From eb3fd68203fee7c63245c702914c2acd3332d65a Mon Sep 17 00:00:00 2001
From: Julian Blake Kongslie
Date: Thu, 22 Sep 2022 11:29:07 -0700
Subject: Initial commit.

---
 frontend/decode.h | 128 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 frontend/decode.h

(limited to 'frontend/decode.h')

diff --git a/frontend/decode.h b/frontend/decode.h
new file mode 100644
index 0000000..717d0f6
--- /dev/null
+++ b/frontend/decode.h
@@ -0,0 +1,128 @@
+#pragma once
+
+#include "frontend/bundle.h"
+#include "frontend/fetch.h"
+#include "infra/port.h"
+#include "inst.h"
+#include "memory/line.h"
+
+namespace frontend {
+    struct decode : public infra::sim {
+        struct restart {
+            unsigned int new_generation;
+            std::uint64_t new_pc;
+            std::uint64_t from_pc;
+        };
+        infra::port<restart> restartp;
+        infra::port<fetch::restart> *fetch_restartp = nullptr;
+
+        infra::port<bundle> bundlep;
+        infra::port<inst> *instp = nullptr;
+
+        inst next_inst;
+        unsigned int generation_up = 0;
+        unsigned int generation_down = 0;
+        std::uint64_t pc = 0;
+
+        static constexpr unsigned int MAX_INST_SIZE = 64;
+        static constexpr unsigned int BYTES_PER_CYCLE = 4;
+
+        void clock() {
+            if (restartp.can_read()) {
+                auto r = restartp.read();
+                generation_down = r.new_generation;
+                pc = r.new_pc;
+                next_inst.size = 0;
+                for (auto &f : next_inst.field)
+                    f = 0;
+                fetch::restart fr;
+                fr.new_generation = ++generation_up;
+                fr.new_next_line_address = pc >> memory::LINE_BYTES_LOG2;
+                fr.previous_line_address = r.from_pc >> memory::LINE_BYTES_LOG2;
+                fetch_restartp->write(std::move(fr));
+                return;
+            }
+            if (next_inst.size >= MAX_INST_SIZE)
+                return;
+            if (bundlep.can_read()) {
+                const auto &b = bundlep.peek();
+                for (unsigned int i = 0; i < BYTES_PER_CYCLE; ++i) {
+                    auto line = pc >> memory::LINE_BYTES_LOG2;
+                    auto offset = pc & memory::LINE_BYTE_OFFSET_MASK;
+                    if (b.generation == generation_up && b.line_address == line && instp->can_write()) {
+                        decodebyte byte;
+                        std::memcpy(&byte, b.data.data() + offset, sizeof(byte));
+                        pte(b.transaction, "d", fmt::format("decode gen={} pc={:x} byte={:02x}", generation_up, pc, *reinterpret_cast<std::uint8_t *>(&byte)));
+                        ++next_inst.size;
+                        if (byte.invert)
+                            next_inst.field[byte.field] = ~next_inst.field[byte.field];
+                        next_inst.field[byte.field] = next_inst.field[byte.field] << 4;
+                        next_inst.field[byte.field] |= byte.bits;
+                        ++pc;
+                        if (!byte.hold) {
+                            next_inst.transaction = infra::pt::child(b.transaction);
+                            next_inst.generation = generation_down;
+                            next_inst.linear_next_pc = pc;
+                            next_inst.predicted_next_pc = pc;
+                            pte(next_inst.transaction, "D", fmt::format("decode gen={}", generation_down));
+                            bool jump = false;
+                            std::optional<std::uint64_t> target;
+                            std::optional<bool> taken;
+                            switch (next_inst.field[OPCODE]) {
+                                case OP_JUMP_ABS_IF_ZERO:
+                                    jump = true;
+                                    if (next_inst.field[FLAGS_DST] & FLAG_IMM1)
+                                        target = next_inst.field[SRC1];
+                                    if (next_inst.field[FLAGS_DST] & FLAG_IMM2)
+                                        taken = next_inst.field[SRC2] == 0;
+                                    break;
+                                case OP_JUMP_ABS_IF_NONZERO:
+                                    jump = true;
+                                    if (next_inst.field[FLAGS_DST] & FLAG_IMM1)
+                                        target = next_inst.field[SRC1];
+                                    if (next_inst.field[FLAGS_DST] & FLAG_IMM2)
+                                        taken = next_inst.field[SRC2] != 0;
+                                    break;
+                            }
+                            std::optional<std::uint64_t> redirect;
+                            bool unpredictable = false;
+                            if (jump) {
+                                if (target.has_value()) {
+                                    if (taken.has_value()) {
+                                        if (taken.value())
+                                            redirect = target;
+                                    } else if (target.value() < pc) {
+                                        redirect = target;
+                                    }
+                                } else if (!taken.has_value() || taken.value()) {
+                                    unpredictable = true;
+                                }
+                            }
+                            if (redirect.has_value()) {
+                                pte(next_inst.transaction, "", fmt::format("fe predicts jump to {:x}", redirect.value()));
+                                next_inst.predicted_next_pc = pc = redirect.value();
+                            } else if (unpredictable) {
+                                pte(next_inst.transaction, "", "frontend halt due to unpredictable jump");
+                                next_inst.predicted_next_pc.reset();
+                            }
+                            instp->write(std::move(next_inst));
+                            next_inst.size = unpredictable ? MAX_INST_SIZE : 0;
+                            for (auto &f : next_inst.field)
+                                f = 0;
+                        }
+                    }
+                }
+                auto line = pc >> memory::LINE_BYTES_LOG2;
+                if (b.generation == generation_up && b.line_address != line && b.next_line_address != line) {
+                    fetch::restart fr;
+                    fr.new_generation = ++generation_up;
+                    fr.new_next_line_address = pc >> memory::LINE_BYTES_LOG2;
+                    fr.previous_line_address = b.line_address;
+                    fetch_restartp->write(std::move(fr));
+                }
+                if (b.generation != generation_up || b.line_address != line)
+                    bundlep.discard();
+            }
+        }
+    };
+}
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