Tuning the GNU instruction scheduler to superscalar microprocessors

In the past, the GNU C compiler (GCC) has been successfully ported to several superscalar microprocessors. For that purpose, the instruction timing of the target processor was usually modeled in a straightforward manner. Unfortunately, in our experience, this is likely to lead the instruction scheduler astray. In this paper, we describe some of our experiments that revealed such flaws, concerning the DEC Alpha 21064 as well as other superscalar RISC processors. We analyze the circumstances that led to poorly scheduled code and demonstrate how the machine description supplied for a superscalar processor can be modified to fit some of these problems without hampering the portability of the GCC. On the other hand, we show situations for which we do not have a solution within the given framework.