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734e4c4a52
This adds an optional 16 bit x 16 bit signed multiplier and uses it for multiply instructions that return the low 64 bits of the product (mull[dw][o] and mulli, but not maddld) when the operands are both in the range -2^15 .. 2^15 - 1. The "short" 16-bit multiplier produces its result combinatorially, so a multiply that uses it executes in one cycle. This improves the coremark result by about 4%, since coremark does quite a lot of multiplies and they almost all have operands that fit into 16 bits. The presence of the short multiplier is controlled by a generic at the execute1, SOC, core and top levels. For now, it defaults to off for all platforms, and can be enabled using the --has_short_mult flag to fusesoc. Signed-off-by: Paul Mackerras <paulus@ozlabs.org>