Begin to implement the Simple-V loop
authorCesar Strauss <cestrauss@gmail.com>
Sat, 6 Mar 2021 19:29:34 +0000 (16:29 -0300)
committerCesar Strauss <cestrauss@gmail.com>
Sat, 6 Mar 2021 19:29:34 +0000 (16:29 -0300)
After returning from executing an instruction, decide whether to return
to Fetch, or go repeat Execute again.

1) If PC or SVSTATE were updated, go directly to Fetch, without updating
   either
2) If there is no vector output, or it's the last VL loop iteration, go
   back to Fetch as well, but update the PC. In the latter case, also
   reset SRCSTEP
3) Otherwise, we are still in the loop, so increment SVSTEP, and go back
   to Execute. But, first, pass through a new state, DECODE_SV, so the new
   register numbers can be decoded.

src/soc/simple/issuer.py

index f1e197eac06e7f2faa043993fb66b6d4a9f38df2..6ba8b39ddcb3bbb9950d0a1ded49d2ceefeb8c9a 100644 (file)
@@ -310,29 +310,57 @@ class TestIssuerInternal(Elaboratable):
                 with m.If(~dbg.core_stop_o & ~core_rst):
                     comb += exec_pc_ready_i.eq(1)
                     with m.If(exec_pc_valid_o):
                 with m.If(~dbg.core_stop_o & ~core_rst):
                     comb += exec_pc_ready_i.eq(1)
                     with m.If(exec_pc_valid_o):
-                        # TODO: update SRCSTEP here (in new_svstate)
-                        #       and set update_svstate to True *as long as*
-                        #       PC / SVSTATE was not modified.  that's an
-                        #       exception (or setvl was called)
-                        # TODO: loop into INSN_EXECUTE if it's a vector
-                        #       instruction and SRCSTEP != VL-1 and
-                        #       PowerDecoder.no_out_vec is True
-                        #       unless PC / SVSTATE was modified, in that
-                        #       case do go back to INSN_FETCH.
-
-                        # before fetch, update the PC state register with
-                        # the NIA, unless PC was modified in execute
-                        with m.If(~pc_changed):
+                        # precalculate srcstep+1
+                        next_srcstep = Signal.like(cur_state.svstate.srcstep)
+                        comb += next_srcstep.eq(cur_state.svstate.srcstep+1)
+                        # was this the last loop iteration?
+                        is_last = Signal()
+                        cur_vl = cur_state.svstate.vl
+                        comb += is_last.eq(next_srcstep == cur_vl)
+
+                        # if either PC or SVSTATE were changed by the previous
+                        # instruction, go directly back to Fetch, without
+                        # updating either PC or SVSTATE
+                        with m.If(pc_changed | sv_changed):
+                            m.next = "INSN_FETCH"
+
+                        # also return to Fetch, when no output was a vector
+                        # (regardless of SRCSTEP and VL), or when the last
+                        # instruction was really the last one of the VL loop
+                        with m.Elif(pdecode2.no_out_vec | is_last):
+                            # before going back to fetch, update the PC state
+                            # register with the NIA.
                             # ok here we are not reading the branch unit.
                             # TODO: this just blithely overwrites whatever
                             #       pipeline updated the PC
                             comb += self.state_w_pc.wen.eq(1 << StateRegs.PC)
                             comb += self.state_w_pc.data_i.eq(nia)
                             # ok here we are not reading the branch unit.
                             # TODO: this just blithely overwrites whatever
                             #       pipeline updated the PC
                             comb += self.state_w_pc.wen.eq(1 << StateRegs.PC)
                             comb += self.state_w_pc.data_i.eq(nia)
-                        m.next = "INSN_FETCH"
+                            # reset SRCSTEP before returning to Fetch
+                            with m.If(~pdecode2.no_out_vec):
+                                comb += new_svstate.srcstep.eq(0)
+                                comb += update_svstate.eq(1)
+                            m.next = "INSN_FETCH"
+
+                        # returning to Execute? then, first update SRCSTEP
+                        with m.Else():
+                            comb += new_svstate.srcstep.eq(next_srcstep)
+                            comb += update_svstate.eq(1)
+                            m.next = "DECODE_SV"
+
                 with m.Else():
                     comb += core.core_stopped_i.eq(1)
                     comb += dbg.core_stopped_i.eq(1)
 
                 with m.Else():
                     comb += core.core_stopped_i.eq(1)
                     comb += dbg.core_stopped_i.eq(1)
 
+            # need to decode the instruction again, after updating SRCSTEP
+            # in the previous state.
+            # mostly a copy of INSN_WAIT, but without the actual wait
+            with m.State("DECODE_SV"):
+                # decode the instruction
+                sync += core.e.eq(pdecode2.e)
+                sync += core.state.eq(cur_state)
+                sync += core.bigendian_i.eq(self.core_bigendian_i)
+                m.next = "INSN_EXECUTE"  # move to "execute"
+
         # check if svstate needs updating: if so, write it to State Regfile
         with m.If(update_svstate):
             comb += self.state_w_sv.wen.eq(1<<StateRegs.SVSTATE)
         # check if svstate needs updating: if so, write it to State Regfile
         with m.If(update_svstate):
             comb += self.state_w_sv.wen.eq(1<<StateRegs.SVSTATE)
@@ -384,9 +412,6 @@ class TestIssuerInternal(Elaboratable):
                     sync += pc_changed.eq(1)
                 with m.If(~core_busy_o): # instruction done!
                     comb += insn_done.eq(1)
                     sync += pc_changed.eq(1)
                 with m.If(~core_busy_o): # instruction done!
                     comb += insn_done.eq(1)
-                    sync += core.e.eq(0)
-                    sync += core.raw_insn_i.eq(0)
-                    sync += core.bigendian_i.eq(0)
                     comb += exec_pc_valid_o.eq(1)
                     with m.If(exec_pc_ready_i):
                         m.next = "INSN_START"  # back to fetch
                     comb += exec_pc_valid_o.eq(1)
                     with m.If(exec_pc_ready_i):
                         m.next = "INSN_START"  # back to fetch