺)
That is, just say the information being asked.
If it is not replying and declaring I eat (meal) (though it's a bit hard to imagine such contexts), then it almost invariably translates to ご飯を食う, regardless of which part you want to emphasize. Emphasis would be put by saying the word a little louder, pronouncing each syllable (= hiragana) distinctly.
In a similar way, 俺はご飯を食う is not really natural because 俺は will be omitted even in a neutral statement. A possible context to put 俺は explicitly would be that the person being talked to is going out, and the speaker says "You go out, on the other hand I eat."
A natural question would be how to emphasize 俺, for example to answer Who eats the meal? The answer is 俺がご飯を食う = It is me that eat the meal. This emphasizing works whether or not it is a reply to a question.
BTW ご飯を食う is not really right in terms of collocation. Either ご飯を食べる or 飯(めし)を食う