Branch prediction

[Willian-Nunes]

This implements static branch prediction. It takes an instruction and its PC and determines if it's a branch or a jump and calculates its target. For jumps it will always predict taken. For branches it will predict taken if the PC offset is negative.

Improved performance by about 14%.

Coremark ticks for an iteration W/o Branch Prediction --> 13090 With Branch Prediction --> 11467

Coremark Iterations per second W/o Branch Prediction --> 76 With Branch Prediction --> 87

Normalized performance score is now 0.872 iterations/Mhz, this is similar to IBex core that has a normalized performance score of about 0.904 Iterations/Mhz.

Comparing with 1.0.0 version (TCC release) that presented 0.670 Iterations/Mhz the RS5 core improved a lot in performance until now (about 30%) with branch prediction mechanism and with one stage forwarding.

[Willian-Nunes]

with branch prediction

Without

Not a big area overhead but inserted timing problems.

TO DO: Study a better way to overcome timing issues.

[Willian-Nunes]

This method presented bad timing and was replaced by a more efficient with new decoder