code compile using -mcpu for ARM platform

When using gcc to compile c code for ARM platform, we set object platform by using:

     -mcpu = xxxxxx

To what extent will that affect results of compiling ?

For example:

     -mcpu = cortex-a8

and

     -mcpu = cortex-a9

When I use results compiled with -mcpu=cortex-a8 to run on A9 platform, will it cause some problems?

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