ISA change proposal: Allow branches in the second issue slot

[Emoun]

This issue will track the discussion into changing the Patmos ISA to allow branches in the second issue slot.

Motivation

The limitation of not allowing branch instructions in the second issue slot is slightly "artificial" in that there are no technical reasons for it (as there is for load instructions.)
However, allowing branches in the second issue slot can provide significant benefit. One example is string copying, where we can use it in an implementation that only uses 3 cycles per character (amortized):

         pmov   $p1 = $p0
loop:
{  ($p1) lbuc   $r1 = [$r2]     ; Load next char
   ($p1) br     loop           }; Loop if not done
{  ($p1) add    $r2 = $r2, 1    ; Increment source pointer
   ($p1) add    $r3 = $r3, 1   }; Increment target pointer
{  ($p1) sbc    [$r3-1] = $r1   ; Save char
         cmpneq $p1 = $r1, 0   }; Check whether null was reached

Note: There might be a better way to do this by loading 4 chars at a time, but this is the fastest way to do it while loading only 1 char at a time.

Todo:

• Implement in simulator
• Benchmark on simulator