A method and a device for abstracting instruction sequences in a computer program. First, a control flow graph of the program is generated and analysed in order to detect multiple occurrences of a same instruction sequence (504, 506). Then, a function including the longest sequence common to at least two instruction sequences from a plurality of sequences having a common instruction sequence of equal or shorter length compared to the longest sequence is created (512). Finally, the original occurrences of the instruction sequences in the plurality of sequences with a reference to a proper position in the newly created function are deleted and a reference to a proper position in the created function inserted instead (514).