Memoize physical address/main-memory map

Spike spends a surprising amount of time searching the device map.

riscv/sim.cc

@@ -402,10 +402,20 @@ void sim_t::set_rom()
 }
 
 char* sim_t::addr_to_mem(reg_t paddr) {
-  auto desc = bus.find_device(paddr >> PGSHIFT << PGSHIFT, PGSIZE);
-  if (auto mem = dynamic_cast<abstract_mem_t*>(desc.second))
-    return mem->contents(paddr - desc.first);
-  return NULL;
+  auto page_offset = paddr % PGSIZE;
+  auto page_addr = paddr - page_offset;
+
+  if (auto it = addr_to_mem_cache.find(page_addr); it != addr_to_mem_cache.end())
+    return it->second + page_offset;
+
+  auto desc = bus.find_device(page_addr, PGSIZE);
+  if (auto mem = dynamic_cast<abstract_mem_t*>(desc.second)) {
+    auto res = mem->contents(page_addr - desc.first);
+    addr_to_mem_cache.insert({page_addr, res});
+    return res + page_offset;
+  }
+
+  return nullptr;
 }
 
 const char* sim_t::get_symbol(uint64_t paddr)

riscv/sim.h

@@ -13,6 +13,7 @@
 #include <fesvr/htif.h>
 #include <vector>
 #include <map>
+#include <unordered_map>
 #include <string>
 #include <memory>
 #include <sys/types.h>
@@ -73,6 +74,7 @@ private:
   std::vector<std::pair<reg_t, abstract_mem_t*>> mems;
   std::vector<processor_t*> procs;
   std::map<size_t, processor_t*> harts;
+  std::unordered_map<reg_t, char*> addr_to_mem_cache;
   std::pair<reg_t, reg_t> initrd_range;
   std::string dts;
   std::string dtb;