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

---
 backend/regfile.h       |  1 +
 frontend/decode.h       | 15 ++++++++++++---
 inst.h                  |  1 +
 tests/loop-indirect.hex |  1 +
 4 files changed, 15 insertions(+), 3 deletions(-)
 create mode 100644 tests/loop-indirect.hex

diff --git a/backend/regfile.h b/backend/regfile.h
index 276504c..4cf328a 100644
--- a/backend/regfile.h
+++ b/backend/regfile.h
@@ -33,6 +33,7 @@ namespace backend {
                 auto i = writebackp.read();
                 if (i.generation == generation_down) {
                     pte(i.transaction, "W", fmt::format("writeback gen={} pc={:x}", generation_down, pc));
+                    assert(pc == i.init_pc);
                     auto old_pc = pc;
                     pc = i.linear_next_pc;
                     switch (i.field[OPCODE]) {
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;
diff --git a/inst.h b/inst.h
index 15ae9d7..5f96fe3 100644
--- a/inst.h
+++ b/inst.h
@@ -8,6 +8,7 @@ struct inst {
     infra::transaction transaction;
     unsigned int generation;
     unsigned int size = 0;
+    std::uint64_t init_pc = 0;
     std::uint64_t linear_next_pc;
     std::optional<std::uint64_t> predicted_next_pc;
     std::uint64_t field[4] = {};
diff --git a/tests/loop-indirect.hex b/tests/loop-indirect.hex
new file mode 100644
index 0000000..c8b07f7
--- /dev/null
+++ b/tests/loop-indirect.hex
@@ -0,0 +1 @@
+00      # 0: jmp r0 if r0 == 0
-- 
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