From ce8be82f047a15edd86f5844492ed63705e99914 Mon Sep 17 00:00:00 2001
From: Julian Blake Kongslie
Date: Thu, 22 Sep 2022 13:31:06 -0700
Subject: Add a trivial indirect jump predictor and matching test

---
 frontend/decode.h | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

(limited to 'frontend/decode.h')

diff --git a/frontend/decode.h b/frontend/decode.h
index 717d0f6..cb7bedd 100644
--- a/frontend/decode.h
+++ b/frontend/decode.h
@@ -24,15 +24,20 @@ namespace frontend {
         unsigned int generation_down = 0;
         std::uint64_t pc = 0;
 
+        std::uint64_t indirect_jump_origin = ~(std::uint64_t)0;
+        std::uint64_t indirect_jump_destination = ~(std::uint64_t)0;
+
         static constexpr unsigned int MAX_INST_SIZE = 64;
-        static constexpr unsigned int BYTES_PER_CYCLE = 4;
+        static constexpr unsigned int BYTES_PER_CYCLE = 1;
 
         void clock() {
             if (restartp.can_read()) {
                 auto r = restartp.read();
                 generation_down = r.new_generation;
-                pc = r.new_pc;
+                next_inst.init_pc = pc = r.new_pc;
                 next_inst.size = 0;
+                indirect_jump_origin = r.from_pc;
+                indirect_jump_destination = r.new_pc;
                 for (auto &f : next_inst.field)
                     f = 0;
                 fetch::restart fr;
@@ -95,7 +100,10 @@ namespace frontend {
                                         redirect = target;
                                     }
                                 } else if (!taken.has_value() || taken.value()) {
-                                    unpredictable = true;
+                                    if (next_inst.init_pc == indirect_jump_origin)
+                                        redirect = indirect_jump_destination;
+                                    else
+                                        unpredictable = true;
                                 }
                             }
                             if (redirect.has_value()) {
@@ -106,6 +114,7 @@ namespace frontend {
                                 next_inst.predicted_next_pc.reset();
                             }
                             instp->write(std::move(next_inst));
+                            next_inst.init_pc = pc;
                             next_inst.size = unpredictable ? MAX_INST_SIZE : 0;
                             for (auto &f : next_inst.field)
                                 f = 0;
-- 
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